Electrical motor 2

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Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramcos

employees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,

or disclosed to third parties, or otherwise used in whole, or in part,

without the written permission of the Vice President, Engineering

Services, Saudi Aramco.

Chapter : Electrical For additional information on this subject, contact

File Reference: EEX20302 W.A. Roussel on 874-1320

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Selecting A Motor Type


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Engineering Encyclopedia Electrical

Selecting Motor Type

Saudi Aramco DeskTop Standards

CONTENTS PAGE

Operating Characteristics and Applications 1

of Three-Phase AC Motors and Single-Phase AC Motors

Operating Characteristics and Typical Applications of DC Motors 35

Selecting the Appropriate Types of Three-Phase AC Motors 45

Selecting the Appropriate Types of Single-Phase AC Motors 52

Selecting the Appropriate Types of DC Motors 54

WORK AID:

Work Aid 1: Procedure and Technical and Economic Factors From 57

SADP-P-113 for Selecting the Appropriate Types of

Three-Phase AC Motors

Work Aid 2: Procedure and Technical and Economic Factors from 61

Established Engineering Practices for Selecting the

Appropriate Types of Single-Phase AC Motors

Work Aid 3: Procedure and Technical and Economic Factors from 64

Established Engineering Practices for Selecting the

Appropriate Types of DC Motors

GLOSSARY 64


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Saudi Aramco DeskTop Standards 1

OPERATING CHARACTERISTICS AND APPLICATIONS OF THREE-PHASE AC

MOTORS AND SINGLE-PHASE AC MOTORS

The operating characteristics of three-phase and single-phase alternating current (AC) motors

are very different from each other and will be separately discussed. Each type of AC motor

also has a typical application that is based on the motor's operating characteristics. The

following topics will be discussed:

Operating Characteristics of Three-Phase AC Motors

Typical Applications of Three-Phase AC Motors

Operating Characteristics of Singe-Phase AC Motors

Typical Applications of Single-Phase AC Motors

Oper ating Char acteristics of Thr ee-Phase AC Motor s

All three-phase AC motors can be supplied from the same power network. The difference in

three-phase motors is in the characteristics that the motor displays when the motor is in

operation. The following types of three-phase AC motor operating characteristics will be

discussed:

Squirrel-Cage Induction Motors

Wound Rotor Induction Motors

Synchronous Motors

Squirr el-Cage Induction Motors

The squirrel-cage induction motor is the simplest and most rugged of the three-phase AC

motors. The squirrel-cage induction motor can be used in a variety of applications due to the

motors design. The following topics of squirrel-cage induction motors will be discussed in

this section:

Uses and Classifications

Operating Characteristics


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Uses and Classifications- The standard squirrel-cage motor is a general-purpose motor.

The squirrel-cage induction motor is for use in driving loads that require a variable

torque at a relatively constant speed and a high full-load efficiency.

There are different types of squirrel-cage induction motors. The main difference

between types of squirrel-cage induction motors is the construction of the rotor. A

change in the construction of the rotor causes a change in the resistance characteristics

of the rotor; a change in the resistance characteristics of the rotor causes a change in

the torque and current characteristics of the motor.

The National Electrical Manufacturers Association (NEMA) classifies squirrel-cage

motors in accordance with the motor's electrical characteristics. Squirrel-cage motors

have the following classifications:

NEMA Class A motors are the most popular motors. Class A motors have anormal starting torque, a normal starting current, and a low slip.

NEMA Class B motors are built to develop a normal starting torque with a

relatively low starting current.

NEMA Class C motors have a high starting torque, a low starting current, and a

low slip.

NEMA Class D motors are special purpose motors. Class D motors have a very

high starting torque, a high slip (15-20%), a low starting current, and a low

efficiency.

Operating Char acteristics - Figure 1 shows the basic torque/speed characteristics of an

induction motor. Figure 1 shows two curves: curve 1 represents the load torque and

curve 2 represents the motor torque. The following specific torques that are associated

with the operating characteristics of an AC induction motor are identified on Figure 1:

Locked-rotor torque - The minimum torque that is developed by a motor at the

instant that rated power is supplied to the motor terminals. Locked-rotor torque

also is called breakaway or starting torque. The motor must have enough locked-

rotor torque to start turning the load. A motor cannot start a connected load whenthe connected load has a higher torque rating than the motor.

Breakdown torque - The maximum torque that a motor can develop when the

motor is supplied with its rated input power.


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Accelerating torque - The torque that a motor develops between zero speed and full

rated speed when the motor is supplied with its rated input power. Accelerating

torque is the net difference between the motor torque and the load torque.

Accelerating torque determines the rate at which the motor can accelerate a load tofull rated speed.

Full-load torque - The torque that a motor can develop when the motor is at rated

speed and the motor is supplied with its rated input power. The previous motor

torques normally are expressed as a percent of the full-load torque value.

The torque of a motor is a rudimentary operating characteristic. Analysis of torque

fluctuations during the stages of motor operation will provide an understanding of the

types of conditions in which a particular type of motor can be utilized. The torque that

a motor produces depends on a set of operational variables. The following proportion

shows how the operational variables relate to motor torque. The operatingcharacteristics of a squirrel-cage induction motor can be derived through an analysis of

the operational variables.

where: _ = Motor torque

I = Motor current

V = Motor terminal voltage

L = Load on the motor

N = Motor speed

R = Resistance of the motor windings


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Basic Torque/Speed Characteristics of an Induction Motor

Figure 1


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The best way to analyze how each of the variables effect the torque of the squirrel-

cage induction motor is to look at the different phases of motor operation. There are

three phases of motor operation to analyze:

_How the motor responds at starting.

_How the motor responds to changing loads.

_How the motor responds to an overload.

A motor at standstill must produce enough starting torque to cause rotation of the

motor and the connected load. The development of motor start torque can be seen

through an analysis of the variables in the following torque relationships:

The relative values of the variables in the torque relationship at the moment a squirrel-

cage induction motor start are as follows:

_I - Current - When the motor is energized, the starting

current that is drawn will be high. The high starting current is

due to the fact that no counter electromotive force (CEMF) is

being produced in the motor.

_V - Voltage - The applied voltage will equal line voltage and

will not fluctuate.

_L - Load - The load is constant at this point.

_N - Speed - The motor speed at the instant of start is zero.The speed of the motor at start is as low as possible, which

causes torque to be high.

_R - Resistance of the Rotor - This value will not vary with a

particular motor. The only way that the resistance will vary is to

design the motor differently.

As the variables change, so does the motor torque. At the moment of start, the torque

is high because current is high and speed is low. The starting torque that is developed

by a motor must be larger than the torque that is required by the load. Starting torque

that is equal to or less than load torque will not cause rotation of the motor and load.

Figure 2 shows the minimum starting torque for a squirrel-cage induction motor as a

percentage of full load torque for various numbers of motor poles. The minimum

starting torques are established by the National Electrical Manufacturers Association

(NEMA).


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Minimum Starting Torque for a Squirrel-Cage Induction Motor

Figure 2


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After the motor has been energized and the motor develops starting torque, the rotor

will start to rotate. The rate at which the motor accelerates depends on the motor's

developed torque and the torque that is required by the load. The difference in these

torques is known as net accelerating torque. The change in motor torque from

standstill to full rated speed can be analyzed through reference back to the torque

proportion.

_I - Current - The motor current will continue to increase

initially until there is sufficient rotation to produce CEMF that

will limit the current flow. The increase in current will cause an

increase in torque.

