EE6351

EE6351/ELECTRICAL DRIVES AND CONTROLS

UNIT-I INTRODUCTION

PART-A1. Draw the block diagram of an electrical drive.

(AU 2013)-2 2. Define cooling time constant of an electrical machine.

(AU 2012)-2

It is defined as the ratio between C and A. Cooling time constant is denoted as .

' = C/A

Where, C=Amount of heat required to rise the temperature of the motor body by 1 degree

Celsius in J/C.

A=Amount of heat dissipated by the motor per unit time per degree Celsius in

J/S/C.3. Define the thermal overload factor for a motor working on short time duty. (AU 2012)-2When motor operates, heat is produced due to losses inside the machine and its temperature rise. As the temperature beyond ambient value, a portion of heat produced flows out to the surrounding medium. The amount of outflow of heat is a function of temperature rise of motor above ambient value. As the motor temperature rises, the heat outflow increases and equilibrium ultimately sets in when the heat generated becomes equal to heat dissipated into the surrounding medium. Motor temperature then reaches a steady state value. Steady state temperature depends on power loss, which in turn depends on the output power of the machine. Since temperature rise has a direct relationship with the output power, it is termed thermal overload factor on the machine.4. Name the four commonly used methods for the determination of power rating of motors.

(AU 2012)i. Thermal loading

ii. Steady state temperature

iii. Motor duty

iv. Temperature withstanding limits on insulating materials

5. Mention the various factors that influence the choice of electrical drives. (AU 2011)-2i. Steady state operation requirements.

ii. Transient operation requirements.

iii. Requirements related to the source.

iv. Capital and running cost, maintenance needs life.

6. Explain heating and cooling curves.

(AU 2011)

A homogenous machine developing heat internally at a uniform rate and gives it to the surroundings proportional to temperature rise. It can be proved that the temperature

rise of a body obeys exponential law.

When the machine is switched off from the main supply or when the load on the motor

is reduced, the machine cools.7. Define group drive system.

(AU 2010)If several groups of mechanisms are organized on one shaft and driven by one motor, the system is called group drive or shaft drive.

Example: Flour mill and Rice mill drives8. Define heating time constant.

(AU 2010)Heating time constant is defined as the time taken by the machine to attain 0.632 of its final steady temperature rise. The heating time constant of the machine is the index of the time taken by the machine to attain its final steady temperature rise.

9. What is meant by continuous rating?

(AU 2010)-2Continuous rating of a motor is denotes operation at constant load (full load) of sufficient duration. Centrifugal pumps, fans, compressors and conveyors are some examples of equipment which run continuously with constant load.10. What is meant by intermittent duty?

(AU 2009)There are certain loads which require motors to be run for a certain period of time and them switched off for a small period. This type duty is called intermittent duty.PART-B

1. Explain the thermal model of an electric motor for

i) Heating the electric motor when starting from cold

ii) Cooling the electric motor when it is switched off from the mains. (AU 2013)2. i) A constant speed drive operating at a speed of 500 rpm has a cyclic loading as given below:

200 Nm for 10 minutes



No load for 10 Minutes

Estimate power rating of the motor.

ii) What are the different classes of motor duty?

(AU 2013)3. Explain various classes of motor drives.

(AU 2013)

4. Describe the selection of motor rating for continuous duty load. (AU 2013)

5. Discuss in details the various factors which affect the selection of electrical drive for particular applications.

(AU 2010)

6. i) Explain the disadvantages of using a motor of wrong rating.

ii) The temp rice of a motor when operating for 25min on full load 25c and becomes of 40c when the motors operate another 25min on the same load .determine heating time constant and steady state temp rise.

(AU 2010)

7. Explain what is meant by a group drive. What are its advantages and disadvantages? (AU 2012)8. Show that for an electric motor, the relationship between temperature rise and time is an exponential function.

(AU 2012)

9. i) Briefly explain the various factors that will influence the choice of elective drive. ii) Explain the methods of estimating equivalent continuous power rating of a motor for

short time load applications.

