Deck 14: Controllers for Three-Phase Motors

Question 1

In reference to a wye-delta controller, what is the meaning of closed transition?

Answer 1

A method of controlling the wye-delta controller called closed transition provides a way to change from the wye connection to the delta connection without completely disconnecting the motor from the power line.

Question 2

Briefly describe the method of stopping a motor known as plugging.

Answer 2

When any two of the line leads feeding a three-phase motor are reversed, a countertorque is set up that brings the motor to a quick standstill before it begins to rotate in the reverse direction. If the circuit is interrupted at the instant the motor begins to turn in the opposite direction, the rotor just stops. This method of bringing a motor to a quick stop is called plugging.

Question 3

The voltage across each winding is only 58% of the line voltage when the windings of a three-phase motor are connected in ____________________ at the start position.

Answer 3

wye

Question 4

____________________ is the quickly repeated closure of a controller circuit to start a motor from rest for the purpose of accomplishing small movements of the driven machine.

Question 5

What are the three main components of a programmable logic controller (PLC)?

Question 6

An automatic compensator is used to automatically connect an autotransformer in series with the motor during the starting cycle, which results in a reduced starting current.

Question 7

Adjustable frequency (static solid-state) drives are also commonly called ____________________.

Question 8

Dynamic braking is a method used to help bring a motor to a quicker stop with the extensive use of friction brakes.

Question 9

Briefly describe how reduced torque that results from reduced-voltage starting methods may be an advantage as well as a disadvantage.

Question 10

The starting current of a squirrel-cage induction motor can be reduced by using a resistance starter controller, which inserts equal resistance values in each line wire at the instant the motor is started.

Question 11

When ____________________ is applied to induction motors, a high rotor current reacts with the stationary field of the stator to produce a countertorque that slows and stops the motor.

Question 12

In ____________________ starters, the control functions are performed by semiconductors.

Question 13

When an electronic controller for wound-rotor motors includes power resistors inserted into the secondary rotor circuit, the more resistance inserted, the slower the speed of the motor.

Question 14

Several sections of the National Electrical Code® are concerned with motor controllers and remote control circuits. List two of the topics that are addressed in these sections.

Question 15

Local building and electrical code authorities should be consulted before installations are made with motors and controllers that do not comply with National Electrical Code® rulings.