_N - Speed - The speed will continue to rise as the motor

accelerates. The rise in speed will tend to lower torque, but until

the motor reaches about 80-85% of full rated speed, the rise incurrent will greatly outweigh the rise in speed.

_V-L-R - Voltage, load, and resistance will all remain relatively constant

during motor acceleration.

The net accelerating torque of a squirrel-cage induction motor will be large at the

moment of starting. Net accelerating torque will continue to increase until the motor

reaches about 80-85% of the motor's rated speed. After reaching 80-85% of rated

speed, the net accelerating torque of the motor will start to decrease.

The next phase of motor operation is how the motor responds to a change in load.Analysis of how torque and the variables of torque vary during a change in load will

explain the operating characteristics of a running squirrel-cage induction motor. The

torque proportion for use in this analysis remains the same as during starting.

A motor that is running at rated speed will develop just enough torque to maintain the

load rotation at a predetermined speed. As the variables of torque change during

operation, the changing variable will cause other variables to change, which keeps the

proportion balanced and the load running. Figure 3 shows a graphic representation of

how the variables of torque change during operation.


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Speed of a squirrel-cage induction motor will vary as load is added or subtracted from

the motor. A squirrel-cage induction motor's operating range is from approximately

90% synchronous speed to 100% synchronous speed. A load (L) increase will cause

motor speed (N) to decrease. The decrease in motor speed will cause the current (I) of

the motor to increase. The resultant increase in current will cause torque of the motor

to increase to a level that is high enough to support operation of the added load. This

relationship between load, speed, and torque will continue until the point of

breakdown torque is reached. The changes in speed on the curves of Figure 3. The

bottom axis shows that as speed decreases, both current and torque will increase. The

amount of motor slowdown for a load increase is a characteristic of a particular

squirrel-cage induction motor design. Torque and current of a squirrel-cage induction

motor will decrease when speed increases as load is removed.

Because the torque of a squirrel-cage induction motor also varies with the square of

the terminal voltage that is applied to the motor, low terminal voltage will significantlyreduce a squirrel-cage induction motor's torque.

The final phase of motor operation is how a squirrel-cage induction motor responds to

an overload. All squirrel-cage induction motors are designed to operate under a

certain amount of overload; however, the overload cannot exceed the breakdown

torque of the motor. The breakdown torque is the point at which the torque that is

required to run the load at overload exceeds the maximum torque that the motor can

produce.

A squirrel-cage induction motor will react to the increase in load (overload) as

previously discussed. Every time more load is added to the motor, the motor's speedwill decrease and the motor's current will increase. The resultant change will be an

increase in motor torque. Torque will continue to increase as load is added to the

maximum value of torque that the motor can produce. A motor that operates at

maximum torque will operate just to the right of the breakdown torque point on Figure

3. Any more load that is added to the motor will cause the motor's speed to drop more

and will cause current to increase. The torque that is produced by the motor will not

be large enough to continue operation of the motor, and the motor will stop.


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Typical Relationship Between Current, Torque, and Speed

in a Squirrel-Cage Induction MotorFigure 3


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Figure 4 shows the comparative torque speed characteristics of the different

classifications of squirrel-cage induction motors. Typically, starting torque is 150% to

250% of the full load torque. All of the NEMA classes of motors will respond to

operational changes in the same manner. The difference in the four types of squirrel-

cage induction motor classes is the construction of the rotor. A change in the squirrel-

cage induction motor's rotor construction will change the resistance of the motor's

rotor circuit. A change in the rotor circuit's resistance will cause the motor's torque

characteristics to change.

Comparative Torque-Speed Characteristics of

Different Classifications of Squirrel-Cage Induction Motors

Figure 4


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In addition to torque, the following operating characteristics are important to an

understanding of the operation of a squirrel-cage induction motor:

_Slip

_Power factor

_Efficiency

For a given motor, slip is the difference between synchronous speed and motor speed.

Slip is expressed as a percentage of the synchronous speed. The amount of slip of the

motor depends on the amount of load. The slip of the motor will increase and the

motor will run slower when the load is increased. At full load, the motor only slows

slightly, which amounts to one to four percent of synchronous speed. Because of the

small changes in speed from no load to full load, a squirrel-cage induction motor is

considered to be a constant speed motor. The actual speed of the motor rotor will

never reach the motor's synchronous speed. A difference between the speed of aninduction motor and synchronous speed is necessary because of the way the rotor field

is developed in an induction motor.

The most common method for calculation of slip in induction motors is through use of

the following formula:

The synchronous speed of a motor is found through use of the following formula:

where: Ns = Synchronous speed

The following is an example of how to determine the slip of an induction motor. Athree-phase, squirrel-cage induction motor with four poles is operating on a 60 Hz, AC

power circuit at a motor speed of 1,728 rpm. The slip of this squirrel-cage induction

motor can be determined through substitution of the following values into the previous

formulas:

Because of the natural slip characteristic of squirrel-cage induction motors, the

conclusion can be made that the squirrel-cage induction motor is not suitable in

industrial applications where a great amount of speed regulation is required. The

reason for non-selection of a squirrel-cage induction motor is that the speed only can

be controlled by a change in frequency, the number of poles of the rotor, or the motor

slip. Speed of a motor is seldom changed through change of the frequency. Thenumber of poles can be changed either through use of two or more distinct windings or

through reconnection of the same winding to establish a different number of poles.

Slip is an inherent characteristic of the motor's design.


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A squirrel-cage induction motor will operate most efficiently when the power factor

range is maintained in the design range of the motor. A squirrel-cage induction

motor's power factor will vary as the load on the motor changes. The power factor of

the squirrel-cage induction motor will be lowest at no load and will increase to the

highest value at rated full load of the motor. Load that is added to the motor beyond

full load will cause the power factor to start to decrease.

The power factor of a squirrel-cage induction motor also is a factor of the motor's

design speed. The power factor of a slow-speed squirrel-cage induction motor will be

lower than the power factor of a squirrel-cage induction motor that operates at a higher

rated speed. The change in power factor over the range of motor speed is due to the

high leakage reactance of the squirrel-cage induction motor at lower speeds.

The efficiency of a squirrel-cage induction motor is the last characteristic that must be

discussed. Efficiency is the ratio between the input and the output of a motor. Theefficiency of a motor can be described by the following equation:

This equation can be restated as:

The losses of a squirrel-cage induction motor will vary with the exact construction and

application of the motor. Some examples of the losses that are experienced by a

squirrel-cage induction motor are:

_I2R

_Winding

_Bearing friction_Hysteresis

_Eddy currents

The efficiency of a squirrel-cage induction motor also will vary with the load on the

motor. A lightly-loaded squirrel-cage induction motor will have a lower efficiency

than an identical squirrel-cage induction motor that is supplied with rated load.

Because a motor is more expensive to operate when the efficiency is lower, each

motor that is installed should be selected so that the actual load and the rated load of

the motor are as close as possible.

Figure 5 shows a comparison of AC squirrel-cage induction motor curves. Thecomparison shows how the values of power factor, amps, watts, and efficiency of the

motor vary as a percent of the motor load. Efficiency and power factor of a squirrel-

cage induction motor are maximum at full load. Note how rapidly efficiency and

power factor decrease when the motor is operated at less than 100% motor load.


