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Third Period Electrician > Transformers > Flashcards

Transformers Flashcards

Transformers stuff

1
Q

Induction is described as the property of a circuit that _____

A

Opposes change in current

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2
Q

Mutual Induction occurs when a change of _____ in one circuit causes _____ to be induced into another circuit

A

Current, voltage

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3
Q

Mutual induction is also called

A

Transformer action

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4
Q

The coil of a transformer which is connected to a source of potential difference is the _____ (Primary or Secondary)

A

Primary

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5
Q

Once primary current in a DC connected transformer reaches a steady value, what happens in the secondary coil?

A

No voltage is induced.

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6
Q

A self or mutually induced voltage only occurs when the current is _____.

A

Changing (as with AC)

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7
Q

What does reluctance do? What does it have to do with transformers?

A

Opposes magnetic flux on a magnetic circuit.

The soft iron core is low-relunctance.

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8
Q

Is the efficiency of a transformer high or low?

A

High. 95-98%

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9
Q

Which type of core has the most leakage flux? And the least?

A

Core type - most

H TYPE - least

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10
Q

An H type center is commonly found on what type of transformers?

A

High voltage distribution

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11
Q

Hysteresis losses are _____ to the frequency of the source voltage

A

Proportional

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12
Q

Power losses due to eddy currents and hysteresis losses are also called _____

A

Core losses

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13
Q

How do you determine %Z of a transformer?

A

A short circuit test

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14
Q

What is a common use for a step up transformer?

A

To transfer power over long distances, such as power station switchyard for transmission.

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15
Q

What happens to a transformer beyond 110% of rated voltage?

A

Primary current increase and transformer burns out.

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16
Q

If the secondary voltage on a transformer doubles, what does the current do?

A

Reduces to half

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17
Q

If the current on a secondary winding is halved, what happens to the voltage?

A

Doubles

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18
Q

Transformer polarity is the relative direction of _____ voltage to the direction of _____

A

Primary voltage, Secondary voltage

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19
Q

With power or distribution transformer, polarity is important when _____

A

Paralleled

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20
Q

A tap labelled -5% would go on the primary if the supply voltage is _____

A

5% lower than rated

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21
Q

A transformer whose capacity is reduced in proportion to the reduction of primary voltage has a _____

A

Reduced capacity tap

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22
Q

A full capacity tap transformer has higher overall capacity so that ____

A

Rated kVA is available at any tap setting

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23
Q

FCBN taps would be used when primary supply voltage is _____

A

5% lower than rated

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24
Q

How does an automatic tap changer work?

A

Feedback control circuit that changes taps when secondary V is above or below a preset value.

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25
Q

What is % Voltage Regulation?

A

Change of the secondary output Voltage from no-load to full-load.

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26
Q

Formula for % Voltage Regulation

A

%VR = “v(secondary no load) - v(secondary full load) “/ v(secondary full load) x 100

27
Q

When E(s) is reduced, the secondary output voltage V(s) is (reduced or increased).

A

Reduced

28
Q

When E(p) is reduced, secondary induced voltage E(s) is (reduced or increased).

A

Reduced

29
Q

Volt drops from secondary current, R(s) and XL(s) of the secondary will (reduce or increase) secondary output voltage

A

Reduce

30
Q

Secondary output voltage V(s) at full load is (less or more) than at no load.

A

Less

31
Q

Core losses in a transformer can be determined by _____.

A

Open circuit test

32
Q

Copper losses are proportional to the _____ and vary from no to full load conditions (true or false)

A

Square of the currents.

True

33
Q

If a transformer’s load is decreased, what happens to the %efficiency?

A

Also decreases

34
Q

Increased power factor means (increase, decrease) %efficiency

A

Increase

35
Q

Increased copper and core losses means (increase, decrease) %efficiency

A

Decrease

36
Q

Short-circuit current is limited by the _____ of the transformer as well as ____ of the remainder of the circuit.

A

Impedances

37
Q

Impedance Voltage of a transformer is the primary voltage required to circulate rated full-load _____ current in a transformer when the _____ is _____ at the terminals

A

Secondary, secondary, short circuited

38
Q

Copper losses in a transformer are determined by _____

A

Short circuit test

39
Q

Core losses are are dependant on applied voltage (true or false)

A

True

40
Q

With constant rated voltage applied, core losses remain constant from no to full load (true or false)

A

True

41
Q

%Z =

A

(Impedance Voltage / Primary rated voltage) x 100

42
Q

Impedance Voltage =

A

Primary rated voltage x ( %Z/100 )

43
Q

When transformers are connected in parallel, the impedance (increases, decreases) and what issue can this cause?

A

Decreases. The available short-circuit current can increase considerably.

44
Q

When paralleling transformers, the no-load secondary voltages must be _____

A

Equal

45
Q

Should you use different tap settings on paralleled transformers? Why or why not?

A

No. Leads to different secondary voltages.

46
Q

Transformers in parallel should have a %Z within _____ of each other

A

5%

47
Q

What happens if one paralleled transformer is disconnected from the source but the other is not?

A

The disconnected transformer backfeeds (induces) voltage into the primary due to the connection to the transformer that is still connected to source.

48
Q

What happens if an emergency generator is connected to a home’s electrical system before disconnecting the utility?

A

Induced voltage in generator backfeeds to utility transformer

49
Q

Formula for %Z

A

%Z = Impedance Voltage / E (p)

50
Q

Transformer efficiency formula

A

(P(out) / P(in) + copper & core losses) x 100

51
Q

What two pieces of info are obtained by an Open Circuit test?

A

Exciting current, core loss

52
Q

How do core losses change in reference to a change of supply voltage?

A

Exponentially

53
Q

What is a harmonic? What is a third harmonic in a transformer?

A

Integer product of supply frequency. 3 x 60hz = 180hz, can induce a current which causes excessive heat.

54
Q

A Wye/Wye connection without a primary neutral is normal, true or false?

A

False. It is rare because it makes it hard to maintain balanced output voltage.

55
Q

What should a voltmeter read in a properly connected Delta open corner test?

A

0V

56
Q

An advantage to the Wye/Delta connection is _____.

A

Primary winding conductors do not need to be insulated for full three-phase line voltage.

57
Q

Are the primary and secondary line voltages of a Delta/Wye connection in phase with each other?

A

No

58
Q

If the polarity of one of the secondary windings in a Delta/Wye bank is reversed, will the correct three phase line voltages be available?

A

No

59
Q

The new neutral on the secondary of a step down transformer bank is typically called the _____

A

XO point

60
Q

3, 5 and 7th Harmonics (low, odd) are the worst because _____. What does it cause regarding voltage and current?

A

They are in phase and stack on the sine waveform. Can cause extremely high dangerous voltages and current can flow in neutral.

61
Q

If you remove one transformer of a 3 phase Delta Delta system, how much of the power capacity will you have between the remaining two transformers?

A

57.7% of the original VA

62
Q

The primary and secondary line voltages of a Delta/Wye 3 phase transformer are in phase with each other, true or false?

A

False. 30 degrees separate

63
Q

What are the coordinating closed contacts for each phase when using a voltmeter transfer switch? And for the lines?

A

Phase A: 2, 3

Phase B: 2, 7

Phase C: 2, 11

Line AB: 3, 6

Line BC:: 6, 11

Line CA: 3, 10

Third Period Electrician (4 decks)

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