(AU 2010)10. Explain the different classes of motor duty with the equation.

(AU 2010)UNIT-II DRIVE MOTOR CHARACTERISTICS PART-A1. Draw the mechanical characteristics of three phase induction motor. (AU 2013)-2

2. What are the methods of breaking of electric motors?

(AU 2013)-2

i. Regenerative braking

ii. Dynamic (or) Rheostatics braking

iii. Plugging (or)Reverse voltage braking3. Write down the equation for the back e.m.f of DC motor in terms of flux per pole and speed.

(AU 2012)N=(V-Ia Ra)/ (or) Eb/

Where,N=Speed of dc motor

Ia=Armature current

Ra=Armature resistance

=flux per poleEb=back emf of dc motor

4. List the types of single phase induction motors.

(AU 2012)i. Split phase induction motorii. Capacitor start inductor motor

iii. Capacitor start capacitor run induction motor

iv. Shaded pole induction motor5. Single phase induction motor is not a self starting motor. Why? (AU 2012)Single phase supply is fed to the single phase induction motor. Its stator winding produces a flux which is only alternates along one space axis only. It is not a synchronously revolving flux, as in the case of a two or a three phase stator winding, fed from a 2 or 3 phase supply. Now alternating or pulsating flux acting on a stationary squirrel cage rotor cannot produce rotation. That is why a single-phase motor is not self-starting.6. Why is DC series motor used for traction purpose?

(AU 2011)For traction purpose, we require high starting of DC motor. The DC series motor has high starting torque. That is why, series motor used in traction purpose.7. Draw the speed torque characteristics of DC shunt and series motor. (AU 2010)-2

8. State the advantages of electrical braking.

(AU 2010)

i. High efficient method

ii. Low maintenance

iii. Braking is very smooth9. What is meant by dynamic braking?

(AU 2010)

In this method the armature of shunt motor is disconnected from the supply and is connected across a variable resistance R. The braking torque is controlled by the resistance R.10. When plugging is employed for stopping an induction motor, why is it necessary to disconnect it from supply when the speed reaches close to zero? (AU 2009)In plugging method, by changing the phase sequence (RBY), the motor rotate in the reverse direction.

Now, the motor comes to zero, we necessary to disconnect induction motor from the supply.

PART-B

1. Explain the modifications to the speed-torque characteristics of a dc shunt motor for the following

(AU 2013)i) With increase in armature resistance ii) By field weakening.

2. A 220 V dc series motor runs at 1200 rpm (clockwise) and takes an armature current of 80 A when driving a load with constant torque. Armature resistance is 0.05 and field resistance is 0.05. Find the magnitude and direction of motor speed and armature current if the motor terminal voltage is reversed and the number of turns in field winding is reduced to 80%. Assume linear magnetic circuit.

(AU 2013)

3. a) Explain with necessary circuit diagram, the reverse current braking and the braking Characteristics of the following:

(AU 2012)i) DC shunt motor ii) DC series motor

b) A 250V DC shunt motor has an armature resistance of 0.05 and with rated field excitation has a back emf of 245V. At a speed of 1200rpm. It is coupled to an overhauling

load with a torque of 200N-m determine the lowest speed at which a motor can hold the

load by regenerative braking.

4. a) Sketch the speed- torque characteristics of a three phase induction motor and explain its

motoring mode, generating mode and braking mode of operation.

(AU 2012)

b) A 15 kW, 415V, three-phase, 4 pole, 50 Hz induction motor has a speed of 1455 rpm at

full load. At this load, the mechanical losses are 600 watt and the stator losses are

750watts. Find

i) Full load slip ii) Total input power to the motor.

iii) Current drawn at a full load, if the power factor is 0.8 lagging. iv) Net torque developed at output at full load. 5. Describe various methods of electrical braking of dc shunt motor a compare their relative merits and demerits.

(AU 2010)6. A DC series motor runs at 1000 rpm taking 100 A at 400V. A diverter having doubled the resistance of the field winding is then connected in parallel with it. Estimate the change in speed if the torque varies as the square of the speed. Assume unsaturated field and neglect losses.

