VFDs and Motor Control

Question 1

What are the two main types of three-phase induction motors?

Answer 1

Squirrel cage induction motor (SCIM)

Wound rotor induction motor (WRIM)

Question 2

What is the difference in construction of the rotors of the two main types of three
phase induction motor?

Answer 2

The SCIM has a non-insulated bar conductors fitted into the rotor slots and short
circuited by connecting rings.

The slots in the rotor of a WRIM are wound with insulated conductors and brought out
to slip rings.

Question 3

What happens to current carrying conductors placed within a magnetic field?

Answer 3

A Lorentz force is exerted upon it in such a way as to tend to move the conductor in a
direction from the current 90° away from the orientation of the magnetic field.

Question 4

Does a wound rotor induction motor have brushes?

Answer 4

Yes but a SCIM does not.

Question 5

A wound rotor motor has high starting current and low starting torque. T/F

Answer 5

False; A wound rotor motor has low starting current and high starting torque.

Question 6

A WRIM requires more maintenance than a SCIM.

Answer 6

True

Question 7

Name six electrical input requirements that may be found on a motor nameplate.

Answer 7

Voltage, current, frequency, phase, code, power factor.

Question 8

Name three information sections on a motor nameplate that describe the mechanical
output of the motor.

Answer 8

Horsepower, rpm (motor speed or rotational frequency), design.

Question 9

What power losses can occur in a SCIM motor?

Answer 9

Hysteresis, Eddy currents, heat losses, friction (bearings)

Question 10

The counter-torque produced by a dynamometer or pony brake is the product of which two mechanical quantities?

Answer 10

Force and radius

Question 11

The number of poles set the speed of the motor; the more poles the higher the speed. (T/F)

Answer 11

False

Question 12

What section of the VFD converts an ac supply voltage to a dc voltage for the VFD?

Answer 12

Rectifier

Question 13

What is the most common semi conductor active switching element used in the inverter to produce PWM in a Variable frequency drive?

Answer 13

IGBT

Question 14

Which section of the VFD alters the pulsating dc to smooth dc?

Answer 14

Filter

Question 15

What is a major undesirable issue associated with operating a VFD for the power system?

Answer 15

Harmonics and special cabling requirements

Question 16

Why are bonding rings installed on the shafts of some motors that are operated by a VFD?

Answer 16

Reduce voltage induction between shaft and bearings

Question 17

What section of the VFD converts a dc bus voltage to a pulse width modulated output?

Answer 17

Inverter

Question 18

What is the major concern if a standard motor is operated by a VFD at slow speeds?

Answer 18

Overheating

Question 19

What are the advantages of a VFD?

Answer 19

Energy savings
Reduced noise
Reduced maintenance costs
Control flexibility

Question 20

What are the three main components of a VFD?

Answer 20

Rectifier, filter, and an inverter

Question 21

What does a PWM VFD alter to change the speed of a SCIM?

Answer 21

Frequency

Question 22

What are the three ratings of induction motors for VFD use?

Answer 22

Inverter duty: specifically designed for VFD use

Inverter ready: better insulation

Vector duty Rated: like inverter duty but with shaft feedback for the rotor

Question 23

What are three types of non-induction motors suitable for VFD use?

Answer 23

Permanent magnet motors
Wound field brushless excited synchronous motors
Reluctance motors