Deck 36: Variable Frequency Control

Question 1

How can the speed on a three phase induction motor be controlled?

Answer 1

The speed of a three phase induction motor can be controlled by either changing the number of stator poles per phase, as is the case with consequent pole motors, or by changing the frequency of the applied voltage.

Question 2

The output frequency of the alternator is determined by the speed of the ____________________.

Answer 2

rotor

Question 3

How do most variable frequency drives operate?

Answer 3

Most variable frequency drives operate by first changing the AC voltage into DC and then changing it back to AC at the desired frequency.

Question 4

Alternators are often used to control the speed of several induction motors that require different changes in speed.

Question 5

Many transistor controlled variable frequency drives now employ a special type of transistor called a(n) ______________________________.

Question 6

Changing frequency causes a corresponding change in the inductive reactance of the windings.

Question 7

The Insulated Gate Bipolar Transistor (IGBT) is a current controlled device, not a voltage controlled device.

Question 8

Variable frequency drives intended to control the speed of motors up to 500 hp generally use ____________________.

Question 9

Drives using Insulated Gate Bipolar Transistors (IGBTs) generally used ____________________ to rectify the AC voltage into DC.

Question 10

What is an advantage of using Insulated Gate Bipolar Transistors (IGBTs) for drives?

Question 11

What are two basic methods of achieving variable frequency?

Question 12

Inverter rated motors are specifically designed to be operated by variable frequency drives.

Question 13

Ramping permits the motor to bring the load up to speed slowly as opposed to simply connecting the motor directly to the line.

Question 14

When bipolar junction transistors are employed as switches, they are generally driven into ____________________ by supplying them with an excessive amount of base-emitter current.

Question 15

What are thyristors?