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Electronics > VFD’s > Flashcards

VFD’s Flashcards

1
Q

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

A

Synchronous Speed

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2
Q

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

A

Slip
More load = more slip

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3
Q

What does Slip affect?

A

Torque

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4
Q

What are the two parameters that can change synchronous speed?

A

Frequency
Number of poles

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5
Q

Why is it important to keep a constant V/Hz ratio?

A

To not exceed FLA
To maintain constant torque at varying speeds

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6
Q

What are the 3 ratings of induction motors for VFD use?

A

Inverter ready rated
Inverter duty rated
Vector duty rated

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7
Q

What are the 3 ratings of NON-induction motors for VFD use?

A
  1. Permanent magnet motors
  2. Reluctant motors
  3. Wound field brushless excited synchronous motors
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8
Q

What are the properties of an Inverter Ready rated motor?

A

Good insulation that can handle voltage spikes.
5:1 VFD operating speed for constant torque loads.
20:1 VFD operating speed for variable torque loads.

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9
Q

What are the properties of an Inverter Duty rated motor?

A

TEBC (totally enclosed blower cooled) motor that contains shaft bonding rings, thin steel laminations.

Operates with constant torque loads with a speed ratio of 1000:1 (can achieve very slow operating speeds)

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10
Q

What are the properties of a Vector Duty rated motor?

A

They are like an inverter duty with an added shaft encoder that gives feedback to the drive about the rotor speed and position.

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11
Q

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

A

Overheating of the motor.

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12
Q

What is the formula for synchronous speed?

A

Nsync = 120 x f / N

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13
Q

What is the basic formula applied to VFD’s?

A

I = E / f

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14
Q

What are the 3 main components for a VFD?

A
  1. Rectifier
  2. Dc Link Filter (L-Section)
  3. Inverter
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15
Q

What are the advantages of using a VFD?

A

Energy savings
Reduced noise
Low maintenance costs
Control flexibility

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16
Q

What are the disadvantages of using a VFD?

A

Harmonics
Special cabling is required

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17
Q

Where is a reactor placed in the cct?

A

On the line and load conductors of the VFD

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18
Q

What type of cable should be used for analog control wiring to a VFD?

A

Shielded cable with only one end grounded to eliminate the potential of a loop for current to flow.

19
Q

What is harmonics?

A

Electrical noise that distorts the voltage and current waveforms

20
Q

What effects do harmonics have on a system?

A

Causes overheating
Voltage spikes
Equipment damage

21
Q

How would you describe THD (total harmonic distortion)?

A

Waveforms within the original waveform that are disaligned with each other in a single cycle.

2nd harmonic: 120hz
3rd harmonic: 180hz
5th harmonic: 300hz
Etc…..

22
Q

At what level of THD do remedial measures need to be taken?

A

THD must be below 3%

23
Q

Does a VFD require 3 phase input?

A

A VFD does not necessarily need a 3 phase input. Most can take single phase and turn it into a 3 phase output.

24
Q

What can be used to reduce harmonics in a system?

A

Line and load reactors.

25
Q

What are the considerations to take when choosing a location for a VFD?

A

No direct sunlight.
Keep away from high vibrations.
Make sure location is clean and dust free.

26
Q

What is installed on the motor shaft to prevent a build up of induced voltages between the shaft and the bearing?

A

Bonding rings to avoid premature bearing failure from arcing.

27
Q

When wiring the controls, what is required by code?

A

Minimum Class 1 wiring method.
Power and control wires cannot be run in the same pipe.

28
Q

What is the most common type of switching device used in the inverter section?

A

IGBT
(isolated gate bipolar transistor)

29
Q

What is the gate voltage requirement of an IGBT?

A

15V

30
Q

How fast can an IGBT turn on and off?

A

On: 400ns
Off: 500ns

31
Q

What is the purpose of the control logic board in the PWM VFD?

A

Controls IGBT to regulate output voltage

32
Q

Why is it so important to maintain the V/Hz ratio to a VFD connected motor?

A

So FLA is not exceeded.

33
Q

What are the advantages of the V/Hz PWM control technique?

A

Uses open-loop control
Less expensive than a flux vector

Offers basic drive adjustments like torque limiting, voltage boost, min and max speed control, and adjustable acceleration rates.

34
Q

Refer to VFD drives, how is the V/Hz ratio held constant when the frequency is varied?

A

Voltage is varied proportionally to the frequency

35
Q

How does the Flux Vector control optimize motor operation?

A

By controlling the rotor and the stator flux angle and V/Hz ratio.

Field and armature matching.

36
Q

What are the advantages of the Flux Vector control method?

A

Very precise speed regulation and torque control.

37
Q

What is the difference between Open Loop and Closed Loop Control?

A

Open Loop has no feedback and runs off the parameters set on the VFD.

Closed loop provides feedback via shaft encoder on the motor and uses direct input to adjust the system to varying conditions.

38
Q

What is PID control?

A

P: Proportional
I: Integral
D: Derivative

A control application that regulates temperature, flow, pressure, speed and other process variables.

39
Q

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

A

Dynamic Breaking

40
Q

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

A

DC Injection Breaking

41
Q

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

A

Line Regenerative Breaking

42
Q

What are the properties of a 6-pulse rectifier?

A

3 phase full wave
6 pulses in 360 degrees

43
Q

What are the properties of a 12-pulse rectifier?

A

2 x 3 Phase full wave rectifiers
Wye/Delta Secondary
30 degrees out of phase from each other

44
Q

What are the properties of an 18-pulse rectifier?

A

3 x 3 phase full wave rectifiers
20 degrees out of phase with each other
18 pulses in 360 degrees

Electronics (4 decks)

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