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4th Year Electrical - Electronics > UPS and VFD's > Flashcards

UPS and VFD's Flashcards

1
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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2
Q

What are three (3) ratings of induction motors for VFD use?

A
  • inverter ready
  • inverter duty
  • vector duty rated
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3
Q

What are three (3) types of non-induction motors suitable for VFD use?

A
  • permanent magnet motors
  • wound field brushless excited synchronous motors
  • reluctance motors
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4
Q

When controlling the speed of an ac motor, what factor does a VFD change to achieve different speeds?

A

Frequency

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5
Q

Name two (2) types of equipment that can achieve cost saving benefits by being operated by a motor connected to a VFD?

A

Pumps

Fans

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6
Q

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

A

Harmonics

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7
Q

What is the 7th harmonic on a 60Hz system?

A

420 Hz

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8
Q

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

A

To prevent the build-up of induced voltages between shaft and bearings

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9
Q

What are the four (4) main components of a PWM type VFD?

A
  • rectifier
  • DC Link
  • inverter
  • control logic (CPU)
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10
Q

What are the three rectifier types?

A

6-pulse
12-pulse
18-pulse

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11
Q

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

A

IGBT - insulated Gate bipolar transistor

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12
Q

What circuit is used in the DC link section of the VFD?

A

L-Section filter

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13
Q

How is an IGBT turned on?

A

+15V applied to Gate

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14
Q

How fast can an IGBT turned on?

A

400 nanoseconds

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15
Q

How fast can an IGBT turned off?

A

500 nanoseconds

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16
Q

What are the two (2) types of PWM techniques?

A

Volts/Hertz (common)

Flux vector

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17
Q

What is the purpose of the control logic board?

A
  • brains of the VFD
  • controls the output
  • stores the software program
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18
Q

What are some common software functions (basic drive adjustments) of the V/Hz control for VFD’s?

A
  • toque limiting
  • voltage boost at low speeds (low speed boost)
  • min/max speeds (frequency)
  • ramp up/down
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19
Q

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

A

Maintain max torque while not exceeding the FLA

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20
Q

What happens to toque as V/Hz ratio is maintained?

A

Stays constant

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21
Q

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

A

DC link section

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22
Q

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

A

Rectifier

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23
Q

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

A

Inverter

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24
Q

What is the most common semi-conductor switching device to produce PWM output?

A

IGBT

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25
Q

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

A
  • V/Hz control technique

- Flux vector control technique

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26
Q

What is the volts/hertz ratio of a 600V motor with a 60 Hz frequency rating?

A

10 V/Hz

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27
Q

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

A

Open-Loop

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28
Q

What type of control does a system have if a heater is energized by a contractor contactor by a thermostat?

A

Closed-Loop

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29
Q

What basic type of control requires feedback?

A

Closed-Loop control

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30
Q

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

A

DC injection braking

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31
Q

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

A

Dynamic braking

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32
Q

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

A

Line regenerative braking

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33
Q

List the three (3) key components in a basic UPS system

A
  • rectifier
  • battery
  • inverter
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34
Q

What are the two (2) applications of a rectifier dc output in a UPS?

A
  • store energy in the batteries

- provide dc to inverter

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35
Q

DC batteries in a UPS system change from charging to discharging immediately when the supply fails or deteriorates

True or False?

A

True

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36
Q

A UPS system with a bypass switch can still maintain supply to a load when

a) system has a high temp
b) system load is excessive
c) the inverter fails
d) all the above

A

d) all the above

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37
Q

Normal battery operating time in a UPS system

A

5-30 minutes

38
Q

How many primary and secondary windings are in the output transformer of a redundant UPS system?

A

3 primary - 1 secondary

39
Q

What are the advantages of installing a common storage battery in a redundant UPS system?

A
  • one battery and and one rectifier

- less maintenance and lower cost

40
Q

A static switch has two SCRs connected in anti-parallel.

True or False?

A

True

41
Q

A static switch can operate in a high speed because it has no moving parts

True or False?

A

True

42
Q

The usual switching time of a static switch is measured in

A

Microseconds

43
Q

The most common type of inverters used in UPS systems

A

PWM - pulse width modulation

44
Q

A PWM inverter can produce pulses of

A

Variable frequency and positive and negative voltages

45
Q

A PWM produces a true sinusoidal waveform

True or False?

A

False; simulated

46
Q

The average voltage between any two pulses from a PWM inverter output is equal to the _____________ voltage of a counterpart sinusoidal voltage.

A

Instantaneous

47
Q

List the two (2) factors in determining the supply conductor ampacity of a UPS system.

A
  • input rating

- battery charge demand

48
Q

What is the recommended UPS battery ambient temperature rating?

