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4th Period Electronics SAIT Alberta Electrician > Variable Frequency Drive > Flashcards

Variable Frequency Drive Flashcards

1
Q

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

A

True

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2
Q

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

A

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

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3
Q

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

A

Inverter Ready
Inverter Duty
Vector Duty

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4
Q

A reluctance motor is what type of motor?

A

Non-induction

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5
Q

Name 4 advantages of VFD’s

A

Energy Savings
Reduced Noise
Reduced Maintenance Costs
Control Flexibility

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6
Q

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

A

Microprocessors

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7
Q

What distorts the voltage and current waveforms?

A

Harmonics

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8
Q

Why does a VFD produce harmonic distortion?

A

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

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9
Q

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

A

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

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10
Q

VFD’s are best operated in what temperature range?

A

10°C to 40°C

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11
Q

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

A

Class 1 wiring method

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12
Q

Skip t

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

A

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

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13
Q

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

A

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

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14
Q

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

A

Emitter
Gate
Collector

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15
Q

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

A

V/Hz Control

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16
Q

What is Flux Vector technology sometimes called instead?

A

Field Oriented Control

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17
Q

What is an autotune in regards to a VFD?

A

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

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18
Q

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

A

Potentiometer

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19
Q

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

A

Closed Loop Control

20
Q

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

A

Transistor

21
Q

What is THD?

A

Total Harmonic Distortion

22
Q

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

A
  • Recifier
  • DC Link
  • Inverter
23
Q

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

A

DC Link

24
Q

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

A

Rectifier

25
Q

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

A
  • V/Hz Control
  • Flux Vector Control
26
Q

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

A

10V/Hz

(600V divided by 60Hz)

27
Q

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

A

Open-Loop

28
Q

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

A

DC Injection Braking

29
Q

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

A

Dynamic Braking

30
Q

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

A

Dynamic Braking

31
Q

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

A

Line Regenerative Braking
(Regenerative Braking)

32
Q

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

A

180Hz

(60Hz x 3)

33
Q

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

A

420Hz
(60Hz x 7)

34
Q

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

A

Overheating of the Motor

35
Q

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

A
  • Permanent Magnet
  • Reluctance
  • Wound Field Synchronous
    (Fancy term for Synchronous Motor)
36
Q

How does a VFD change the speed of a motor?

A

Changing the Frequency

37
Q

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

A

“VT” (Variable Torque) for Inverter Ready

“CT” (Constant Torque) for Inverter Duty

“Precise Control” for Vector Duty

38
Q

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

A

Synchronous Speed

39
Q

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

A

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

40
Q

What does slip affect in a rotor?

A

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

41
Q

What two parameters change synchronous speed?

A

Frequency and Poles

42
Q

If Frequency Increases synchronous speed __________

A

Increases

43
Q

What is the basic formula for VFDs?

A
44
Q

What is Low Speed Boost and when is it used?

A

Increases starting Torque.

Only used at Startup

45
Q

What are the disadvantages of using VFDs?

A
  • Harmonics
  • Special Cabling
46
Q

Which Harmonics are the bad Harmonics?

Even or Odd?

A

Odd

47
Q

What is a Reactor?

A

It’s another name for Harmonic filter.

Filters out the harmonics.

4th Period Electronics SAIT Alberta Electrician (8 decks)

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