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Comparison of AC Induction Motor Curves

Figure 5


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Wound Rotor Induction M otors

The wound rotor induction motor is another form of the three-phase induction motor. The

wound rotor induction motor has operating characteristics that are similar to the squirrel-cage

induction motor. The only real difference in the operating characteristics of the two types of

induction motors is that some of the operating characteristics of the wound rotor induction

motor can be varied. The operating characteristic that can be varied are torque, current,

speed, and efficiency. These characteristics are varied through a change in the amount of

external resistance that is connected in series with the wound rotor windings.

Figure 6 shows typical torque, current, and speed relationships of wound rotor induction

motor with different amounts of external resistance added. Curve 1 shows the torque speed

characteristics of the wound rotor motor with no external resistance added to the rotor. Curve

2 is the torque speed characteristics of the wound rotor motor with 10% external resistance

added to the rotor. The external resistance is given as a percentage of the external resistancevalue required to give full load torque at standstill.

The starting torque of the wound rotor induction motor with no external resistance adds is

approximately 90% of full load torque. Through addition of 10% external resistance to the

rotor circuit, the starting torque produced by the wound rotor induction motor can be raised to

approximately 200% of full load torque. The starting torque required by the load can be

achieved through change in the amount of external resistance that is added to the circuit.

Also, the addition of the resistance in the rotor circuit will cause the starting current of the

motor to drop.

During operation, the wound rotor induction motor will produce the necessary running torquethat is required to support the operation of the load. The variations in running torque of the

wound rotor induction motors shown in curve 1 and curve 2 are the rate of change in running

torque as compared to speed. A wound rotor induction motor with no external resistance

added to the rotor circuit will develop more running torque for a given drop in speed than a

wound rotor induction motor with 10% external resistance added. The difference in the rate

of development of running torque is due to the current change between the motor in curve 1

and the motor in curve 2. The resistance added to the rotor circuit of the motor in curve 2 will

limit the rise in current as motor speed decreases. The lower increase in current will cause

less running torque to be produced.


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As mentioned above, the breakdown torque of a motor is the maximum torque that a motor

can produce when the motor is supplied with its rated input power. Through change of the

amount of external resistance added to the rotor circuit of a wound rotor induction motor, the

value of breakdown torque can be varied and the speed at which the motor reaches

breakdown torque can be varied. The breakdown torque of the wound rotor induction motor

with no external resistance shown by curve 1 has a breakdown torque of approximately 250%

of full load torque; the breakdown torque of the motor is reached at approximately 83% of

synchronous speed. Addition of 10% external resistance the rotor circuit will cause the

motor's breakdown torque to change as shown in curve 2. The value of the breakdown torque

will only slightly vary by a few percentage points of full load value. The biggest change is

the speed of the motor when breakdown torque is reached. The motor in curve 1 reached

breakdown torque at approximately 83% of synchronous speed, but, if 10% external

resistance is added to the rotor circuit, the motor will not reach breakdown torque until the

motor slows to approximately 50% of synchronous speed.


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Typical Torque, Current, and Speed Relationship of Wound Rotor

Induction Motors With Different Amounts of External Resistance

Figure 6


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The speed and efficiency of the wound rotor induction motor are dependent upon each other.

The speed of the wound rotor motor can be varied by about 50 to 75 percent. To change the

speed of the wound rotor induction motor under a constant load condition, resistance is added

or removed from the rotor circuit. The speed of the motor is decreased through the addition

of resistance to the rotor circuit. The resistance will cause the current flow to drop in the

rotor; the torque produced will be reduced; and the speed of the motor will slow. Conversely,

the speed of the wound rotor induction motor is increased through a removal of resistance

from the rotor circuit. The wound rotor motor is not designed to run at speeds that are slower

than rated speed for extended periods of time. The addition of resistance to the rotor circuit to

lower speed will generally only be done for short duration duties.

A consequence of the addition of resistance to the wound rotor induction motor is the change

in the motor's efficiency. The addition of resistance to the motor rotor circuit to lower the

motors speed will cause the efficiency of the motor to drop. Operation of a wound rotor

induction motor with external resistance added for extended periods of time will significantlyadd to the operating cost of the motor due to the drop in efficiency of the motor. With all the

external resistance removed from the motor's rotor circuit, the wound rotor induction motor's

overall efficiency will be about 2 to 3% less than the overall efficiency of a comparable

squirrel-cage induction motor because of a difference in the motor's construction.

The power factor of a wound rotor induction motor is a factor of the motor's design. The

power factor of the motor will vary over the load of the motor just as the power factor varied

on the squirrel-cage induction motor.

Synchr onous Motors

Synchronous motors have many of the same relationships and characteristics as induction

motors; however, there are differences. Figure 7 shows the relationship between the speed,

torque, and current in a synchronous motor. Note the location of the torque points on Figure

7 and the high starting current and low running current at different percents of synchronous

speed. The high starting current and low running current at different percents of synchronous

speed are typical of the following torques in a synchronous motor:

_Starting Torque is the torque that is developed when full voltage is applied to

the armature windings and when there is no motion of the motor rotor.

Because the synchronous motor itself has very little starting torque, an alternate

starting method must be used to develop a large enough starting torque.

_Pull-In Torque is the torque that is developed during the transition from slip

speed to synchronous speed. Pull-in torque is the maximum constant torque

with which the motor will pull its connected load into synchronism, with rated

power input, when field excitation is applied.


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_Pull-Out Torque is the value of the torque when the rotor will fall out of

synchronism with the rotating stator field. With increases in the motor load,

the rotor will fall behind the rotating stator field but not out of synchronism. If

the load is increased beyond the pull-out torque point, the motor will "slip a

pole" or pull-out of synchronism. The mechanical pull-out point of a

synchronous motor is approximately half of the distance between adjacent

poles.

Synchronous Motor Speed/Torque/Current Curves

Figure 7


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The speed of a synchronous motor is determined through the frequency of the power supply

and the number of poles of the motor. The operating speed of a synchronous motor will be

constant for a given frequency and the number of poles. The following formula is for use in

the determination of synchronous motor speed in revolutions per minute (RPM).

Because synchronous motor speed is controlled by the number of poles in the motor, a

synchronous motor can be designed for a specific speed application. Figure 8 shows

synchronous motor speeds in rpm for motors that are designed with different numbers of

poles for different supply frequencies (Hertz).

Synchronous Motor Speeds (rpm)

Figure 8


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The speed of a synchronous motor must always remain constant no matter how the load

changes. The angle between the rotation of the field and the rotation of the rotor will increase

as load is increased on a synchronous motor. This increase in angle will cause torque to

increase, but the speed of the synchronous motor will remain constant. Load can be added to

the synchronous motor until the developed torque of the motor reaches pull-out torque. The

addition of any more load to a motor that is operating at pull-out torque will cause the motor

to lose synchronism and stall.

Power factor and power factor correction are important aspects of a synchronous motor's

operation. Power factor is defined as the ratio of real power to apparent power and is usually

expressed as a percent leading when the current in the circuit leads the voltage in the circuit,

or as a percent lagging when the current in the circuit lags the voltage in the circuit. Power

factor is a measure of the efficiency of a circuit. Power factor takes into account inductive

and capacitive reactance that dissipates power that is not available to do real work. The

capacitive reactance in a capacitive circuit causes the current in the circuit to lead the voltagein the circuit; therefore, capacitive circuits have leading power factors. The inductive

reactance in an inductive circuit causes the current in the circuit to lag the voltage in the

circuit; therefore, inductive circuits have lagging power factors.