(AU 2010) 7. Explain speedtorque characteristics of different types of load with graph. (AU 2013)8. Explain with speed-torque characteristics of DC series motor under dynamic braking. (AU 2013)

9. Explain the braking methods used in three phase induction motors. (AU 2010)

10. Explain the speed-torque and braking characteristics of single phase induction motor. (AU 2009)UNIT-III STARTING METHODSPART-A1. State the necessity of starter for a dc motor.

(AU-2012)-3A DC motor is directly switched on, at the time of starting; the motor back emf is zero. Due to this, the armature current is very high. Due to the very high armature current, the motor become damaged. The starting current can be limited by using starter.2. Mention the types of starter suitable for three phase squirrel cage induction motor. (AU-2012)

i. DOL starter ii. Stator resistance starter

iii. Star delta starteriv. Auto transformer starter3. Name the type of induction motor which produces highest starting torque? (AU 2008)Slip ring induction motor.4. What is the basic principle in starting 3-phase induction motor using rotor resistance

starter?

(AU-2012)In this method, the motor is always started with full line voltage, applied across the stator terminals. The value of starting current is adjusted by introducing a variable resistance in the rotor circuit. At starting, the full resistance is included and hence the starting current is reduced. The resistance is gradually cut out of the rotor circuit as the motor gathers speed5. What are the two types of rotor for three phase induction motor? (AU-2013)i. Squirrel cage induction motor

ii. Slip ring induction motor.6. What are the functions of starter of a dc motor?

(AU-2012)

i. To limit the starting current

ii. To protect the motor against low voltage and over load condition. 7. What is the advantage of three phase slip ring induction motor? (AU-2012)i. Pull-out torque can be achieved even from zero RPM.ii. It has a high starting torque when compared to squirrel cage induction motor. Approximately 200 - 250% of its full-load torque.

iii. A squirrel cage induction motor takes 600% to 700% of the full load current, but a slip ring induction motor takes a very low starting current approximately 250% to 350% of the full load current.8. What is the basic difference between a three- point starter and a four- point starter?

(AU 2011)-2

Three point starter:

i. The NVR coil is connected in series with the shunt field winding.

ii. The exiting current through the NVR coil in three point starter is same as shunt field

of the motor.

Four point starter:

i. The NVR coil along with a high resistance is connected across the supply voltage.

ii. The exciting current through NVR coil of a four point starter is directly proportional to the supply voltage and independent on shunt field current.9. What are the different types of protection provided in starter used for a three phase induction motors?

i. No voltage release coil

ii. Over load relay coil.10. What is the advantage of squirrel cage induction motor over dc motors? (AU 2009)i. Low cost

ii. Ruggedness, smaller size and weight

iii. They can operate in dusty and explosive environment, because the brushes are being not used. Therefore there is no possibility of sparking.

PART-B

1. i) Explain the function and working overload realease in a 3 point shunt motor. (ii) A 250V, 37kW, DC shunt motor is allowed to exert a maximum of 150 percent of the full

load torque during the starting period. the resistance of armature is 0.2 ohm and the full

load efficiency is 84 percent. Number of section of resistance in the starter. Determine a) The upper and lower limit of current during starting

b) The values of resistance of each section of the starter.

(AU 2012)

2. i) With the help of the neat diagram, explain the working of star delta starter. ii) A 3 phase induction motor has the ratio of maximum torque as 2.5:1.the rotor resistance

and standstill reactance per phase are 0.4 ohm respectively. Determine the ratio of

Starting torque to full load torque, if the star delta starter is used.

(AU 2012)3. A starter is required for a 220V shunt motor .the maximum and minimum range of current values 50A &30A respectively. Find the number of section of starter resistance required and the resistance of each section .the armature resistance of the motor is 0.5ohm.

(AU 2013)

4. Explain the different starting method for three phase squirrel cage induction starter. (AU 2013)5. Discuss different types of D.C motor starters.

