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OBJECTIVE:

� Describe types of d.c. motor and their characteristics.

� state typical applications of d.c. motors

� describe a d.c. motor starter

� describe methods of speed control of d.c. motors

� describe types of d.c. motor and their characteristics

Overview:

Operating Principle of DC Motors:

The basic principle of a dc motor is the creation of a rotating magnet inside the

mobile part of the motor, the rotor. This is accomplished by a device called the

commutator which is found on all dc machines. The commutator produces the

alternating currents necessary for the creation of the rotating magnet from dc power

provided by an external source. Figure 8-1 illustrates a typical dc motor rotor with its

main parts. This figure shows that the electrical contact between the segments of the

commutator and the external dc source is made through brushes. Note that the rotor

of a dc motor is also referred to as the armature.

Figure 8-1. The Main Parts of a DC Motor Rotor (Armature).

The construction of a d.c. motor is the same as a d.c. generator. The only difference is

that in a generator the generated e.m.f. is greater than the terminal voltage, whereas in a

motor the generated e.m.f. is less than the terminal voltage.

D.c. motors are often used in power stations to drive emergency stand-by pump systems

which come into operation to protect essential equipment and plant should the normal a.c.

supplies or pumps fail.

Islamic University of Gaza Faculty of Engineering Electrical Engineering department

Electric Machines Lab Eng. Mohammed S. Jouda

Eng. Omar A. Qarmout

Eng. Amani S. Abu Reyala

Experiment

8 Direct Current Motors

Separately Exited, Shunt and Compound Connection

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Back e.m.f:

When a d.c. motor rotates, an e.m.f. is induced in the armature conductors. By Lenz’s law

this induced e.m.f. E opposes the supply voltage V and is called a back e.m.f .,

and the supply voltage, V is given by:

Torque and Speed of a d.c. machine:

In the circuit of Figure 8.2, EA is the voltage applied to the motor brushes, IA

is the current flowing through the brushes, and RA is the resistance between the brushes.

Note that EA, IA, and RA are usually referred to as the armature voltage, current, and

resistance, respectively. ERA is the voltage drop across the armature resistor. When the

motor turns, an induced voltage ECEMF proportional to the speed of the motor is

produced. This induced voltage is represented by a dc source in the simplified equivalent

circuit of Figure 8.2 . The motor also develops a torque T proportional to the armature

current IA flowing in the motor. The motor behaviour is based on the two equations given

below. The first relates motor speed n and the induced voltage ECEMF, and the second

relates the motor torque T and the armature current IA.

n = K1 × ECEMF and T = K2 × IA

where K1 is a constant expressed in units of r/min/V,

K2 is a constant expressed in units of NAm/A or lbfAin/A.

Figure 8-2. Simplified Equivalent Circuit of a DC Motor

The dc motor is the machine that converts the electric energy into mechanical

energy, in other meaning, it is considered a voltage to speed converter figure 8.3

(a, b) and a current to torque converter figure 8.3 (c, d).

(a) DC Motor as a Voltage-to-Speed Converter (c) DC Generator as a Torque-to-Current Converter

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(b) Output Speed versus Voltage (d) Output Torque versus Current

Figure 8.3: DC Motor

The magnet of the generator is electrical magnet which depends on its field current. The

properties of the DC Motor are changed with changing the field current Figure 8.4.

Figure 8.4: Decreasing Current �� below its Nominal Value Increases �� and Decreases ��

Therefore, when a fixed armature voltage EA is applied to a dc motor, the voltage drop

ERA across the armature resistor increases as the armature current IA increases, and

thereby, causes ECEMF to decrease. This also causes the motor speed n to decrease

because it is proportional to ECEMF. This is shown in Figure 8.5 which is a graph of

the motor speed n versus the armature current IA for a fixed armature voltage EA.

Figure 8.5. Motor Speed Drop as the Armature Current Increases (Fixed Armature Voltage EA).

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Any of the methods of excitation used for generators can also be used for motors.

Typical steady-state dc-motor speed-torque characteristics are shown in Fig. 8.6, in

which it is assumed that the motor terminals are supplied from a constant-voltage

source. In a motor the relation between the emf Ea generated in the armature and the

armature terminal voltage Va is

where Ia is now the armature-current input to the machine. The generated emf Ea is now

smaller than the terminal voltage Va, the armature current is in the opposite direction to

that in a generator, and the electromagnetic torque is in the direction to sustain rotation

of the armature.

