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

UPS and VFD's Flashcards

(91 cards)

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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
What are the two (2) most common control techniques for producing a PWM output of a VFD?
- V/Hz control technique | - Flux vector control technique
26
What is the volts/hertz ratio of a 600V motor with a 60 Hz frequency rating?
10 V/Hz
27
What type of control does a system have if a heater is energized by a contactor by a start stop station?
Open-Loop
28
What type of control does a system have if a heater is energized by a contractor contactor by a thermostat?
Closed-Loop
29
What basic type of control requires feedback?
Closed-Loop control
30
What type of braking is done by putting a dc voltage on the motor stator windings?
DC injection braking
31
What type of braking uses a resistor to dissipate the regenerative voltage as heat?
Dynamic braking
32
What type of braking recycles the energy produced by a decelerating motor back to the electrical system?
Line regenerative braking
33
List the three (3) key components in a basic UPS system
- rectifier - battery - inverter
34
What are the two (2) applications of a rectifier dc output in a UPS?
- store energy in the batteries | - provide dc to inverter
35
DC batteries in a UPS system change from charging to discharging immediately when the supply fails or deteriorates True or False?
True
36
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
d) all the above
37
Normal battery operating time in a UPS system
5-30 minutes
38
How many primary and secondary windings are in the output transformer of a redundant UPS system?
3 primary - 1 secondary
39
What are the advantages of installing a common storage battery in a redundant UPS system?
- one battery and and one rectifier | - less maintenance and lower cost
40
A static switch has two SCRs connected in anti-parallel. True or False?
True
41
A static switch can operate in a high speed because it has no moving parts True or False?
True
42
The usual switching time of a static switch is measured in
Microseconds
43
The most common type of inverters used in UPS systems
PWM - pulse width modulation
44
A PWM inverter can produce pulses of
Variable frequency and positive and negative voltages
45
A PWM produces a true sinusoidal waveform True or False?
False; simulated
46
The average voltage between any two pulses from a PWM inverter output is equal to the _____________ voltage of a counterpart sinusoidal voltage.
Instantaneous
47
List the two (2) factors in determining the supply conductor ampacity of a UPS system.
- input rating | - battery charge demand
48
What is the recommended UPS battery ambient temperature rating?
20-25 degrees Celsius
49
What is the purpose of maintenance bypass circuit breakers?
Total shutdown in a UPS system
50
Most UPS systems can still operate if one cooling fan fails. True of False?
True
51
The normal cooling air path in a UPS cabinet is that cool air enters in the
Lower front side
52
A sealed battery has a longer serviceable life than a vented type battery. True or False?
True
53
The best method to identify a failed solid state power device is by comparing the dc voltage between the _________ ______ ___________ terminals.
Anode and cathode
54
The most easy to isolate section for troubleshooting a UPS system is the
Battery bank
55
The purpose of the inverter in a UPS is to provide what two (2) values?
- rated voltage | - rated frequency
56
What is the purpose of a UPS redundant system?
Higher reliability by use of two UPS systems
57
The devices used for high speed switching of inverters
IGBT
58
Two anti-parallel SCRs are used in what device to switch utility power to the load when the UPS fails.
Bypass; static switch
59
During a load test it is found that the UPS cannot supply rated output for the rated time. The problem could be
Batteries or rectifiers
60
What type of batteries are used in UPS systems?
Lead acid; either sealed or vented
61
Installation of a UPS can be divided into four areas. What are they?
- input - dc - output - auxiliaries
62
If a standard motor (TEFC) is operated by a VFD at low speeds the motor may
Overheat
63
Analog control wiring to a VFD should be of what type and installed how
Twisted shielded pair (TSP) in its own conduit and bonded at one end only
64
With a VFD connected motor, if the voltage increased and frequency decreased, the current would.
Increase
65
Two factors that determine the speed of a squirrel cage motor are
- Frequency | - Number of poles
66
A PWM drive (VFD) is fed with three phase 208V, the dc link voltage would have an approximate value of
208 / 0.707 = 294V
67
The VFD changes the speed of a motor by changing the
Frequency
68
A VFD motor voltage is held constant, frequency is increases. The current of the motor would
Decrease
69
Low speed boost in a VFD provides
Voltage boost at low speeds to overcome resistance
70
Why is it important to maintain a steady ratio of voltage and frequency?
Max torque without exceeding FLA
71
What are three braking methods of a VFD connected motor
Dynamic braking DC injection braking Line regenerated braking
72
Which inverter rating type of a VFD connected induction motor would come with shaft encoders?
Vector duty; used in closed loop systems
73
What are three types of non-induction motors that can be connected to a VFD
- Permanent magnet motors - wound field brushless (sync) - reluctance motors
74
__________ __________ is an example of a semiconductor compound.
Gallium arsenide
75
Two disadvantages of VFDs
- harmonics | - special cabling requirements
76
Limit of total harmonic distortion (THD) allowed back on supply lines
3%
77
Filtering THD on the line side can be accomplished by using a
Input line reactor
78
Used between the drive and the motor; to reduce reflected waves due to unmatched cable surge impedance and motor surge impede.
Load reactors
79
What causes damaged motor insulation such as pitting
Reflected waves causing high voltage spikes
80
Five (5) primary install considerations
``` Location Utility Supply Bonding Control wiring Harmonics ```
81
VFDs are best operated between _____-______ degrees Celsius
10 - 40
82
Line voltage to the supply should not vary more than plus or minus
10%
83
Sometimes installed on the motor shafts to prevent buildup of induced voltage between the shaft and bearing
Bonding rings
84
Control wiring at a minimum is a Class ___
Class 2
85
A VFD can be fed with single phase and output three-phase to the motor True or False?
True
86
Full wave three phase is ___ pulse rectifier
6 pulse
87
Wye-Delta; two full wave three phase is ___ pulse rectifier
12 pulse; 30 degree out of phase
88
A dynamic breaking resistor is often included in what section
Dc link section
89
The IGBT receives its gate pulse from the
Control logic circuit
90
The V/Hz PWM control technique offers drive adjustments such as
- torque limiting - voltage boost at low speeds - minimum and maximum speed control - adjustable acceleration and deceleration rates
91
To achieve 10% motor speed, the VFD should supply ___% frequency
10%

4th Year Electrical - Electronics (5 decks)

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