Power factor is expressed as a unitless fraction. Power factor equals one for a purely resistive

circuit (no inductance or capacitance) and is less than one for circuits with inductive or

capacitive reactance. Power factor is essentially a ratio of the pure resistance of a circuit to

the circuit's total impedance. The power factor of a synchronous motor is controlled by the

amount of field excitation that is supplied to the motor.

In a synchronous motor that is pulling a constant load, a variation of the stator current isaccomplished through variation of the field current. Figure 9 shows synchronous motor "V"

curves for no load, 1/2 load, 3/4 load, and full load conditions. The V curves describe the

relationship between stator current and field current. The curves are called V-curves because

of their shape. For any given load and any given motor, there is a single value of field

current that will give a unity power factor at the motor terminals. An increase in the field

current above the point for unity power factor (moving right) will cause a corresponding

increase in stator current that will cause the power factor to become increasingly leading. A

decrease of the field current from the unity point will cause the stator current to increase, and

the power factor will become more lagging (moving left).


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Synchronous Motor "V" Curves

Figure 9


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The motor field current is set at the value that is stamped on the nameplate and is kept at this

point for all loads during operation. Maximum pull-out torque is maintained through

sustenance of rated field current. Sustenance of rated field current provides the maximum

level of power factor correction. In a motor that is operated at reduced load for a long period

of time, reduction in the field current may be desired. Such a reduction of the field current

would increase the motor efficiency. For a motor that operates at part load with a unity power

factor, the field current can be adjusted until the stator current is at a minimum value.

The following equation is for use in the determination of the required stator current for a

given pf:

A motor that operates at other than unity power factor will supply the system with either

leading or lagging kVA. The amount of kVA that is supplied to the system can be

determined, but first the correct stator current to achieve a desired power factor must be

determined. The amount of kVA that is supplied to a system by a synchronous motor must beknown to allow protective devices and operating mechanisms to be set. A synchronous motor

that operates at full load and rated excitation delivers to the power system a leading kVA

equal to:

where:

hp rating = The horsepower of the synchronous motor

Eff = The efficiency of the synchronous motor

cos _ = Power factor

Electrical Engineers should note that more leading kVA is supplied at partial loads and rated

excitation. The curves in Figure 10 show the reactive kVA for synchronous motors at fourdifferent power-factor ratings and at varying load conditions. These curves are based on

maintenance of full-load rated field current at all loads. For example, a 100 hp (74.6 kW)

80% power factor synchronous motor operated at 75% load supplies a leading reactive kVA

equal to approximately 66 percent of the motor's horsepower rating, or 66% reactive kVA.

The unity power factor synchronous motor (100% pf motor), whose curve is shown in Figure

10, only supplies a leading reactive kVA when the load is less than 100%. The unity power

factor synchronous motor, although providing no leading reactive kVA at full load, still

improves the system power factor through addition of kilowatt load without increase to the

system reactive-kVA load. A synchronous motor that operates at 90, 80, or 70% of power

factor will provide a leading reactive kVA at all loads.


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Reactive kVA for Synchronous Motors

Figure 10


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The efficiency of a motor is a ratio of the input power to the output power of the motor.

Because a synchronous motor has no slip as load is added, the synchronous motor will have a

higher efficiency than a corresponding induction motor. The full load efficiency of a

synchronous motor is generally one to three percent higher than that of an induction motor.

Typical Applications of Thr ee-Phase AC Motors

The squirrel-cage motor is one of the most widely used machines because the squirrel-cage

motor can be built with electrical characteristics to suit almost any industrial requirement.

Another reason the squirrel-cage induction motor is widely used is the motor's simplicity of

construction. Squirrel-cage motors are not suitable in situations where a high starting torque

is required, but, when the starting-torque requirements are of a medium or low value, the

squirrel-cage induction motor is very suitable.

Typical applications of the squirrel-cage induction motor include blowers, centrifugal pumps,and fans. Because of the absence of any exposed electrical connections, the squirrel-cage

induction motor is suitable for use in areas with hazardous environments.

The wound rotor induction motor is very similar to the squirrel-cage induction motor in

application, but the wound rotor induction motor has the ability to start extremely heavy

loads. The following are specific applications of the wound rotor induction motor:

_To drive various types of machinery that require development of considerable

starting torque to overcome friction.

_To accelerate extremely heavy loads that have a flywheel or inertial effect.

_To overcome back pressures set up by fluids and gases in the case of

reciprocating pumps and compressors.

_When motors must be started frequently without overheating the motor.

The advantages of a wound rotor motor over a squirrel-cage induction motor are:

_High starting torque.

_Moderate starting current.

_Smooth acceleration under heavy load._No excessive heating during starting.

_Good running characteristics.

_Adjustable speed control.


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The main disadvantage is that both initial and maintenance costs of a wound rotor motor are

greater than those costs of the squirrel-cage rotor motor. Also, the efficiency of the wound

rotor induction motor is lower than the efficiency of a squirrel-cage induction motor.

A synchronous motor can be used for almost any application for which a squirrel-cage

induction could be used. The main applications of synchronous motors fall into three areas:

_Power-factor correction

_Constant-speed, constant-load drives

_Voltage regulation

Synchronous motors have two advantages over AC induction motors:

_A constant speed with no variation due to changes in load.

_An ability to improve power factor when operated with high DC excitation.

Another factor that must be taken into account in the decision between an induction or

synchronous motor is cost. The cost of the higher-speed, low-horsepower, squirrel-cage

induction motor and control is lower than the cost of the corresponding synchronous motor.

The motor costs are reversed for higher horsepower and lower speeds; the synchronous

machines are less costly.

Running cost also must be considered in selection between a synchronous motor and an

induction motor. The full-load efficiency of an induction motor is generally one percent to

three percent lower than that of a synchronous motor of the same horsepower and speed

rating. The greater efficiency of the synchronous motor over the induction motor can pay

cost dividends over the life of the motor operation.

The synchronous motor should not be used where fluctuations in torque are violent. As a

general rule, synchronous motors also are not used in small sizes (under 50hp) because they

require DC excitation and are more difficult to start than induction motors. Synchronous

motors also fall out of step quite readily when system disturbances occur.


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Oper ating Char acteristics of Single-Phase AC Motors

Single-phase motors were one of the first types of motors developed for use on AC circuits.

Single-phase motors have been perfected over the years from the original repulsion type into

many improved types. The following are the types of single-phase motors that will be

covered:

_Split-phase motor

_Repulsion induction motor

_Capacitive start motor

_Universal motor

Split-Phase Motor

The split-phase induction motor is the most popular of all the single-phase motors. The split-phase motor consists of a squirrel-cage rotor and two stator windings, a main winding, and a

starting winding. Current that is applied to the motor will cause both windings to produce a

magnetic field. The magnetic fields that are produced by the main winding and the starting

winding will be mechanically and electrically displaced. The mechanical displacement is

produced through position of the windings in the stator. The electrical displacement is

produced through the use of windings with different electrical properties.

The main winding is produced to have a low resistance and a high inductance. The starting

winding will have a high resistance and a low inductance. The different characteristics of the

two windings produce a weak rotating electric field. The interaction of the two fields that are

produced by the windings produce the motor's starting torque. In a split-phase motor, thestarting torque is 150 to 200 percent of the full-load torque, and the starting current is six to

eight times of the full-load current.