A

20-25 degrees Celsius

49
Q

What is the purpose of maintenance bypass circuit breakers?

A

Total shutdown in a UPS system

50
Q

Most UPS systems can still operate if one cooling fan fails.

True of False?

A

True

51
Q

The normal cooling air path in a UPS cabinet is that cool air enters in the

A

Lower front side

52
Q

A sealed battery has a longer serviceable life than a vented type battery.

True or False?

A

True

53
Q

The best method to identify a failed solid state power device is by comparing the dc voltage between the _________ ______ ___________ terminals.

A

Anode and cathode

54
Q

The most easy to isolate section for troubleshooting a UPS system is the

A

Battery bank

55
Q

The purpose of the inverter in a UPS is to provide what two (2) values?

A
  • rated voltage

- rated frequency

56
Q

What is the purpose of a UPS redundant system?

A

Higher reliability by use of two UPS systems

57
Q

The devices used for high speed switching of inverters

A

IGBT

58
Q

Two anti-parallel SCRs are used in what device to switch utility power to the load when the UPS fails.

A

Bypass; static switch

59
Q

During a load test it is found that the UPS cannot supply rated output for the rated time. The problem could be

A

Batteries or rectifiers

60
Q

What type of batteries are used in UPS systems?

A

Lead acid; either sealed or vented

61
Q

Installation of a UPS can be divided into four areas. What are they?

A
  • input
  • dc
  • output
  • auxiliaries
62
Q

If a standard motor (TEFC) is operated by a VFD at low speeds the motor may

A

Overheat

63
Q

Analog control wiring to a VFD should be of what type and installed how

A

Twisted shielded pair (TSP) in its own conduit and bonded at one end only

64
Q

With a VFD connected motor, if the voltage increased and frequency decreased, the current would.

A

Increase

65
Q

Two factors that determine the speed of a squirrel cage motor are

A
  • Frequency

- Number of poles

66
Q

A PWM drive (VFD) is fed with three phase 208V, the dc link voltage would have an approximate value of

A

208 / 0.707 = 294V

67
Q

The VFD changes the speed of a motor by changing the

A

Frequency

68
Q

A VFD motor voltage is held constant, frequency is increases. The current of the motor would

A

Decrease

69
Q

Low speed boost in a VFD provides

A

Voltage boost at low speeds to overcome resistance

70
Q

Why is it important to maintain a steady ratio of voltage and frequency?

A

Max torque without exceeding FLA

71
Q

What are three braking methods of a VFD connected motor

A

Dynamic braking
DC injection braking
Line regenerated braking

72
Q

Which inverter rating type of a VFD connected induction motor would come with shaft encoders?

A

Vector duty; used in closed loop systems

73
Q

What are three types of non-induction motors that can be connected to a VFD

A
  • Permanent magnet motors
  • wound field brushless (sync)
  • reluctance motors
74
Q

__________ __________ is an example of a semiconductor compound.

A

Gallium arsenide

75
Q

Two disadvantages of VFDs

A
  • harmonics

- special cabling requirements

76
Q

Limit of total harmonic distortion (THD) allowed back on supply lines

A

3%

77
Q

Filtering THD on the line side can be accomplished by using a

A

Input line reactor

78
Q

Used between the drive and the motor; to reduce reflected waves due to unmatched cable surge impedance and motor surge impede.

A

Load reactors

79
Q

What causes damaged motor insulation such as pitting

A

Reflected waves causing high voltage spikes

80
Q

Five (5) primary install considerations

A 
Location
Utility Supply 
Bonding
Control wiring 
Harmonics
81
Q

VFDs are best operated between _____-______ degrees Celsius

A

10 - 40

82
Q

Line voltage to the supply should not vary more than plus or minus

A

10%

83
Q

Sometimes installed on the motor shafts to prevent buildup of induced voltage between the shaft and bearing

A

Bonding rings

84
Q

Control wiring at a minimum is a Class ___

A

Class 2

85
Q

A VFD can be fed with single phase and output three-phase to the motor

True or False?

A

True

86
Q

Full wave three phase is ___ pulse rectifier

A

6 pulse

87
Q

Wye-Delta; two full wave three phase is ___ pulse rectifier

A

12 pulse; 30 degree out of phase

88
Q

A dynamic breaking resistor is often included in what section

A

Dc link section

89
Q

The IGBT receives its gate pulse from the

A

Control logic circuit

90
Q

The V/Hz PWM control technique offers drive adjustments such as

A
  • torque limiting
  • voltage boost at low speeds
  • minimum and maximum speed control
  • adjustable acceleration and deceleration rates
91
Q

To achieve 10% motor speed, the VFD should supply ___% frequency

A

10%

4th Year Electrical - Electronics (5 decks)

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