Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Level 2 Electrical, Canada > Transformers > Flashcards

Transformers Flashcards

You may prefer our related Brainscape-certified flashcards:
Certified Sommelier Exam (CMS 2) flashcards
1
Q

What does the transformer ‘ratio’ represent?

A

Turns of wire per winding

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Which two elements do not change in transformers?

A

Applied voltage, frequency

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Power In = Power Out
T/F

A

True

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is the main benefit of an isolation transformer?

A

The reduce current spikes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Which three methods are transformers classified under?

A
  1. Cooling type
  2. Application
  3. Phases
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

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

A

PCB’s

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What are the properties of how a TX is built?

A

High voltage windings, low voltage windings, core

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What are the properties of low voltage windings?

A

Thicker wire for higher current, fewer turns, thinner insulation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is hysteresis?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What are eddy currents?

A

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

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What can prevent eddy currents?

A

Laminating the core.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Which winding has higher current?

A

Whichever winding has lower voltage.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Which winding has higher voltage?

A

Step down TX - primary
Step up TX - secondary

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Which winding has a higher VA?

A

They should be equal.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
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)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions