Important Short Questions and Answers: Converter, Chopper Fed DC Motor Drive

1. What is the use of flywheel? Where it is used?

It is used for load equalization. It is mounted on the motor shaft in compound motor.

2.     What are the advantages of series motor?

The advantages of series motors are,

a.     High starting torque

b.     Heavy torque overloads.

3.    Define and mention different types of braking in a dc motor?

In breaking the motor works as a generator developing a negative torque which opposes the motion. Types are regenerative braking, dynamic or rheostat braking and plugging or reverse voltage braking.

4. How the D.C. motor is affected at the time of starting?

A D.C. motor is started with full supply voltage across its terminals, a very high current will flow, which may damage the motor due to heavy sparking at commuter and heating of the winding. Therefore, it is necessary top limit the current to a safe value during starting.

5. List the drawbacks of armature resistance control?

In armature resistance control speed is varied by wasting power in external resistors that are connected in series with the armature. since it is an inefficient method of speed control it was used in intermittent load applications where the duration of low speed operations forms only a small proportion of total running time.

6.  What is static Ward-Leonard drive?

Controlled rectifiers are used to get variable d.c. voltage from an a.c. source of fixed voltage controlled rectifier fed dc drives are also known as static Ward-Leonard drive.

7.  What is aline commutated inverter?

Full converter with firing angle delay greater than 90 deg. is called line commutated inverter. such an operation is used in regenerative braking mode of a dc motor in which case a back emf is greater than applied voltage.

8. Mention the methods of armature voltage controlled dc motor? When the supplied voltage is ac,

Ward-Leonard schemes

Transformer with taps and un controlled rectifier bridge Static Ward-Leonard scheme or controlled rectifiers

when the supply is dc:

Chopper control

9.     How is the stator winding changed during constant torque and constant horsepower operations?

For constant torque operation, the change of stator winding is made form series - star to parallel - star, while for constant horsepower operation the change is made from series-delta to parallel-star. Regenerative braking takes place during changeover from higher to lower speeds.

10.  Define positive and negative motor torque.

Positive motor torque is defined as the torque which produces acceleration or the positive rate of change of speed in forward direction. Positive load torque is negative if it produces deceleration.

11. Write the expression for average o/p voltage of full converter fed dc drives?

Vm=(2Vm/pi)cospi.........continuous conduction

Vm=[Vm(cos alpha-cos beta)+(pi+alpha+beta)]/pi] discontinuous conduction

12. What are the disadvantages of conventional Ward-Leonard schemes?

Higher initial cost due to use of two additional m\cs. Heavy weight and size.

Needs more floor space and proper foundation. Required frequent maintenance

13. Mention the drawbacks of rectifier fed dc drives?

Distortion of supply. Low power factor.

Ripple in motor current

14. What are the advantages in operating choppers at high frequency?

The operation at a high frequency improves motor performance by reducing current ripple and eliminating discontinuous conduction.

15. Why self commutated devices are preferred over thyristors for chopper circuits?

Self commutated devices such as power MOSFETs power transistors, IGBTs, GTOs and IGCTs are preferred over thyristors for building choppers because they can be commutated by a low power control signal and don't need commutation circuit.

16. State the advantages of dc chopper drives?

DC chopper device has the advantages of high efficiency, flexibility in control, light weight, small size, quick response and regeneration down to very low speed.

17. What are the advantages of closed loop c of dc drives?

Closed loop control system has the adv. of improved accuracy, fast dynamic response and reduced effects of disturbance and system non-linearities.

18.            What are the types of control strategies in dc chopper?

·        Time ratio control.

·        Current limit control.

19.            What are the adv. of using PI controller in closed loop ctrl. of dc drive?

Stabilize the drive

·        Adjust the damping ratio at the desired value

·        Makes the steady state speed error close to zero by integral action and filters out noise again due to the integral action.

20.            What are the different methods of braking applied to the induction motor?

Regenerative braking Plugging, Dynamic braking.

21. What are the different methods of speed control of IM?

Stator voltage control, Supply frequency control, Rotor resistance control, Slip power recovery control.

22. What is meant by stator voltage control.?

The speed of the IM can be changed by changing the stator voltage. Because the torque is proportional to the square of the voltage.

23. Mention the application of stator voltage control.

This method is suitable for applications where torque demand reduced with speed, which points towards its suitability for fan and pump drives.

24. Mention the applications of ac drives.

AC drives are used in a no. of applications such as fans, blowers, mill run-out tables, cranes, conveyors, traction etc.

25. What are the three regions in the speed-torque characteristics in the IM?

Motoring region (0<=s<=1) Generating region(s<0)

Plugging region (1<=s<=2) where s is the slip.

26.            What are the advantages of stator voltage control method?

·        The control circuitry is simple

·        Compact size

·        Quick response time

·        There is considerable savings in energy and thus it is economical method as compared to other methods of speed ctrl.

27.            What is meant by soft start?

The ac voltage controllers show a stepless control of supply voltage from zero to rated voltage they are used for soft start for motors.

28.            List the adv of squirrel cage IM?

·        Cheaper

·        light in weight

·        Rugged in construction

·        More efficient

·        Require less maintenance

·        It can be operated in dirty and explosive environment

29.            Define slip

The difference between the synchronous speed (Ns)and actual speed(N)of the rotor is known as slip speed. the % of slip is gn by,

%slip s=[(Ns-N)/Ns]x 100

30.            Define base speed.

The synchronous speed corresponding to the rated freq is called the base speed.