DC Drives: Review Questions

1) A speed-controlled D.C. motor drive is running light at 50% of full speed. If the speed reference was raised to 100%, and the motor was allowed to settle, how would you expect the new steady-state values of armature voltage, tacho voltage and armature current to com pare with the corresponding values when the motor was running at 50% speed?

2) A d.c. motor drive has a PI speed controller. The drive is initially running at 50%speed with the motor unloaded. A load torque of 100%is then applied to the shaft. How would you expect the new steady-state values of armature voltage, tacho voltage and armature current to compare with the corresponding values before the load was applied?

3) An unloaded d.c. motor drive is started from rest by applying a sudden 100%speed demand. How would you expect the armature voltage and current to vary as the motor runs up to speed? 4)What would you expect to happen to a d.c. drive running with 50% torque at 50%speed if:

a) the mains voltage fell by 10%;

b) the tacho wires were inadvertently pulled off;

c) the motor seized solid;

d) a short-circuit was placed across the armature terminals;

e) the current feedback signal was removed.

5) Why is discontinuous operation generally undesirable in a d.c. motor?

6) What is the difference between dynamic braking and regenerative braking?

7) Explain why, in the drives context, it's often said that the higher the armature circuit inductance of d.c. machine, the better. In what sense is high armature inductance not desirable?

8) The torque-speed characteristics shown in ill. Q.8 relate to a d.c. motor supplied from a fully controlled thyristor converter.

Identify the axes . Indicate which parts of the characteristics display 'good‘ performance and which parts indicate ‘bad‘ performance, and explain brie fl y what accounts for the abrupt change in behavior. If curve A corresponds to a firing angle of 5 8 , estimate the firing angle for curve B. How might the shape of the curves change if a substantial additional inductance was added in series with the armature of the motor?

9) The 250 kW drive for a tube-mill draw bench had to be engineered using two motors rated at 150 kW,1200 rev/min and 100 kW, 1200 rev/min respectively, and coupled to a common shaft. Each motor was provided with its own speed-controlled drive. The specification called for the motors to share the load in proportion to their rating, so the controls were arranged as shown in ill Q.9 (the load isn't shown).

a) Explain brie fl y why this scheme is referred to as a master/slave arrangement.

b) How is load sharing achieved?

c) Discuss why this arrangement is preferable to one in which both drives have active outer speed loops.

d) Would there be any advantage in feeding the current reference for the smaller drive from the current feedback signal of the larger drive?