Pretest Module 13 Single-phase Transformers
1. What is the definition of a transformer? Device used to transfer electrical energy
from one circuit to another.
2. What is the principle transformers work on? Mutual induction.
3. What is the purpose of coiling the source conductor in a transformer? Intensify
magnetic field.
4. What is the purpose of coiling the load conductor in a transformer? Increase
active length of conductor being cut by field.
5. How efficient is a transformer? 96-99%
6. What is the primary winding in a transformer? Coil connected to the source.
Secondary? Coil connected to the load.
7. What is voltage ratio? Ratio of EP to ES. Turns ratio? Ratio of NP to NS.
8. What is the voltage in the secondary of a transformer if the primary is 600 V with
200 turns, and the secondary has 50 turns? 150V.
9. Is the transformer in question 8 a step-up or a step-down transformer? Step-down.
10. What is the current in the secondary of a transformer with 20 turns if the primary
has a current of 6 amps and 100 turns? 30A
11. How are transformers classified? Cooling method, winding insulation, number of
phases, type of service, and type of winding.
12. What is the turns ratio for a transformer with a primary voltage of 120 V and a
secondary voltage of 24 V? 5 to 1
13. What is the turns ratio of a transformer with a primary current of 5 amps and a
secondary current of 35 amps? 7 to 1
14. What are the three main parts of a transformer? High-voltage winding, lowvoltage winding, and core.
15. How is the high-voltage winding physically distinguished from the low-voltage
winding in a transformer? Many turns of smaller wire.
16. What are the characteristics of a good core material? High permeability, low
retentivity, and high electrical resistance.
17. What is the purpose of the silicone in the silicone steel used for cores in
transformers? Reduce eddy currents.
18. What is the purpose of laminating the core in a transformer? Reduce eddy
currents.
19. What are the three most common core designs in transformers today? Core, shell,
and cross (or H) type.
20. What is the purpose of placing the high-voltage windings between the lowvoltage winding in a cross type transformer? Reduce insulation required.
21. How does temperature rise affect insulation aging? Insulation aging doubles for
every 10oC above normal operating temperature.
22. What are the two most common mediums for transferring heat away from
transformers? Air and oil.
23. What is additive and subtractive polarity in transformers? Relative position of
high-voltage terminals to low-voltage terminals.
24. What is the formula for efficiency of a transformer? Power out / Power in.
(x100%)
25. What is the kVA rating of a transformer? Total kVA the transformer can
transform.
26. What does it mean when a transformer has two primary HV winding ratings? It is
a multi-winding transformer.
27. What are the two basic types of three-phase transformers? Individual single-phase
transformers connected together or three windings on a common core.
28. What is the result of operating a transformer at a lower frequency than it is rated
for? Excessive current, overheating, and insulation damage.
29. What are three situations where polarity of transformers is important? Paralleling
transformers, instrument connections, or building a three-phase bank.
30. What does the temperature rise rating on a transformer indicate? Maximum
ambient temperature rise above 40oC to which the insulation may be continuously
subjected without shortening its life.
31. Where is the wiring diagram typically found for a transformer? In tables or
schematic diagrams.
32. What is the characteristic that determines if a transformer is a control transformer
or not? Step down voltage to supply control or signal circuits for electrically
operated switches.
33. How are signal transformers different from control transformers? Signal
transformers are typically smaller. (8-25 volts)
34. What is the definition of a distribution transformer? Take power from the supply
authority lines and deliver it directly to the customer.
35. What is the purpose of an instrument transformer? Reduce current and voltage
levels to a value that can safely and effectively be used to operate instruments
such as ammeters and voltmeters.
36. What is the unique characteristic of an autotransformer? Primary and secondary
windings are one single winding.
Unit 2
1. Is a transformer additive or subtractive polarity when X1 is adjacent to H1?
Subtractive.
2. What do the white dots indicate on instrument transformers? Polarity.
3. What is the first step in determining the polarity of a transformer with a
voltmeter? Determine high and low voltage leads.