In shunt- and separately-excited motors, the field flux is nearly constant. Consequently,

increased torque must be accompanied by a very nearly proportional increase in rmature

current and hence by a small decrease in counter emf Ea to allow this increased current

through the small armature resistance. Since counter emf is determined by flux and

speed, the speed must drop slightly.

In the series motor flux increases with load, speed must drop in order to maintain the

balance between impressed voltage and counter emf; moreover, the increase in

armature current caused by increased torque is smaller than in the shunt motor

because of the increased flux. The series motor is therefore a varying-speed motor

with a markedly drooping speed-torque characteristic of the type shown in Fig. 8.6.

Figure. 8.6

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SEPARATELY EXITED DC MOTOR AND FIELD CURRENT EFFECTS:

� OBJECTIVE:

After completing this part, you will be familiar with:

- The relation between the input voltage and the output speed of the DC Motor.

- The relation between the input current and the output torque of the DC Motor.

- The relation between the output speed and the input current.

- The effects of the magnetic current on the characteristics of the DC Motor.

� INTRODUCTION:

At first a separated source will be used to obtain the current in the magnetic circuit of

the stator that is the magnetic circuit is not connected electrically with input of the DC

Motor so it's not affected with the changing of the input current of the DC Motor and

has approximately constant current.

The armature resistance �� of the DC Motor will be measured. It is not possible to

measure the armature resistance �� directly with a conventional ohmmeter because

the non-linear characteristic of the motor brushes causes incorrect results when �� is

too small. The general method used to determine the armature resistance �� consists

in connecting a dc power source to the motor armature and measuring the voltage

required to produce nominal current flow in the armature windings. Power is not

connected to the stator electromagnet to ensure that the motor does not turn, thus

��� equals zero. The ratio of the armature voltage �� to the armature current ��

yields the armature resistance �� directly or the slope of the relation between the

voltage and current.

(a) Separately-Excited DC Motor Coupled to a

Dynamometer (b) Separated Magnetic Circuit

Figure 8.7: Separately-Excited DC Motor.

PRACTICAL 8.1.A: MEASURING THE ARMATURE RESISTANCE

� PROCEDURE:

1- Ensure that the power supply (unit 8821-25) is switched off. Then connect the

circuit shown in Figure 8.7.

2- Let the Magnetic circuit open.

3- Increase the input voltage according to Table 8.1.

Input Voltage � 0 10 20 30 40

Input Current �� Table 8.1

4- Draw the graph of the relation between input voltage and input current.

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5- Calculate the slope which is the armature resistance of the DC Motor.

6- Switch off the power supply.

PRACTICAL 8.1.B: OUTPUT SPEED VERSUS INPUT VOLTAGE CHARACTERISTIC

� PROCEDURE:

1- Make the necessary connecting between the Dynamometer and the Data

Acquisition Interface in order to receive the result on the computer.

2- Set the LOAD CONTROL knob to the MIN. position (fully CCW).

3- On the DC Motor set the Field Rheostat so that the field current is 190��.

4- On power supply, increase the input voltage of the DC Motor 10% and

measure the speed in every time according to (table 8.2).

Voltage

(V) 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Speed

(r/min)

Table 8.2

5- Turn of the power supply after finishing recording.

6- Draw the graph of the output speed as function of input voltage (what do you

observe from the resulting graph? Why?) then determined the value of �� from

the slope.

PRACTICAL 8.1.C: OUTPUT TORQUE VERSUS CURRENT CHARACTERISTIC

� PROCEDURE:

1- Turn on the power supply then adjust the input voltage to reach the speed to

1500 r/min.

2- In the Metering window, record the dc motor output torque T, armature

voltage E�, armature current I�, field current I�, and speed n ( indicated by

meters T, E1, I1, I2, and n, respectively ) in the Data Table. On the Prime Mover /

Dynamometer, set the DISPLAY switch to the TORQUE (T) position then adjust

the LOAD CONTROL knob so that the torque indicated on the module display

increases by 0.2 �. � increments up to 2.0 �. � .For each torque setting, readjust

the voltage control knob of the Power Supply so that the armature voltage EA

remains equal to the value recorded in the previous step, then record the data in

table 8.3.

Output

Torque

�. �

Input

Voltage

�

Input

Current

��

Field

Current

��

Output

Speed

�/�

���

= ��

− ����

Calculate

d Speed

�����

Input

Power

� �

Output

Power 2!"#/60

Efficiency &'

&

0.0 1500

0.2

0.4

0.6

0.8

1.0

1.2 Table 8.3

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3- Turn off the power supply.

4- Draw the graph of the output torque as function of input current (what do you observe from

the resulting graph? Why?) Then determined the value of �� from the slope.