(AU 2010)6. Discuss the various methods of starting of induction motors and compare their relative merits and demerits.

(AU 2010)7. Explain the different types of starters used in DC shunt and series motor. (AU 2009)8. Explain the different starters used in 3 phase induction motors.

(AU 2009)9. With the neat diagram explain the operation of four point starter. Also mention the advantage of its over a 3 point starter.

(AU 2010)10. i) Describe the operation of suitable starter for 3-phase slip ring induction motors.

ii) Draw the control circuit for DOL starter.

(AU 2010)UNIT-IV CONVENTIONAL AND SOLID STATE SPEED CONTROL OF D.C. DRIVESPART-A1. What are the various methods of speed control in dc motor?

(AU-2012)i. Armature control ii. Flux control iii. Voltage control2. Write the application of DC chopper.

(AU 2008)

i. Electric traction

ii. Steel mills

iii. Paper mills

iv. Machine tools

v. Printing mills3. State the advantage of ward- Leonard control system.

(AU 2008)

i. Full forward and reverse speed can be achieved.ii. Short time over load capacity is large.

iii. The armature current of the motor is smooth.

iv. A wide range of speed control is possible. 4. Draw the speed-torque characteristics of DC series motor for different values of armature resistances.

(AU 2012)-2

5. Draw the block schematic of the phase controlled rectifier fed DC drives. (AU 2012)

6. State the different methods of speed control of DC series motor (AU 2010)

i. Variable resistance in series with motor.ii. Flux control method

a) Field divertor b) Armature divertor c) Tapped field control d) Paralleling field coils7. Enumerate the limitation of field control in dc motor?

(AU 2012)i. The flux cannot be brought to zero level.

ii. If certain occasions, the flux becomes zero. i.e., field gets open, the motor tries to run with infinite speed. It causes damage to the machine. 8. What is a DC chopper?

(AU 2012)

It converts the fixed dc voltage in to variable dc voltage. It is known as dc chopper. 9. What is time ratio control?

(AU 2012)

In time ration control method, duty cycle Ton /T is varied. By varying the duty cycle output voltage is also varied. 10. Field control is employed for getting speeds higher than rated and armature voltage is employed for getting speeds less than rated why?

(AU 2013)-2By using this formula N (Eb/)

For field control method, by maintaining voltage constant and by decreasing the flux the motor speed increases above rated speed.

For armature control method, by maintaining flux constant by decreasing the flux motor speed decreases below rated speed.PART-B

1. i) Explain the different methods of speed control used in DC series motors.

ii) A 250V shunt motor with armature resistance of 0.5ohm runs at 600rpm on full load and

takes an armature current of 20A.if resistance of 1.5ohm is placed in the armature circuit

find the speed on (a) full load torque (b) half full load torque. (AU 2009)2. Explain the voltage, frequency and slip power control used in 3 phase induction motors.

(AU 2009)3. Describe with the help of a connection diagram the ward lonard methods of speed control. Explain in operation at starting, stopping and reversing. States the merits and demerits of the scheme.

(AU 2010)

4. A 220V,24A,1000rps separately excited DC motors has an armature of 2Amps.motor is controlled by a chopper operating at 600 Hz from a 230V supply .calculate at the duty ratio for 1.2 times rated torque and 500 rpm. Draw the power circuit for the Dc motors drive. (AU 2010) 5. i) Explain the ward-leonard method of speed control of DC motors with a sketch showing the circuit. State also the adv of the method.

ii) A DC series having a armature resistance of 1 ohm, runs at a speed of 800 r.p.m at 200V

with a current of 15A. Find the speed at which it will when a 5 ohm resistance is connected

in series, at a same supply and taking the same same current. (AU 2012)6. i) Describe the working of step down DC chopper, with the help of a suitable circuit diagram

and waveform diagram state the relation between output and input voltages. How the

speed of a DC motor is controlled using a step down chopper.

ii) A 100v shunt motor has a armature resistance and field resistance of 0.4 ohm

respectively. At a particular constant torque load, it takes a current of 25A at the speed of

1200 r.p.m. A chopper is used to control the speed of the motor. Find Ton to reduce the

Speed to 800r.p.m at the chopper frequency of 500Hz.