4. Where is the voltmeter connected for a DC inductive kick test? X1 and X2.
5. When a transformer with a dual primary winding is connected to the higher voltage,
which terminals are connected together? (X1 and H4 or whatever) H2 and H3.
6. How many different turns ratios are available with dual primary and dual
secondary windings? Four (Fig 7)
7. What is the most common winding configuration for distribution transformers?
Single primary and dual secondary.
8. Why is it important to keep loads balanced in a dual winding transformer? So one
transformer is not overloaded.
9. What are circulating currents in transformers? Currents that circulate between
windings but not out into the load.
10. What is the result of high circulating currents in transformers? Overheating
resulting in overloading.
11. What are three values or characteristics of transformers that must be considered
when paralleling transformers? Impedance, voltage, polatity.
12. When testing closure voltage, what voltage should be measured if proper polarity
is maintained paralleling transformers? 0 V
13. What should be done if the voltage closure test indicates a significant voltage? Do
not make final connection. Recheck polarities then test again.
14. Briefly describe “back-feed” in parallel transformers? When a transformer is
disconnected for repairs, etc. there can still be power feeding from the other
transformer.
15. When a transformer has one primary winding and one secondary winding, what
percentage of the kVA rating can be handled by each winding? (50% or 100%)
16. Why is a transformer rated in kVA instead of kW? KVA takes the power factor
into account. KW would have to be at a specific power factor.
17. Does the nameplate on a transformer typically have a current value? Why? No. It
is easily calculated. (kVA/V)
18. Why should the voltage rating of the HV winding never be exceeded for a
transformer? It will over stress the insulation leading to insulation failure.
19. When two HV winding ratings are given, how should the windings be connected
to supply the higher value? Series.
20. Where is indication provided if a transformer contains oil with PCBs? Nameplate.
21. What are three types of wiring information? Schematics, Tables, and Voltage
vector diagrams.
22. What are the two types of energy losses in transformers? Define each. Core
losses- overcoming eddy currents and hysteresis losses. Copper losses- I2 R losses
in the conductors of primary and secondary windings.
23. What is the test for each? (Question above) Open-circuit test for core losses and
short-circuit test for copper losses.
24. The magnetizing current for a transformer is typically what percentage of the full
load current? About 2-5% of full load current.
25. How does the source voltage affect the magnetizing current of a transformer?
Directly proportional.
26. How does the load affect the magnetizing current of a transformer? Remain
constant.
27. How does the load affect copper losses? Directly proportional.
28. What is the formula for efficiency of a transformer? Power out / Power in x 100
29. What is the result of paralleling transformers with unequal impedances? Unequal
load sharing resulting in overloading the transformer with the smaller impedance.
30. What is the available fault current for a transformer with a FLA of 250 and an
impedance of 3 %? 8333 A
31. What is the available fault current for a transformer with a FLA of 135 and an
impedance of 4 %? 3375 A
32. What is the available fault current for a 600/120 V transformer with a 2 %
impedance, rated at 100 kVA? 41,666 A
33. What is the rating of a bank of two paralleled transformers with ratings of 50 kVA
, 1 % impedance and 75 kVA, 1 % impedance? (125 kVA, 2 % impedance, etc)
125 kVA 1% impedance.
34. How are fault currents calculated for banks of transformers? Add individual fault
currents.
Unit 3
1. What is an unloaded transformer? Primary connected to a source and secondary
left open. Acts like an inductor.
2. What is the relationship between the applied emf in the primary of a transformer,
the counter emf in the primary and the induced emf in the secondary? Counter
emf in the primary and induced emf in the secondary are in phase and 1800 out of
phase with the applied voltage.
3. Why is the counter emf in the primary always slightly less than the applied emf in
a transformer? Primary current is self-adjusting.
4. Which value in a transformer is always equal in the primary and secondary
windings? kVA
5. What is the formula for percent voltage regulation in a transformer? Voltage No Load
– Voltage Full Load/Voltage Full load x 100%.
6. What is the benefit of a transformer with an adjustable turns ratio? Compensate
for voltage regulation and line drop.