5- Draw the graph of the output torque as function of output speed from table

6- Calculate the input and output power of the DC Motor to determine its efficiency.

7- What are effects of changing the field current on the DC Motor characteristic?

Types of d.c. motor and their characteristics ( SERIES, SHUNT

AND COMPOUND) :

� OBJECTIVE:

After completing this part, you will also be able to demonstrate the main operating

characteristics of series, shunt, and compound motors.

a) Shunt-wound motor: In the shunt wound motor the field winding is in parallel with the armature across

the supply as shown in Figure 8.8 .

Figure 8.8

Supply voltage: V = E + IaRa

or generated e.m.f: E = V − IaRa

Supply current: I = Ia + If ,

Characteristics:

The two principal characteristics are the torque/armature current and speed/armature current

relationships. From these, the torque/speed relationship can be derived.

i. The theoretical torque/armature current characteristic can be derived from the

expression T ∝(Ia, For a shunt-wound motor, the field winding is connected in

parallel with the armature circuit and thus the applied voltage gives a constant field

current, i.e. a shunt-wound motor is a constant flux machine. Since (is constant, it

follows that T ∝Ia, and the characteristic is as shown in Figure 8.9.

Figure 8.9

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ii. Conductors are rotating in a magnetic field, a voltage, E ∝(ω, is generated by the

armature conductors. From equation : V ====E ++++IaRa or E ====V –IaRa

speed of rotation:

iii. For a shunt motor, V, _ and Ra are constants, hence as armature current Ia increases,

IaRa increases and V −IaRa decreases, and the speed is proportional to a quantity

which is decreasing and is as shown in Figure 8.10.

Figure 8.10

iv. Since torque is proportional to armature current, the theoretical speed/torque

characteristic is as shown in Figure 8.11.

Figure 8.11

b) Series-wound motor: In the series-wound motor the field winding is in series with the armature across the supply

as shown in Figure 8.12.

Supply voltage V = E + I (Ra + Rf )

OR generated e.m.f. E =V –I (Ra + Rf )

Figure 8.12

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Characteristics: In a series motor, the armature current flows in the field winding and is equal to the

supply current, I.

i. The torque/current characteristic It is shown in Figure 8.13 that torque T ∝(Ia.

Since the armature and field currents are the same current, I, in a series machine.

Thus (∝I and T ∝��.

Figure 8.13

ii. The speed/current characteristic an approximate relationship for the speed is n∝V/I

n∝1/I since V is constant. as shown in Figure 8.14.

Figure 8.14

iii. The theoretical speed/torque characteristic as shown in Figure 8.15

Figure 8.15

c) Compound -wound motor:

It is possible to combine shunt and series windings to obtain a particular speed

versus torque characteristic. For example, to obtain the characteristic of

decreasing speed when the motor torque increases, a series winding can be

connected in series with the armature so that the magnetic flux it produces adds

with the magnetic flux produced by a shunt winding. As a result, the magnetic

flux increases automatically with increasing armature current. This type of dc

motor is referred to as a cumulative compound motor because the magnetic fluxes

produced by the series and shunt windings add together. Shunt and series

windings can also be connected so that the magnetic fluxes subtract from each

other. This connection produces a differential compound motor, which is rarely

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used because the motor becomes unstable when the armature current increases.

Figure 8.16 shows a compound motor and its speed versus torque characteristic

(cumulative compound).

Figure 8.16: Compound Motor and its Speed versus Torque Characteristic.

PRACTICAL 8.2.A: SERIES DC MOTOR CHARACTERISTICS

1- Ensure that the power supply (unit 8821-25) is switched off.

2- Make the needed change on the previous connection to get the connection in

figure 8.17.

Figure 8.17: Series Motor Coupled to a Dynamometer.

3- In the Metering window, record the dc motor output torque T, armature

voltage E�, armature current I�, and speed n (indicated by meters T, E1, I1,

and n, respectively) in the Data Table. On the Prime Mover / Dynamometer,

set the DISPLAY switch to the TORQUE (T) position then adjust the LOAD

CONTROL knob so that the torque indicated on the module display increases

by 0.2 �. � increments up to 1.2 �. �. For each torque setting, readjust the

voltage control knob of the Power Supply so that the armature voltage E�

remains equal to the value recorded in the previous step, then record the data

in table 8.4.

