Transformers

Question 1

Induction is described as the property of a circuit that _____

Answer 1

Opposes change in current

Question 2

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

Answer 2

Current, voltage

Question 3

Mutual induction is also called

Answer 3

Transformer action

Question 4

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

Answer 4

Primary

Question 5

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

Answer 5

No voltage is induced.

Question 6

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

Answer 6

Changing (as with AC)

Question 7

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

Answer 7

Opposes magnetic flux on a magnetic circuit.

The soft iron core is low-relunctance.

Question 8

Is the efficiency of a transformer high or low?

Answer 8

High. 95-98%

Question 9

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

Answer 9

Core type - most

H TYPE - least

Question 10

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

Answer 10

High voltage distribution

Question 11

Hysteresis losses are _____ to the frequency of the source voltage

Answer 11

Proportional

Question 12

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

Answer 12

Core losses

Question 13

How do you determine %Z of a transformer?

Answer 13

A short circuit test

Question 14

What is a common use for a step up transformer?

Answer 14

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

Question 15

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

Answer 15

Primary current increase and transformer burns out.

Question 16

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

Answer 16

Reduces to half

Question 17

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

Answer 17

Doubles

Question 18

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

Answer 18

Primary voltage, Secondary voltage

Question 19

With power or distribution transformer, polarity is important when _____

Answer 19

Paralleled

Question 20

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

Answer 20

5% lower than rated

Question 21

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

Answer 21

Reduced capacity tap

Question 22

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

Answer 22

Rated kVA is available at any tap setting

Question 23

FCBN taps would be used when primary supply voltage is _____

Answer 23

5% lower than rated

Question 24

How does an automatic tap changer work?

Answer 24

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

Question 25

What is % Voltage Regulation?

Answer 25

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

Question 26

Formula for % Voltage Regulation

Answer 26

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

Question 27

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

Answer 27

Reduced

Question 28

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

Answer 28

Reduced

Question 29

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

Answer 29

Reduce

Question 30

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

Answer 30

Less

Question 31

Core losses in a transformer can be determined by _____.

Answer 31

Open circuit test

Question 32

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

Answer 32

Square of the currents. True

Question 33

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

Answer 33

Also decreases

Question 34

Increased power factor means (increase, decrease) %efficiency

Answer 34

Increase

Question 35

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

Answer 35

Decrease

Question 36

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

Answer 36

Impedances

Question 37

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

Answer 37

Secondary, secondary, short circuited

Question 38

Copper losses in a transformer are determined by _____

Answer 38

Short circuit test

Question 39

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

Answer 39

True

Question 40

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

Answer 40

True

Question 41

%Z =

Answer 41

(Impedance Voltage / Primary rated voltage) x 100

Question 42

Impedance Voltage =

Answer 42

Primary rated voltage x ( %Z/100 )

Question 43

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

Answer 43

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

Question 44

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

Answer 44

Equal

Question 45

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

Answer 45

No. Leads to different secondary voltages.

Question 46

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

Answer 46

5%

Question 47

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

Answer 47

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

Question 48

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

Answer 48

Induced voltage in generator backfeeds to utility transformer

Question 49

Formula for %Z

Answer 49

%Z = Impedance Voltage / E (p)

Question 50

Transformer efficiency formula

Answer 50

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

Question 51

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

Answer 51

Exciting current, core loss

Question 52

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

Answer 52

Exponentially

Question 53

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

Answer 53

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

Question 54

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

Answer 54

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

Question 55

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

Answer 55

0V

Question 56

An advantage to the Wye/Delta connection is _____.

Answer 56

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

Question 57

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

Answer 57

No

Question 58

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?

Answer 58

No

Question 59

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

Answer 59

XO point

Question 60

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

Answer 60

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

Question 61

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?

Answer 61

57.7% of the original VA

Question 62

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

Answer 62

False. 30 degrees separate

Question 63

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

Answer 63

Phase A: 2, 3 Phase B: 2, 7 Phase C: 2, 11 Line AB: 3, 6 Line BC:: 6, 11 Line CA: 3, 10