7. How does adding an extra winding in the primary of a transformer affect the
secondary voltage? Lowers.
8. What are the two types of tap changers? No-load and On-load.
9. Which type of tap changer is typically used to compensate for power differences
throughout the day? On-load.
10. Which tap changer is smaller and cheaper to install? No-load.
11. What is the purpose of the reactor in an on-load tap changer? Prevent short-circuit
of tap connections during switching.
12. How is the reactor size reduced in an on-load tap changer? Spring snap-switch
doesn’t allow time for the reactor to heat up.
13. How is the secondary voltage calculated when you know the primary voltage and
the number of primary and secondary turns? Primary voltage times the turns-ratio.
14. What is the turns ratio for a transformer with a primary voltage of 750 V and a
secondary voltage of 120 V? 6.25 to 1.
15. What is the secondary voltage of a transformer with a turns ratio of 15:1 and a
primary voltage of 1200 V? 80 V.
16. What is the purpose of removing a section from the center of a transformer
winding? Reduce damage from fault currents.
17. What is the main precaution when making jumper connections on a large
transformer? Not to short circuit any winding.
Unit 4
1. How is an autotransformer different from other types of transformers? Primary
and secondary windings are not isolated from one another.
2. What are the four main uses of autotransformers? 1. Small increase or decrease in
voltage is required. 2. Reduced-voltage of squirrel-cage motors. 3 Power supplies
where multiple voltages are required. 4. Automatic voltage regulators to
compensate for line drop as load changes occur.
3. What are some advantages of an autotransformer? Smaller, lighter, and less
expensive.
4. Which conductor must be grounded in the secondary of a transformer? The
conductor solidly connected to the identified conductor on the primary.
5. What does it mean when the low-voltage winding is “bucking” the high-voltage
winding in a transformer? Reducing the voltage in increments by switching taps.
6. Which winding in a “buck/boost” voltage regulator is considered the series
winding? Secondary because it is in series with the secondary lines.
7. How is the buck/boost voltage regulator switched live without shorting out the
series winding? Having a reactor connected to the wiper contacts to prevent short
circuits.
8. What is the secondary voltage on an autotransformer connected across 200 turns
if a voltage of 240 volts is connected to 1000 turns at the primary? 48 V.
9. What is the primary voltage of a transformer with a ratio of 2.5/1 if the secondary
voltage is 28 V? 70 V.
10. What is the one value of a transformer every other value hinges on? Turns ratio.
11. What is one major hazard with autotransformers? Should not be use for circuits
where there is a large difference between primary and secondary voltages. Open
in the common part of the winding can cause very high voltages across secondary.
Unit 5
1. What is the main reason for using current and potential transformers? Allow the
use of much smaller, cheaper, and safer control and measuring equipment.
2. Where is a current transformer primary winding placed? In series with the
conductor carrying the current.
3. How are current transformers typically rated? 5 ampere secondary.
4. Why is the turns ratio for current transformers not exactly equal to the calculated
turns ratio? Losses.
5. Why is it important for current transformers to be accurate? Small inaccuracy in
the secondary may be a large inaccuracy in the primary.
6. What is the danger in opening the secondary circuit of a current transformer?
High voltages may be induced in the secondary.
7. What precaution should always be taken before opening the secondary circuit of a
current transformer? Short out the secondary terminals.
8. Which type of current transformer consists of two separate coils wound over the
same coil? Wound.
9. What type of current transformer is a clamp-on ammeter? Window.
10. Which type of current transformer is best suited to comparison measurements?
Window.
11. Which type of current transformer is strongest and best able to handle
overcurrents? Bar.
12. How is a potential transformer connected into the primary circuit? (series or
parallel)
13. What is the typical secondary voltage for a potential transformer? 120 V
14. How are voltages above 100 kV measured using potential transformers? Potential
transformer with a capacitor voltage divider.
15. What is the hazard associated with working on the secondary of potential
transformers? None.
16. What is the conventional method of marking polarity for instrument transformers?
H and X subscripts or a white dot for H1 and X1.