4- Draw the graph of the output torque as function of output speed

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Output

Torque

�. �

Input

Voltage

�

Input

Current

��

Field

Current

��

Output

Speed

�/�

���

= ��

− ����

Calculate

d Speed

�����

Input

Power

� �

Output

Power 2!"#/60

Efficiency &'

&

0.0 1500

0.2

0.4

0.6

0.8

1.0

1.2 Table 8.4

PRACTICAL 8.2.B: SHUNT DC MOTOR CHARACTERISTICS

1- Ensure that the power supply (unit 8821-25) is switched off.

2- Make the needed change on the previous connection to get the connection in

figure 8.18.

Figure 8.18: Shunt Motor Coupled to a Dynamometer.

3- In the Metering window, record the dc motor output torque T, armature

voltage E�, armature current I�, field current I�, and speed n (indicated by

meters T, E1, I1, I2, and n, respectively) in the Data Table. On the Prime

Mover / Dynamometer, set the DISPLAY switch to the TORQUE (T) position

then adjust the LOAD CONTROL knob so that the torque indicated on the

module display increases by 0.2 �. � increments up to 1.2 �. �. For each

torque setting, readjust the voltage control knob of the Power Supply so that

the armature voltage EA remains equal to the value recorded in the previous

step, then record the data in table 8.5.

4- Draw the graph of the output torque as function of output speed.

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Output

Torque

�. �

Input

Voltage

�

Input

Current

��

Field

Current

��

Output

Speed

�/�

���

= ��

− ����

Calculate

d Speed

�����

Input

Power

� �

Output

Power 2!"#/60

Efficiency &'

&

0.0 1500

0.2

0.4

0.6

0.8

1.0

1.2 Table 8.5

Only By using the virtual instrumentation ( LVVL Program ): PRACTICAL 7.2.C: CUMULATIVE COMPOUND DC MOTOR

1- Ensure that the power supply (unit 8821-25) is switched off.

2- Make the needed change on the previous connection to get the connection in figure

8.19.

Figure 8.19: Cumulative Compound DC Motor Coupled to a Dynamometer

3- In the Metering window, record the dc motor output torque T, armature

voltage E�, armature current I�, field current I�, and speed n (indicated by

meters T, E1, I1, I2, and n, respectively) in the Data Table. On the Prime

Mover / Dynamometer, set the DISPLAY switch to the TORQUE (T) position

then adjust the LOAD CONTROL knob so that the torque indicated on the

module display increases by 0.2 �. � increments up to 1.2 �. �. For each

torque setting, readjust the voltage control knob of the Power Supply so that

the armature voltage EA remains equal to the value recorded in the previous

step, then record the data in table 8.6 .

4- Draw the graph of the output torque as function of output speed.

Output

Torque

�. �

Input

Voltage

�

Input

Current

��

Field

Current

��

Output

Speed

�/�

���

= ��

− ����

Calculate

d Speed

�����

Input

Power

� �

Output

Power 2!"#/60

Efficiency &'

&

0.0 1500

0.2

0.4

0.6

13

0.8

1.0

1.2 Table 8.6

PRACTICAL ASPECTS:

- The series motor provides a strong starting torque and a wide range of operating

speeds when it is supplied by a fixed-voltage dc source. However, the speed,

torque, and armature current depend on the mechanical load applied to the

motor. Also, the series motor has non-linear operating characteristics as

suggested by the speed versus torque relationship. As a result, it is difficult to

operate a series motor at a constant speed when the mechanical load fluctuates.

Furthermore, the armature current must be limited to prevent damages to the

motor when it is starting (when power is applied to the motor). Finally, a series

motor must never run with no-mechanical load because the speed increases to a

very-high value which can damage the motor (motor runaway .)

Today, series motors can operate with fixed-voltage power sources, for

example, automobile starting motors; or with variable-voltage power sources,

for example, traction systems.

- The main advantage of a shunt motor is the fact that only a single fixed-voltage

dc source is required to supply power to both the armature and the shunt

winding. Also, speed varies little as the mechanical load varies. However, a

shunt motor has a limited speed range because speed cannot be easily varied by

varying the armature voltage. Furthermore, the armature current must be limited

to prevent damage to the motor when it is starting (when power is applied to the

motor). Finally, when the shunt winding opens accidentally, the field current IF

becomes zero, the motor speed increases rapidly, and motor runaway occurs as

suggested by the speed versus field current characteristic.

- The main feature of cumulative compound motor characteristics is that the

motor speed varies little and linearly as the torque varies. On the other hand, the

series motor characteristic is non linear and shows that the motor speed varies a

lot (wide range of operating speed) as the torque varies. Finally, the

characteristic of a cumulative compound motor is a compromise of the series

and shunt motor characteristics. It provides the compound motor with a fairly

wide range of operating speed, but the speed does not vary linearly as the torque

varies.