Variable Frequency Drive

Question 1

A motor operated with a VFD is subject to voltages higher than the peak of the line. T or F?

Answer 1

True

Question 2

What can high frequency from a VFD do to a motor?

Answer 2

Bearings can become pitted and may prematurely wear out
Create induced current in the rotor core causing overheating

Question 3

What are the three induction motors that can be used with VFDs?

Answer 3

Inverter Ready
Inverter Duty
Vector Duty

Question 4

A reluctance motor is what type of motor?

Answer 4

Non-induction

Question 5

Name 4 advantages of VFD’s

Answer 5

Energy Savings
Reduced Noise
Reduced Maintenance Costs
Control Flexibility

Question 6

VFDs are often controlled by _______________ like the type used in a PC.

Answer 6

Microprocessors

Question 7

What distorts the voltage and current waveforms?

Answer 7

Harmonics

Question 8

Why does a VFD produce harmonic distortion?

Answer 8

Its a result of high frequency solid state switching in the drive.

Question 9

What is the limit of THD allowed and what must be done if the THD is too high on the line side?

Answer 9

3%
If too high, filtering on the line side of the VFD can be accomplised with an input line reactor.

Question 10

VFD’s are best operated in what temperature range?

Answer 10

10°C to 40°C

Question 11

Control wiring of a VFD is, at minimum a Class _ wiring method.

Answer 11

Class 1 wiring method

Question 12

Skip t

How many pulses does 3 Three Phase, Full-Wave rectifiers have? And how many degrees out of phase are their supply voltages?

Answer 12

18-Pulses
20 degrees out of phase, provided by a phase shifting transformer

Question 13

A dc Link section in a VFD is compromised of what?

Answer 13

An L-Section Filter
(An inductor in series with the output, and a capacitor in parallel with the output)

Question 14

An IGBT is a semiconductor consisting of an _______, _________ and a ____________.

Answer 14

Emitter
Gate
Collector

Question 15

What motor speed control technique is the lowest cost and requires no feedback?

Answer 15

V/Hz Control

Question 16

What is Flux Vector technology sometimes called instead?

Answer 16

Field Oriented Control

Question 17

What is an autotune in regards to a VFD?

Answer 17

A process where the VFD learns about the motor it is operating.

Question 18

Manual speed control of the motor is usually accomplished by keypad settings or locally or remotely by a ______________ connected to the VFD.

Answer 18

Potentiometer

Question 19

The type of control system that may be used if direct feedback was desired would be?

Answer 19

Closed Loop Control

Question 20

To control the amount of braking from a VFD, the current through the resistor is controlled by a _____________.

Answer 20

Transistor

Question 21

What is THD?

Answer 21

Total Harmonic Distortion

Question 22

What are the 3 main components or sections of a VFD?

Answer 22

• Recifier
• DC Link
• Inverter

Question 23

What section of the VFD filters the pulsating dc to smooth dc?

Answer 23

DC Link

Question 24

What section of the VFD converts an AC supply voltage to a DC voltage for the VFD?

Answer 24

Rectifier

Question 25

What are the 2 most common control techniques for producing a PWM output of a VFD?

Answer 25

- V/Hz Control - Flux Vector Control

Question 26

What is the volts per Hz ratio for a 600V motor with a 60Hz frequency rating?

Answer 26

10V/Hz (600V divided by 60Hz)

Question 27

What type of control does a system have if a heater is energized by a contactor controlled by a start stop station?

Answer 27

Open-Loop

Question 28

What type of braking is done by putting a DC voltage on the motor stator windings?

Answer 28

DC Injection Braking

Question 29

What type of braking uses a resistor to dissipate the regenerative voltage as heat?

Answer 29

Dynamic Braking

Question 30

What type of braking uses a resistor to dissipate the regenerative voltage as heat?

Answer 30

Dynamic Braking

Question 31

What type of braking recycles the energy produced by a decelerating motor back to the electrical system?

Answer 31

Line Regenerative Braking (Regenerative Braking)

Question 32

What is the Frequency of the 3rd harmonic in a 60Hz system?

Answer 32

180Hz (60Hz x 3)

Question 33

What is the Frequency of the 7th harmonic in a 60Hz system?

Answer 33

420Hz (60Hz x 7)

Question 34

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

Answer 34

Overheating of the Motor

Question 35

**Know these for Roy’s Exam** What are the three types of non-induction motors suitable for VFD use?

Answer 35

- Permanent Magnet - Reluctance - Wound Field Synchronous (Fancy term for Synchronous Motor)

Question 36

How does a VFD change the speed of a motor?

Answer 36

Changing the Frequency

Question 37

What 3 things **could** be on a motor nameplate if it is VFD rated? **only one would be present**

Answer 37

“VT” (Variable Torque) for Inverter Ready “CT” (Constant Torque) for Inverter Duty “Precise Control” for Vector Duty

Question 38

What is the speed at which a rotating field moves around a stator called?

Answer 38

Synchronous Speed

Question 39

What is the difference between synchronous speed and rotor speed called?

Answer 39

Slip Speed - This is the actual speed in rpm **Slip** is represented as a percent the slip speed divided by synchronous speed n=1750 RPM nsync=1800rpm 1800-1750=50RPM 50rpm/1800rpm= 2.78% slip

Question 40

What does **slip** affect in a rotor?

Answer 40

E-Voltage Z-Impedance F-Frequency I-Current X-Inductive Reactance

Question 41

What two parameters change synchronous speed?

Answer 41

Frequency and Poles

Question 42

If Frequency **Increases** synchronous speed __________

Answer 42

Increases

Question 43

What is the basic formula for VFDs?

Answer 43

Question 44

What is **Low Speed Boost** and when is it used?

Answer 44

Increases starting Torque. Only used at Startup

Question 45

What are the disadvantages of using VFDs?

Answer 45

- Harmonics - Special Cabling

Question 46

Which Harmonics are the **bad** Harmonics? Even or Odd?

Answer 46

Odd

Question 47

What is a Reactor?

Answer 47

It’s another name for Harmonic filter. Filters out the harmonics.