Figure 11 shows the speed torque characteristics of a split-phase induction motor. Upon

energization of the motor, the combined windings produce the rotating magnetic field that

will produce the necessary torque to start the motor. The motor accelerates to 75 to 80

percent of synchronous speed. At this speed, a starting switch (usually centrifugally operated)

opens to disconnect the starting winding, and the motor operates with the main winding only.

The function of the starting switch is to prevent the motor from drawing excessive current

from the line and to protect the starting winding from damage due to overheating.

The motor may be started in either direction through reversal of the connections to either the

main or the starting winding, but not to both.


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Speed-Torque Characteristics of a Split-Phase

Induction Motor

Figure 11


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Repulsion In duction Motors

The repulsion induction motor has a combination of a squirrel-cage and a repulsion winding

on the rotor. Because of the combination windings, occasionally the motor is referred to as a

squirrel-cage repulsion motor.

A repulsion induction motor can be designed to have either a constant speed or a variable

speed characteristic. In the repulsion induction motor, the desirable starting characteristics of

the repulsion motor (such as high starting torque) and the constant speed characteristics of the

induction motor are obtained. Unfortunately, the two types of motors are impossible to

combine and obtain only the desirable characteristics of each. Because the combination of

both windings will cause the running torque of the repulsion induction motor to be less than a

comparative split phase induction motor, a larger repulsion induction motor would be

necessary for the same load rating.

Figure 12 shows the torque-speed characteristics of a typical repulsion induction motor. The

rotating magnetic field of the repulsion induction motor is produced in the same way as in the

split phase induction motor. The construction of the repulsion induction motor was discussed

in Module EEX 203.01. The torque-speed curve of the repulsion induction motor is very

similar to that of a repulsion motor. The repulsion induction motor has a high starting torque

(approximately 300-350% full load torque) and can operate at relatively high speeds under

light loads. The similarity between the repulsion induction motor curve and the curve of a

repulsion motor is due to the dominance of the commuted repulsion winding when the

repulsion induction motor is started.


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Torque/Speed Characteristic of a Typical Repulsion-Induction Motor

Figure 12


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Brush position of the repulsion induction motor is very important in determination of the

motor's operating characteristics. Figure 13 shows the characteristic curves of a repulsion

induction motor that illustrates the effects of adjustment of brush position. Through

adjustment of the brushes, the direction of rotation of the motor can be changed from

clockwise to counterclockwise or vice versa. The other main effect of a shift in the brush

position is the effect on motor starting torque. When the brushes are at the 0 brush position,

the repulsion induction motor will produce zero starting torque. Starting torque can be

maximized through shift of the motor brushes to 25 degrees off center. The torque graph is a

quantative analysis of how the motor's torque will change. The line of zero torque shows the

relative amount of motor starting torque as compared to other brush positions. The shift in

brush position also will lower the motor's current.

Characteristic Curves of a Repulsion Induction Motor Showing

the Effect of Adjusting Brush Position

Figure 13


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Capacitive Start Motors

The capacitive-start motor is another form of split-phase induction motor that has a capacitor

that is connected in series with the auxiliary winding. The auxiliary circuit of a capacitive

start motor is opened when the motor has attained a predetermined speed. The net effect of

the capacitor in the auxiliary circuit is to give its motor a starting torque of about four times

the motor's rated torque. Once the capacitive start motor has come up to speed and the

starting winding has been disconnected, the motor will have the same running characteristics

as the split-phase motor.

The rotating magnetic field is produced identically to the way in which this field is produced

in the split-phase motor. The larger starting torque comes from the addition of a capacitor in

series with the starting winding. The addition of the capacitor will cause the electrical

displacement of the two fields to increase. This increase in the displacement of the electrical

fields produces the larger torque.

Figure 14 shows a comparison of the torque slip curves for a capacitor start and a split-phase

motor. Curves are shown for both types of motors to show the comparison. Various starting

capacitor values (200 _F, 300 _F, 400 _F, and 500 _F) also are shown for comparison.

Through change of the value of the starting capacitor, the starting current will be greatly

effected. An increase in the capacitor size will cause an increase in the motor's starting

torque. The capacitor start type of motor has certain advantages over the other single-phase

AC motors in that the motor has a considerably higher starting torque that is accompanied by

a high power factor. Notice how the torque of the capacitive start motor drops at the point

where the centrifugal switch opens. The capacitive start motor operates in the same speed

range as a split-phase induction motor.


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Torque-Speed Curves for Capacitor-Start and Split-Phase Motors

Figure 14


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Universal Motor s

A universal motor is a series wound motor that may be operated on direct current (DC) or

single-phase alternating current (AC). Because a universal motor is a series wound motor, the

universal motor's operating characteristics are very similar to those of a DC series wound

motor. The main difference in the operating characteristics of the universal motor and the

series DC motor is that the universal motor will have a no load speed. The no load speed of

the universal motor will be quite high but not high enough to damage to motor.

Universal motors are very susceptible to changes in speed and these changes in speed must be

considered whenever a universal motor is used. The following three factors change the speed

of a universal motor:

_A change in load.

_A change in frequency of the power supply._A change in applied voltage.

When a load is placed on a universal motor, torque will increase and speed will decrease. The

speed of the universal motor will continue to decrease as load and torque are added. Figure

15 shows the torque-speed characteristics for a typical universal motor with a change in the

frequency of the power supply. The power supply frequencies that are shown are of 25 Hz

AC, 60 Hz AC, and DC power. The curves show that at a 25 Hz supply, the universal motor

will develop the maximum torque and that the minimum starting torque will be developed at

60 Hz.

Adjustment of the speed of a universal motor is very easily accomplished. The speed of theuniversal motor can be adjusted through adjustment of the input voltage to the motor.

Adjustment of the input voltage to the motor is accomplished through use of a variable

resistor. Adjustment of the value of the variable resistor allows the speed of the universal

motor to be adjusted at will.


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Torque/Speed Characteristics of a Typical Universal Motor

Figure 15


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Typical Applications of Single-Phase AC Motor s

The split-phase induction motor is the most popular of the fractional-horsepower motor types.

The split-phase motor is most commonly used in sizes that range from 1/30 hp (24.9 W) to

1/2 hp (373 W) for applications such as fans, business machines, automatic musical

instruments, and buffing machines.

The split-phase motor has the advantage of a very low initial cost. A disadvantage of the

split-phase motor is that the motor has a relatively low starting torque.

The capacitive-start motor is made in sizes from 1/4 hp (150W) to 10 hp (7.5 KW). The

starting capacitor is a dry-type electrolytic cell made for AC use. Typical values of the

capacitors are from 200 to 600_F. The major advantage of the capacitor start motor is the

increase in starting torque. The starting torque of a capacitive start motor can be about four

times the rated torque of the motor. This increase in starting torque makes the capacitive startmotor very useful. The disadvantage in the capacitive start motor is the increased cost over

the split-phase motor. Typical applications of the capacitive start motor would be a

compressor or a pump drive because of the large starting torque that is developed by the

capacitive start motor.

The repulsion induction motor is especially suitable to drive frequently started devices such as

compressors, air pumps, and water systems. The two advantages to the repulsion induction

motor are its low starting current and its constant speed. The low starting current of the

repulsion induction motor is what makes this motor so suitable for applications that require

frequent starting. The motor's constant speed characteristics add to the motor's efficiency.

The only disadvantage to the repulsion induction motor is the increased cost of the motor over

the split-phase induction motor. The repulsion induction motor will generally cost about

twice as much as a split-phase induction motor.

