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Transformers and PCBs > Test Review > Flashcards

Test Review Flashcards

1
Q

Why is speed a desirable quality to have in opening circuit breakers?

A
  • facilitates extinguishing an arc
  • minimizes damage caused by a fault
  • reduces effects of a fault on lines and equipment not directly involved with the fault
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2
Q

What is an isolation transformer?

A

A transformer that is magnetically coupled, but electrically and mechanically isolated.

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3
Q

What are harmonics? When do they occur?

A

Spikes of frequency that occur at multiples of 60 Hz

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4
Q

What must be known about a transformer to determine the different values of voltage and current?

A

Turn ratio

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5
Q

What part of the transformer is connected to the load and what part is connected to the power supply?

A

The secondary winding is connected to the load and the primary winding is connected to the power supply

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6
Q

True or False:

In a step down transformer, the power in does not equal the power out.

A

False

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7
Q

What is a transformer?

A

A magnetically operated machine that can change voltage, current, and impedance without changing frequency

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8
Q

How are transformers rated?

A

kVA

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9
Q

Name the three basic contacts of a PCB?

A

Butt, wedge, and bayonet

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10
Q

What is the meaning of electrically trip free regarding a PCB?

A

The PCB will not allow the closing coil to close under a fault condition.

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11
Q

What is the most common gas used in circuit breakers to quench an arc?

A

Sulfur hexafluoride (SF6)

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12
Q

What is continuous current?

A

The amount of current that a circuit breaker can carry continuously

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13
Q

What is interrupting time?

A

The time it takes a circuit breaker to open after the tripping coil is energized

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14
Q

How many transformers make an open delta connection?

A

Two

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15
Q

What is the frequency of the second harmonic?

A

120 Hz

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16
Q

What is a triplen harmonic?

A

Odd multiple of the third harmonic

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17
Q

What is the primary function of a circuit breaker?

A

Protection

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18
Q

A transformer has a primary voltage of 240 V, a secondary voltage of 180 V, and the windings are connected subtractively. What voltage will appear across the entire connection?

A

240 V - 180 V = 60 V

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19
Q

What is another name for an arc?

A

Plasma

20
Q

Two transformers rated at 100 kVA each are connected in an open delta connection. What is the total output power supplied?

A

.866(100kVA + 100kVA) = 173.2 kVA

Or

1.732(100 kVA) = 173.2 kVA

21
Q

What is a knife switch?

A

The simplest form of a circuit breaker

22
Q

How many seconds does it take for a circuit breaker to open?

A

1/20 of a second

23
Q

A transformer has a primary voltage of 240 V and a secondary voltage of 80 V. What is the turn ratio?

A

Np/Ns = 240V/80V

Np/Ns = 3/1

3:1

24
Q

How are circuit breakers rated?

A
  • interrupting current capacity
  • continuous current capacity
  • voltage class
25
Q

What are the three mediums that extinguish the arc in circuit breakers?

A

Air, oil, and SF6

26
Q

A transformer has a turn ratio of 1:6 and a primary current of 18 A. What is the secondary current?

A 
Np/Ns = Is/Ip 
1/6 = Is/18
Is= 3 A
27
Q

True or False:

Phase voltage and line voltage are 90 degrees out of phase on a three phase system.

A

False

They are 120 degrees out of phase

28
Q

What two things are important about a wye connected three phase system?

A

Line current equals phase current

E(Line) = E(Phase) x 1.732

29
Q

What two things are important about a delta connected three phase system?

A

Line voltage equals phase voltage

I(Line) = I(Phase) x 1.732

30
Q

True or False:

Voltage, current, frequency, and impedance are things that can be changed by a transformer.

A

False

Frequency can’t be changed

31
Q

In a transformer with a power output of 750 VA and a primary voltage of 120 V, what is the primary current?

A

P = I x E
750 VA = I x 120V
I = 6.25 A

32
Q

What is the formula to find the phase voltage if given line voltage in a three phase wye connected system?

A

E(phase) = E(line) / 1.732

33
Q

An open delta three phase transformer system has one transformer center tapped to provide a neutral for single phase voltages. If the voltage from the line to center tap is 277 V, what is the high leg?

Show the math.

A

277 V x 1.732 = 479.76 V

34
Q

What kind of transformer uses one winding for both the primary and secondary windings?

A

An autotransformer

35
Q

What is mutual inductance?

A

When a coil induces voltage into another coil

36
Q

A transformer has a turn ratio of 2.5:1 and a primary voltage of 220 V. What is the secondary voltage?

A 
Np/Ns = Ep/Es
2.5/1 = 220/Es
Es(2.5) = 220(1)
Es = 220/2.5
Es = 88 V
37
Q

A transformer has a primary voltage of 240 V, apparent power of 360 VA, and a secondary voltage of 48 V. What is the secondary current?

A

P = Is x Es
360 VA = Is x 48 V
Is = 7.5 A

38
Q

All values of a transformer are proportional to what?

A

Turn ratio of the windings

39
Q

What is self-induction?

A

When a coil induces voltage onto itself

40
Q

What is the formula to find primary current in a transformer?

A

Np/Ns = Is/Ip = Ep/Es

41
Q

Name three things that dielectric oil does in a transformer.

A

Insulates, cools, and moisture prevention

42
Q

What is the disadvantage of SF6 in circuit breakers?

A

It’s corrosive

43
Q

What is the disadvantage of oil circuit breakers?

A

They are explosive

44
Q

How can a breaker be tripped free?

A

Electrically, mechanically, and pneumatically

45
Q

Why are heaters used with SF6 breakers?

A

It liquifies readily

46
Q

What 8 things are on a transformer nameplate?

A
  1. Name of manufacturer
  2. Rated kilovolt-ampere
  3. Frequency
  4. Primary and secondary voltage
  5. Impedance of transformers rated 25kVA and larger
  6. Required clearances of transformers with ventilated openings
  7. Amount and kind of insulating liquid where used
  8. The temperature class for the insulating system of dry-type transformers

Transformers and PCBs (1 decks)

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