(AU 2012)7. Explain the operation of single phase full converter fed separately excited DC motor drive.

(AU 2013)8. With neat circuit diagram, explain chopper fed four quartrand DC drives. (AU 2013)9. i) Explain the operation of armature control of a DC shunt motor. ii) Draw the explain the speed control of DC motor using chopper drives. (AU 2010)10. i) With the block diagram explain the operation of armature and field control of dc motor

drive using controlled rectifiers.

ii) Name the different flux control methods adopted for DC series motors. (AU 2010)UNIT-V CONVENTIONAL AND SOLID STATE SPEED CONTROL OF A.C. DRIVESPART-A1. State the applications where stator voltage control is employed for three phase induction motors.

(AU 2013)

The stator voltage control method is suitable for applications where torque demand reduces with speed, which points towards its suitability for

i. Fan

ii. Pump drives.2. What is meant by slip power recovery scheme?

(AU 2008)The wasting of slip power can be recovered by various schemes for the speed control of slip ring induction motor. This system is called as slip power recovery system.

3. What is v/f control in three phase induction motor?

(AU 2008)

In voltage/frequency control method, motor speed can be varied by varying voltage and frequency, but we can maintain V/F ratio is constant. 4. Draw the block diagram of conventional scherbius system.

(AU 2013)

5. What are the variable frequency AC drive applications?

(AU 2013)

i. Used in pumps

ii. Used as I.D ,FD fans

iii. Used in lifts and elevator. 6. What are the advantages of v/f control for three phase induction motor? (AU-2012)

i. Smooth speed control

ii. Small input current and improved power factor at low frequency start

iii. Higher starting torque for low cage resistance. 7. Mention the advantages of slip ring recovery scheme of controlling the speed of induction motor?

(AU 2012)

The slip power can be recovered and fed back to the supply due to this overall efficiency

is increased.8. What is meant by V/f control?

(AU 2010)-2If the ratio of voltage to frequency is kept constant, the flux also remains constant. At low frequency the air gap flux is reduced due to the drop in the stator impedance and the voltage has to be increased to maintain the torque level. This type of control in usually called as volts/hertz (v/f) control.

9. Mention some of the merits and demerits of ac drives.

(AU 2009)-2Merits:i. Very fast responseii. Less losses

iii. More reliability and long life due to absence of moving parts.

iv. Pollution free method.

Demerits:i. Very difficult to regeneration of power.ii. It contains low power over load capacity.

10. Draw the block diagram of soft starter.

(AU 2009)

PART-B

1. Explain the static scherbius drive which provides speeds below and above synchronous speed.

(AU 2013)

2. Explain the constant torque mode and constant power mode of operation of voltage source inverter fed induction motor drive with necessary diagram.

(AU 2013)

3. Sketch and explain the circuit using thyristor controller, to control the speed of a 3 phase induction motor by varying the stator voltage. Mention the merits and demerits of this method. Also sketch and explain the torque speed characteristics when stator voltage control is used.

(AU 2012)

4. Explain the following solid state methods of controlling speed of 3 phase induction motor with suitable diagram.

i) Cyclo converter static scherbius drive.ii) Static Kramer drive.

(AU 2012)

5. Explain voltage /frequency control 3 phase induction motor.

(AU 2013)

6. Describe Kramer system slip power recovery system of 3 phase induction motor. (AU 2013)

7. With suitable schematic diagram explain the following solid state methods of controlling speed of 3 phase induction motors

i) Static Kramer drive.ii) DC link scherbius drive.

(AU 2012)

8. Explain the scherbius system of slip energy recovery schemes used in speed control of large induction motors.

(AU 2011)

9. State and explain the various schemes adopted for control of induction motor using voltage source inverter.

(AU 2011) 10. What is meant by slip power recovery scheme? Explain with the necessary diagram. (AU 2010)KCE/ME/QB/IIYR/EDC

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EE6351