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Level 2 Electrical, Canada > Transformers > Flashcards

Transformers Flashcards

1
Q

What does the transformer ‘ratio’ represent?

A

Turns of wire per winding

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2
Q

Which two elements do not change in transformers?

A

Applied voltage, frequency

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3
Q

Power In = Power Out
T/F

A

True

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4
Q

What is the main type of transformer, explain its properties.

A

Isolation transformer, there is zero physical or electrical connection between the primary and secondary windings. Isolation transformers work off of mutual induction.

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5
Q

What is mutual induction?

A

Magnetic coupling. The continuous increasing and decreasing of the magnetic field causes the flux lines to cut the windings, inducing a voltage into the windings.

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6
Q

What is the main benefit of an isolation transformer?

A

The reduce current spikes

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7
Q

Which three methods are transformers classified under?

A
  1. Cooling type
  2. Application
  3. Phases
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8
Q

What are the different types of cooking methods for a TX? Which is most common and why?

A

Air or Oil.

Oil is most common - oil helps disperse heat away from the core, and oil is a great insulator.

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9
Q

What is the most common type of oil used in TX’s?

A

PCB’s

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10
Q

What are the four types of TX applications?

A

Power supply (over 500kVA)
Distribution (500kVA and under)
Control (steps down to 120v)
Instrument (CT’s)

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11
Q

What are the properties of how a TX is built?

A

High voltage windings, low voltage windings, core

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12
Q

What are the properties of the high voltage winding?

A

Thinner wire for lower current, more turns, thicker insulation for higher electrostatic stress

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13
Q

What are the properties of low voltage windings?

A

Thicker wire for higher current, fewer turns, thinner insulation

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14
Q

What are the properties of the TX core?

A

Usually made of iron - high permeability, low retentivity, high resistance. Concentrates the magnetic flux lines.

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15
Q

What is permeability? Which is better, high or low?

A

The ability for flux lines to pass through a material. High permeability means more flux lines can pass through which is better.

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16
Q

What is retentivity? Which is better, high or low?

A

The ability of a material to retain magnetism.

Low retentivity is better. If material was high retentivity then magnetism would still remain once supply was turned off, and low retentivity reduces hysteresis losses.

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17
Q

What is hysteresis?

A

The losses due to constant reversal of magnetic fields. Usually heat loss.

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18
Q

What is resistance? Which is better in a core, high or low?

A

The resistance specific materials have to current.

High resistance is better to reduce eddy currents which leads to heat loss.

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19
Q

What are eddy currents?

A

Whirlpool like currents that are induced on metal materials. Iron cores have high resistance to eddy currents.

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20
Q

What can prevent eddy currents?

A

Laminating the core.

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21
Q

How are the ratios between primary and secondary voltages, turns, and currents related?

A

Primary and secondary volts per turns are equal ratios, primary and secondary currents are equal reciprocal ratios.

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22
Q

Which winding has higher current?

A

Whichever winding has lower voltage.

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23
Q

Which winding has higher voltage?

A

Step down TX - primary
Step up TX - secondary

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24
Q

Which winding has a higher VA?

A

They should be equal.

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25
Q

What is magnetizing current?

A

When there is no load on a secondary, the secondary acts as pure resistance. The primary will self induce a current into the secondary that is 2-5% less than the primary.

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26
Q

What is the most important factor in hooking up multi coil Tx’s?

A

Polarity.

27
Q

Explain the importance of polarities in multi coil TX’s

A

When current flows in the same direction a magnetic flux develops in the same direction.

When current flows in opposite directions, the magnetic flux will develop in opposite directions and cancel itself out creating no current flow.

28
Q

When checking that transformers are correctly connected in parallel, measuring across one lead of each secondary winding will produce what voltage and why?

A

0V, proving that there is zero potential difference between the coils.

29
Q

How do you find the total kVA of a bank of transformers?

A

Add all rated kVAs together.

30
Q

When connecting transformers in parallel, if the polarities are connected incorrectly what will the secondary windings read in volts?

A

Twice the voltage on a secondary coil.

31
Q

In a parallel transformer, if you disconnect the primary without disconnecting the secondary, what safety hazard is present?

A

Backfeed from mutual induction.

32
Q

Formula to calculate %Z

A

%Z = E short circuit / E rated

33
Q

Formula to calculate Efficiency

A

Eff = Power Out / Power Out + Power Losses

34
Q

Formula to calculate fault currents.

A

I short cct = I Full Load / %Z

35
Q

Formula for voltage regulation

A

VR = E no load - E full load / E full load

36
Q

What is the ratio for PT’s

A

______:120V

37
Q

What ratio are CT’s

A

_____:5A

38
Q

Copper losses are found with ______ test

A

Short

39
Q

Core losses are found with ______ test

A

Open circuit

40
Q

Copper losses are losses do to _____

A

Ohmic resistance. Aka I^2R losses.

41
Q

Core losses are losses due to

A

Eddy currents and hysteresis

42
Q

Do current losses increase or decrease as the load increases?

A

Increase. Losses are directly proportional to magnitude.

43
Q

What type of current is needed to measure copper losses?

A

Full load.

44
Q

True power is measured from which test?

A

Open circuit.

45
Q

What is exciting current?

A

Just enough current to induce magnetism in a core. Not full rated.

46
Q

Where do you place a wattmeter to perform an open circuit test?

A

Parallel between source and primary windings

47
Q

On a parallelled transformer, how do you find total fault current?

A

Find individual fult currents of each transformer and then add together, or find total A on both secondary windings of tx’s

48
Q

What is voltage regulation?

A

The change in voltage from no load to full load.

49
Q

Formula for voltage regulation

A

no load - full load / full load

50
Q

Voltage regulation is measured off of which winding?

A

Secondary.

51
Q

You can use a voltage triangle to find voltage regulation Egenerated.
T or F?

A

True

52
Q

What is the purpose of instrumental transformers?

A

To step down current and voltage

53
Q

CT’s stand for what? What is their purpose?

A

Circuit Transformer. To step down current.

54
Q

PT’s stand for what? What are their purpose?

A

Potential Transformer. To step down voltage.

55
Q

How are CT’s connected to a circuit?

A

In series between supply and primary winding

56
Q

How are PT’s connected in a circuit?

A

In parallel with the power supply

57
Q

What is the CT ratio?

A

_____:5

58
Q

What is the PT ratio?

A

____:120v

59
Q

How do you find a wattmeter multiplier ratio?

A

Find lowest form of CT and PT ratios, multiply together.

60
Q

How do you find true power of CT and PT circuit?

A

Multiply watts by wattage multiplier

61
Q

When finding fault current of a dual winding transformer, how do the coils add in series and parallel?

A

Fault current in series, use one current. Fault current in parallel add both currents together

62
Q

What happens if you place an Ammeter in parallel with a winding?

A

Very low ohmic resistance will create a short circuit - current will take path of least resistance.

63
Q

A wattmeter connected in parallel on a primary will read _____ losses

A

Copper losses

64
Q

When performing a voltage test on the secondary side of the transformer, if polarities are correct you will read _____, if polarities are incorrect you will read ______

A

0V, Double the secondary.

Level 2 Electrical, Canada (19 decks)

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