Deck 8: The Dc Cemf Motor Controller and Dc Variable-Speed Motor Drives

Question 1

In a CEMF controller, braking action increases as the armature speed decreases.

Answer 1

False

Question 2

A DC motor is reversed by reversing the shunt field current.

Answer 2

False

Question 3

Name two examples of special equipment that use motor reversal and dynamic braking operations.

Answer 3

Reversal of motors and dynamic braking are operations used in special equipment such as cranes, hoists, railway cars, and elevators.

Question 4

Due to the nature of the control circuit of the electrical and mechanical interlocking system, it is impossible for the reverse contacts to close until the ____________________ contacts are open.

Question 5

Briefly describe what causes the armature to act as a generator in the process of dynamic braking.

Question 6

When the stop button is pressed, the dynamic braking contacts connect the dynamic braking resistor directly across the ____________________.

Question 7

In the absence of a discharge path when using the CEMF controller and dynamic braking, what damage would occur due to the high voltage?

Question 8

A field discharge resistor is a thyristor-type device that conducts when the voltage across it is high enough, but has a high resistance to normal line voltage.

Question 9

What is the cause of the very large "inductive kick" voltage that occurs when using the CEMF controller and dynamic braking?

Question 10

____________________ involves using a DC motor as a generator, taking it off the supply line and applying an energy dissipating resistor to the armature.

Question 11

In a DC reversing motor, the forward and reverse buttons are mechanically interlocked so that it is not possible to operate these buttons at the same time.

Question 12

When using the CEMF controller and dynamic braking, a field discharge resistor must be added.

Question 13

What is the purpose of dynamic braking?

Question 14

A generator converts ____________________ energy into electrical energy.

Question 15

One method of dynamic braking is to change the function of the motor to that of a(n) ____________________.