The universal motor is often preferred because of this motor's ability to operate on direct

current (DC) or on alternating current (AC). In areas where both AC and DC are available,

use of a universal motor increases the flexibility of the motor's application. Most universal

motors are used in high speed applications (such as portable tools) because of the difficulty in

obtaining similar performance from AC and DC power supplies at low speeds.

The ability to adjust the speed of a universal motor at will by adjustment of the resistance isan advantage in uses where speed must be adjusted over a large range.

The disadvantage of a universal motor is the increased cost. The increased cost is due to the

increased winding insulation requirements. The winding insulation requirements increase

over a comparable series wound DC motor because of the peak voltage to which insulation is

exposed in an AC supply.


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OPERATING CHARACTERISTICS AND TYPICAL APPLICATIONS OF DC

MOTORS

Each type of DC motor, although it contains the same basic parts as discussed in EEX 203.01,

has very different operating characteristics. The operating characteristics of the different

types of DC motors will be determined by how the windings of the motors are employed. The

following topics will be covered in this section:

_Operating Characteristics of DC Motors

_Typical Applications of DC Motors

Oper ating Char acteristics of DC Motors

Different types of DC motors have different operating characteristics. Because of these

differences, the proper type of DC motor should be selected when the load to be driven isknown. The following types of DC motors are discussed below:

_Series Motors

_Shunt Motors

_Cumulative Compound Motors

_Differential Compound Motors.

Series Motor s

The series motor has the highest starting torque of all DC motors and is ideal for applications

(such as hoists, cranes, and locomotives), that require high torque and slow speeds. Thespeed of a DC series motor is controlled by the size of its load.

Figure 16 shows the torque/speed characteristics of a DC series motor. Note that the motor

speed varies greatly with respect to the torque. At point 1, there is no load on the motor, and

the motor will overspeed and destroy itself from excessive speed. At point 2, 50% of the load

has been applied to the motor. The torque increases, and speed will be about 150% of full

load speed. At point 3, 100% load has been applied to the motor; the torque has increased to

100% full load torque; and speed is at 100% of full load speed. As load is increased past

100% full rated load, the speed of the series motor will drop rapidly. A heavy load must

always be applied to a series DC motor otherwise, these motors will speed out of control and

destroy themselves.


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Operating Characteristics of A DC Series Motor

Figure 16


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Shunt Motors

The shunt motor, in comparison to the series motor, has a very low starting torque that

requires the shaft load to be relatively small. A DC shunt motor has a no load speed point and

can be operated without a connected load. Operation of a DC shunt motor without load will

not cause the motor to speed out of control.

Figure 17 shows the torque/speed characteristics of a DC shunt motor. Figure 17 shows that

this motor will run at nearly the same speed at any load within the motor's capacity and that

the motor will not slow very much even when it is greatly overloaded. There is only a slight

drop in speed from no load (point 1) to full load (point 2). The slight difference in speed is

called the droop of the motor.

Figure 17 also shows the development of linear torque through addition of load to the motor.

The linear addition of torque allows for very smooth operation of the motor over a varyingload.

Operating Characteristics of a DC Shunt Motor

Figure 17


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The shunt motor's speed can be varied through variance of the amount of current that is

supplied to the shunt field. Control of the current to the shunt field allows the rpm to be

changed by 10 to 20 percent when the motor is at full rpm. A shunt motor's speed control

usually is accomplished through placement of a rheostat in series with the shunt field.

Change in the position of this rheostat will increase or decrease the voltage that is applied to

the field. This change in the voltage that is applied to the field results in a corresponding

change in field current and strength. When the field current is decreased, the motor speed will

increase. Motor speed will increase because of the following chain of events:

_When motor field decreases, CEMF decreases (FC _ N f).

_When CEMF decreases and applied EMF stays the same, armature current

increases.

_When armature current increases in a shunt motor, torque increases (_a __fIa).

_When torque increases with constant load, speed increases (N _ _).

In the above explained sequence:

Fc = Force of the CEMF

N = Speed of rotor

_f = Flux of the field

Ia = Armature current

Ea = Armature voltageEc = CEMF voltage

Ra = Armature resistance

_= Torque

From this sequence, the net effect of a decrease in the shunt motor field current is an increase

in the shunt motor's speed. The opposite is true when shunt motor field current is increased.

The shunt motor's rpm also can be controlled through regulation of the voltage that is applied

to the motor armature. If such regulation is applied, and if the motor is operated on less

voltage than is shown on its nameplate, the motor will run at less than full rpm. The shunt

motor's efficiency will drastically drop off when the shunt motor is operated below themotor's rated voltage. The drop in motor efficiency is caused by an increase in heat loss in

the motor windings. Because the motor will tend to overheat when operated below full

voltage, motor ventilation must be provided. The motor's torque also is reduced when the

motor is operated below the full voltage level.


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Cumulative-Compound Motors

In a cumulative compound motor, the series and shunt windings are connected so that the flux

that is produced by the windings aid each other. The DC cumulative compound motor will

have a combination of the operating characteristics of a series DC motor and a shunt DC

motor. The cumulative compound DC motor will have more starting torque than a

shunt DC motor but not as much starting torque as a series DC motor. The cumulative motor

will have larger speed droop than a shunt DC motor but not as much speed droop as a series

DC motor. The characteristics of the cumulative motor will be determined by the amount of

turns in the series field. The more turns there are in the series winding, the more closely the

operating characteristics will emulate those of a series DC motor. When a cumulative

compound DC motor has few turns in the series field DC motor, the motor will more closely

resemble the operating characteristics of a DC shunt motor.

Figure 18 shows the torque/speed characteristics of a cumulative compound DC motor.Notice how the speed droops as the load is increased. Although the droop is greater than that

of a shunt DC motor, the cumulative compound motor does have a no load speed and will not

runaway. The torque development of a cumulative DC motor is relatively linear. Because

speed is not easily controlled in a cumulative compound DC motor, the cumulative compound

DC motor is not suitable for applications that require adjustable speed.


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Torque/Speed Characteristics of a Cumulative Compound Motor

Figure 18


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Differential Compound Motors

A differential compound DC motor is of a very similar design to a cumulative compound DC

motor. The only difference in the two types of compound motors is that in the differential

compound DC motor the series and shunt windings will be connected so that their individual

flux will be in opposition to each other. A change in the connections of the fields in the

differential compound DC motor will cause the field fluxes to oppose each other. The

differential compound DC motor will have a lower starting torque but a more constant speed

characteristic than the cumulative compound motor.

Figure 19 shows the torque/speed characteristics of a differential compound DC motor.

Notice that the torque will raise in an approximately linear manner as in the cumulative

compound DC motor, but the speed will not droop as much as it will in the cumulative

compound DC motor. Figure 19 also shows that the differential compound DC motor will

have a rising speed characteristic when load is added beyond the full load of the motor.


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Torque/Speed Characteristics of a Differential Compound Motor

Figure 19


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Typical Applicat ions of DC Motor s

A series DC motor is used in applications where a high starting torque is required but where

running speed regulation is of little concern. The advantages of the series DC motor are the

motor's high starting torque and relatively low initial cost. One disadvantage of the series

motor is that the speed will decrease steadily as more load is applied to the motor. Another

disadvantage of the series DC motor is that the motor must be hard-connected to a load

because the series DC motor has no "no load" speed. Loss of load on a DC series motor will

cause the motor to speed out of control and destroy itself.

