Deck 7: Basic Principles of Automatic Motor Control

Question 1

Figure 07-6

Refer to the accompanying figure. Basic electrical theory of a DC motor-generator, variable speed control system. Briefly explain how speeds above the motor base speed (nameplate speed) and speeds below the motor base speed are obtained.

Answer 1

Speeds above the motor base speed (nameplate speed) are obtained by weakening the motor shunt field. Speeds below the motor base speed are obtained by weakening the generator field.

Question 2

A(n) ____________________ contains two circuits. One circuit is in series with the armature and has the armature current flowing through its thermal sensor or heating element; the second circuit is the control circuit with a control contact.

Answer 2

thermal overload relay

Question 3

One of the most advantageous characteristics of the motor-generator set drive is its inherent capability to regenerate.

Answer 3

True

Question 4

Which diagram is most useful to the worker who installs the conduit and the wires between the starter panel and the control panel and motor?

Question 5

Briefly explain why the SCR control has replaced the rheostat.

Question 6

The ____________________ is probably the most popular solid-state device for controlling large and small electrical power loads.

Question 7

Compare the CEMF produced by the rotor at the time when the motor starts to the CEMF levels as the motor accelerates.

Question 8

The external wiring diagram is used to locate problems and failures in the operation of the controller.

Question 9

The ____________________ wiring diagram uses symbols and a simple plan of connections to illustrate the scheme of control and the sequence of operations.

Question 10

Electrically controlled, variable speed motor drives offer a wide choice of speed ranges, torque, and horsepower characteristics.

Question 11

Static DC drives utilize moving parts in the power conversion equipment that converts (rectifies) and controls the AC power.

Question 12

Figure 07-1

Refer to the accompanying figure. When starting the DC motor, as the ____________________ builds across the armature, it acts like a resistor that drops a larger percentage of the applied voltage, thus providing a voltage drop for the A coil and the A contacts 3-4 close to bypass current around the starting resistor.

Question 13

In an SCR-controlled DC motor drive, the feedback ____________________ maintains a preset speed.

Question 14

For some machinery with precise requirements, AC motors provide characteristics that are not available on DC motors.

Question 15

When troubleshooting or checking original installations, which two wiring diagrams would be most beneficial to an electrician?