Multiple Choice Answers

1. What is the efficiency of a transformer that has an input of 600 VA and an output to a load of 580 VA?
A. 20 VA C. 83.8% B. 180 VA D. 96.6%
2. The secondary line-to-line voltage of a three-phase Y-Y–connected transformer bank is 115 VAC. What would be the secondary line to neutral voltage?
A. 199.2 V C. 84.2 V B. 66.4V D. 50V
3. Most of the transformers that you’ll find in your plant are either single-phase or
A. zero-phase. C. three-phase. B. two-phase. D. four-phase.
4. An inductor whose reactance can be varied by supplying current through a separate DC winding is called.
A. a reactance tube. C. an autotransformer. B. a current transformer. D. a saturable reactor.
5. If the turns ratio of a transformer is 4 : 1 and the incoming voltage is 120 VAC, what is the secondary voltage?
A. 30 VAC C. 240 VAC B. 120 VAC D. 480 VAC
6. If the primary in a transformer has more turns than the secondary, the secondary has A. lower current. C. higher power.
B. lower voltage. D. lower ampere-turns.
7. What type of transformer is used to prevent electrical noise from passing from the pri-
mary winding to the secondary winding?
A. Dry-type transformer C. Shielded transformer
B. Oil-filled transformer D. Delta transformer
8. You’ll work at times with a transformer that has one winding connected in series with
the other to form the equivalent of a single winding. This is called
A. a distribution transformer. C. a specialty transformer.
B. an electronic transformer. D. an autotransformer.
9. If the power in the primary circuit of a 4 : 1 transformer is 12 watts, what will be the
power in the secondary of this transformer?
A. 3 watts C. 12 watts
B. 4 watts D. 48 watts
10. What would be the power in the secondary of a transformer that has a voltage of 48
VAC and a current of 2.2 A?
A. 52.8 watts C. 121.8 watts
B. 105.6 watts D. 212.6 watts
11. What is the kVA rating of a transformer that has a secondary winding that’s delivering
10 amps at 480 VAC?
A. 4.8 kVA C. 480 kVA
B. 48 kVA D. 4800 kVA
12. A basic transformer consists of two separate conductive coils wrapped around a(n)
_______ core.
A. copper C. iron
B. oiled paper D. plastic
13. Three single-phase transformers, each having a secondary phase current of 10 A, are
Delta-Delta–connected. The value of the secondary line current is A. 1.73 A. C. 57.8 A.
B. 17.3 A. D. 578 A. 44
14. The primary current drawn by the transformer shown below will be
A. 2 A. C. 10 A. B. 5 A. D. 50 A.

Figure is required to reply this question.
15. What special installation or maintenance requirements are needed for askarel-insulated transformers?
A. Cooling water must be supplied to the transformer core.
B. The transformer must be installed in a vault if the voltage exceeds 35,000 VAC. C. The oil must be changed at least once a year.
D. The radiator must be installed where air can circulate and the fins must remain
clean.
16. Typical potential transformers are used for
A. allowing the connection of low-voltage measuring instruments to high-voltage lines. B. biasing control circuits in high-voltage circuits.
C. powering lighting systems from high-voltage feed lines.
D. powering current-sensitive relays in control circuits.
17. Why are high-quality transformers wound with large diameter wire?
A. To lower the hysteresis losses
B. To lower the I2R losses
C. To lower the eddy current losses
D. To lower the magnetic leakage losses
18. If 120 VAC is applied to a transformer with a turns ratio of 6 : 1, what type of trans- former is present and what is the output voltage of the secondary?
A. Step-up, 720 VAC C. Step-down, 20 VAC B. Step-up, 600 VAC D. Step-down, 60 VAC
19. When a load is suddenly connected to a transformer, the secondary voltage will be reduced. This drop in voltage is called transformer
A. seduction. C. excitation. B. regulation. D. conductance.
20. What type of device is used on a distribution transformer to protect the transformer from voltage spikes?
A. A fuse C. A surge diverter B. A pressure vent D. A ground shield