A shunt DC motor, as compared to the series DC motor, will have a lower starting torque but

much better speed control. Because of the motor's speed control, the shunt DC motor is very

useful in applications where speed accuracy is required but where a large starting torque is not

required. A good example of where speed control would be necessary is on machine tools or

lathes. Because a shunt DC motor also has a no load speed, runaway is not a concern of theshunt DC motor. A no load speed makes the shunt motor very useful in running belt drive

equipment such as a conveyor belt. The main advantage of a shunt DC motor is speed

control. The main disadvantage of a shunt DC motor is the low starting torque.

The use of the cumulative and differential compound DC motors are very similar. Because

both the cumulative and differential compound DC motors are the same except for electrical

connections, cost is not an issue in selection of the motor type. Both of the compound DC

motors cost more than the series or shunt motors. The cumulative compound DC motor

would be used where a higher starting torque is required but where speed control is not a vital

issue; examples of this application would be in some hoisting and conveying machinery. The

differential compound DC motor would be used in situations where a high starting torque isnot required but where speed regulation is more important. Examples of typical applications

of the differential compound DC motor would be in pumps or paper cutting machines. The

main advantage of a compound DC motor is that these motors can be designed to combine the

desired characteristics of the shunt DC motor and the series DC motor. The main

disadvantage of compound DC motors is that these motors cost more than the shunt DC motor

or the series DC motor.

Figure 20 shows an overall composite of the speed, torque, and current (load) characteristics

for compound, shunt, and series DC motors of equal size. This figure provides a comparison

of the torque and speed characteristics that each DC motor type will exhibit. The cumulative

and differential compound motors are shown as one line that is called compound because thecurve that is shown is typical. Figure 20 shows that the series motor has the highest torque,

followed by the compound motors, and by finally the shunt motor. If starting torque is the

most important consideration, the series motor would be the best selection.


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Figure 20 also shows that the series motor speed droop is much greater than that of the

compound orshunt motors. If speed control is the most important consideration, the shunt

motor is the best selection. The DC compound motors are good examples of a compromise in

both torque and speed characteristics. The selection of this compromise will cause costs to

increase due to the complexity of the motor.

Speed-Torque and Current Characteristic Curves for Compound, Shunt, and

Series DC Motors of Equal Size

Figure 20


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SELECTING THE APPROPRIATE TYPES OF THREE-PHASE AC MOTORS

Selection of the appropriate type of three-phase motor for a Saudi Aramco application,

requires that all the motor selection factors to be considered. Early failure may occur in a

three-phase AC motor that does not completely fit the intended application. The selection of

a type of 3_ AC motor basically involves a choice between an induction and a synchronous

motor. The subselection of a squirrel-cage induction versus a wound rotor motor is based on

torque requirements and cost. The selection of a motor also will take into account the area

into which the motor will be installed.

Motor Selection Factors

A designer must weigh all of the factors that bear on the selection of the type of 3_ AC motor

for a particular application. The following factors must be considered:

_Preferred Voltage and Horsepower Ranges

_Load Characteristics

_Motor Starting Characteristics

_Speed

_Power Output Required

_Limitations of the Supply Network

_Cost

Pr eferr ed Voltage and Horsepower Ra nges

Three-phase AC motors come in a variety of standard voltage and horsepower ratings. Aspecial order for a specific rating that is not in a standard rating would cause the cost of the

motor to increase. Saudi Aramco allows a limited choice of voltage and horsepower ranges.

The actual table of the allowed voltage and horsepower ranges for use in Saudi Aramco

installations is in Work Aid 1.

The second table in Work Aid 1 shows the preferred voltage and horsepower ranges of Saudi

Aramco installations. Column 1 of the table in Work Aid 1 shows the nominal system

voltages. Column 2 of the table in Work Aid 1 shows the nameplate voltage or utilized

voltage. The European practice is to quote the nameplate voltage, but the American practice

is to quote nominal system voltage. To avoid confusion of voltage, all motors for use in

Saudi Aramco will be specified by the nameplate voltage only. Column 3 of the table inWork Aid 1 gives the number of phases in the motor. Column 4 of the table in Work Aid 1

shows the available Hp ranges for the specified voltage and phases. Column 5 of the table in

Work Aid 1 shows the type of motor to be selected to meet the necessary requirements.


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The table shows that synchronous motors are only used on high horsepower applications

unless the operating speed is required to be less than or equal to 1200 rpm.

Load Char acteristics

Load characteristics refer to the following:

_Load horsepower

_Required load starting torque

_Speed at which the load must operate

_Steadiness or unsteadiness of a load

Load horsepower will determine the size of the motor that is required for the application. In

actuality, any type of motor can be designed for a specific horsepower requirement. Column

4 of the second table in Work Aid 1 gives the approved Saudi Aramco horsepower ranges formotors.

Required Load Starting Torque of a load will vary with the type of load. For loads that obey a

square-law characteristic, a motor's required load starting torque should be at least 60 percent

of the full-load torque for liquid pumps, and 40 percent of full-load torque for gas-handling

pumps. Pumping requirements over 11,000 kW (15,000 Hp) should be referred to Consulting

Services Department.

A constant speed motor must be applied for loads that must operate at constant speed. The

synchronous motor by design must always run at a constant speed. The speed of a

synchronous motor was designed by its construction and the supply frequency. SaudiAramco uses 60 Hz input frequency for all motors. The synchronous motor would be the best

choice for a load that must be operated at a constant speed.

The steadiness of a load also plays a large role in the selection of a type of 3_ AC motor.

Induction motors account for varying loads through adjustment of the amount of slip. A

synchronous motor does not compensate for a varying load. Where the variations in load are

excessive, such as a compressor application, a synchronous motor may slip out of

synchronism and stall. The pole slippage could be for a short duration such as one pole or a

complete stoppage of the motor. Slippage or stoppage depends on the amount of load

variation.


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Figure 21 shows typical load speed/torque curves for Saudi Aramco equipment. The figure

shows that the profile of the curve for the centrifugal pump can be changed through variance

of the load (valve shut or valve open). Reduced load starting (valve shut) should be done

only when absolutely necessary because of excess heat build up in the motor. The axial

compressor load torque drops initially after the load commences to move because of the

inertia of the load. The axial compressor will develop a smooth torque build up.

Typical Load Speed/Torque Curves

Figure 21


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Motor Starting Char acteristics

All motors that are used in Saudi Aramco applications must be self-starting. Induction motors

are self-starting by design and always can be used in Saudi Aramco. Synchronous motors

must be designed with a method of self-starting. The most common way to accomplish the

self-starting of a synchronous motor is to build a squirrel-cage into the motor's rotor. A

squirrel-cage that is built into the rotor of a synchronous motor provides the necessary starting

torque to start the rotor rolling. At the point where the motor reaches synchronous speed,

there will be no relative motion between the rotating magnetic field and the squirrel-cage; no

voltage, therefore, will be induced into the squirrel-cage.

Motor starting torque is a major consideration in the selection between a squirrel-cage

induction motor and a wound rotor induction motor. The addition of the external resister in a

wound rotor induction motor causes starting torque to increase. Also, motor run up time can

be safely increased because the resisters will limit the amount of current flow. Limitation ofthe amount of current flow will prevent damage to the motor's windings when the load takes a

long period of time to accelerate.

Speed

A motor's speed generally is dictated by the need of the driven equipment, except in cases

where the driven equipment is connected to the motor through use of gear boxes. For

example, if the driven equipment must rotate at 1800 RPM, the motor must rotate at 1800

RPM or the driven equipment must be connected to the motor through use of a gearbox that

changes the speed of the motor to 1800 RPM. Such specifications are referred to the relevant

Project Engineers.

Occasionally the motor application requires that speed be optional: e.g., 600 rpm, 900 rpm or

1800 rpm will perform the job to an equal level of satisfaction. When the speed of the motor

is optional, the 1800 rpm (4-pole) motor should be chosen. Generally, 1800 rpm motors are

lighter in weight and less expensive than lower speed motors and 3600 rpm motors.

Because the speed of a synchronous motor is constant at synchronous speed, and because the

speed of an induction motor will vary slightly with load, a synchronous motor is preferable

when constant speed is required.

Power Output Required

The second table in Work Aid 1 shows the acceptable Hp ranges of motors for Saudi Aramco

applications. The table shows that a motor can be selected for almost any necessary output

range and that synchronous motors are used only for motors that are larger than 15,000 Hp or

for motors that are between 670 - 4000 HP and that operate at 1200 RPM or below.


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Motors should not be oversized for an application. An oversize motor will cause the motor to

run at less than maximum output and, consequently, at less than maximum efficiency.

Limitations of the Supply Network

Motor placement and application depend on the kind of power that can be supplied to the

location. Where the application needs a 3_ AC power and only a single-phase AC power is

available, either the installation must be reconsidered or a 3_ AC power supply must be

supplied to the system.

Consideration must given to the strain that starting of the motor will place on the supply

network when large motors are to be selected. This strain will increase as the size of the

motor increases. Voltage dips upon starting will be discussed in more detail later in this

course.

Cost

All costs of the motor must be taken into account prior to selection of a squirrel-cage

induction motor, wound rotor motor, or a synchronous motor.

The following costs must be taken into account to determine the total cost:

_Initial cost

_Running cost

_Maintenance cost

Initial cost is lowest in the squirrel-cage induction motor. The lower initial cost is due to the

simple design of the motor. The wound rotor induction motor and synchronous motor are

more complicated in design and construction and cost more to buy and install.

Maintenance cost is again lowest in the squirrel-cage induction motor. The lower

maintenance cost can again be attributed to the simple construction. The wound rotor motor

has the addition of slip rings and brushes that require a certain amount of maintenance. The

added maintenance for the brushes and slip rings of the wound rotor motor causes the

maintenance cost to rise. Synchronous motors have the highest maintenance costs because

they have not only brushes and slip rings but also some source of excitation that requires a

certain amount of maintenance.

The running cost of a motor will depend on the size of the motor. All motors improve in

efficiency as their size increases. The efficiency of synchronous motors tends to be higher

than the efficiency of induction motors. The increase in efficiency of running a synchronous

motor can sometimes justify the added initial and maintenance costs of the synchronous

motor. For Saudi Aramco purposes, the critical size for justification of a synchronous motor


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selection is approximately 11,000 kW (15,000 Hp), but the economic power rating may be

lower if there is a need to control the power factor of an installation.


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Motor Selection for Hazar dous Areas

Motors that will not ignite a flammable atmosphere can be constructed in various forms to

meet the different classifications of hazards and can be constructed with different types of

protection. The "Ex d" (flameproof or explosion-proof) type of protection, is usual for Zone 1

(Division 1) applications. The "Ex n" (non-sparking type of protection) of totally enclosed

motors is usual for Zone 2 (Division 2) locations.

Because different types of protection are permitted in Zone (Division) 1 and 2 areas, the final

choice must be based on economic considerations. The economic considerations should take

into account the operating costs, reliability, maintenance, and capital cost. The following

guidelines highlight the main cost parameters of each type of protection. Other factors,

however, may apply to particular installations or motor requirements.

_Zone 1 (Division 1 ) Areas

The Ex d type is preferred in sizes up to several hundred kW because this type

is simple and rugged and requires no special motor protection. Above 750 kW,

the price becomes prohibitive because it is difficult to produce a sufficiently

strong enclosure to withstand an internal explosion.

Large motors should not be located in Zone 1 areas to minimize costs.

_Zone 2 (Division 2) Areas

In Zone 2 areas, Ex n motors should be used because they are cheaper. Ex dtype motors should not be used due to their higher cost.

Figure 22 shows a cost comparison for different Ex type motors. The cost of each of the Ex

type motors is compared to the cost of a standard industrial motor. At lower ratings, Ex d

motors that are explosion-proof are relatively inexpensive. But as motor rating goes up the

cost of an Ex d motor rapidly increases. The increase in cost of Ex d motors prohibit their use

above about 750 kW. The cost of Ex n motors that are non-sparking starts out just slightly

above the cost of a standard industrial motor and decreases as the motor rating increases. The

Ex n motor is the most economical motor to use. Ex p motors are pressurized motors. For

small motors, the cost of the Ex p motor is prohibitively large. The cost of the larger Ex p

motors drops rapidly as the motor size increases. At large motor ratings, the cost of an Ex pmotor is feasible.


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Cost Comparisons for Different Ex Type Motors

Figure 22


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SELECTING THE APPROPRIATE TYPES OF SINGLE-PHASE AC MOTORS

Selection of a single-phase AC motor is much simpler than the selection of a 3_ AC motor.

Fewer factors must be considered because of the size and applications of single-phase AC

motors.

Motor Selection Factors

The following factors are taken into account in the selection of a single-phase AC motor:

_Load Characteristics

_Motor Starting Characteristics

_Speed

_Power Output Requirements

_Cost

Load Char acteristics

The load characteristics must be taken into account in selection of a 1_ AC motor just as with

a 3_ AC motor. The amount of torque that is required to start a load in motion must be

known. The starting torque of the load must be met so that the correct motor can be selected.

A motor with a starting torque that is too low will cause the run up time to increase and can

cause the motor to overheat.

Duty cycle also must be considered. The length of time that the load will be required to run

and the frequency with which the load will be cycled are important factors in the selection ofa single-phase AC motor. The more often a load is cycled, the more times the motor windings

will be subjected to starting current. The starting current also is a concern with long load run

up times. A long load run up time will result in the application of the starting current for a

longer period of time.

Motor Starting Char acteristics

For proper selection of a single-phase AC motor, the motor starting characteristics must be

matched with the required load characteristics. The starting torque that is required by the load

must be met by the selected single-phase motor. The split-phase motor has the lowest starting

torque. The repulsion induction motor has a large amount of starting torque. The capacitorstart and universal both have about the same amount of starting torque.

The motor to be selected must have the necessary amount of starting torque to operate the

load. If the starting torque is too low, damage to the motor can occur through excessive

starting current. The repulsion induction motor will have the lowest starting current. This


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characteristic makes the repulsion induction motor more suitable for a load with a harsh-

cyclic duty.


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Speed

The split-phase, capacitor start, and repulsion induction motors can be designed for a wide

variety of speeds. When operated, these types of single-phase motors will exhibit

approximately the same speed droop characteristics. The universal motor is generally only

designed for speeds of 3500 rpm or higher. Most universal motors will operate between

8,000 and 10,000 rpm. Universal motors operate at high speeds because it is difficult to

obtain similar performance on AC and DC at low speeds.

Power Output Required

The second table in Work Aid 1 shows the accepted ranges of Hp for single-phase AC

motors. Each of the four types of single phase AC motors is manufactured for a range of

power output requirements. The HP ranges for each type of single-phase AC motor is sho

Documents

Electrical motor 2