3-1-13 Transformers—Single-Phase Connections

Question 1

A typical distribution transformer provides 120/240 volts to residential customers and has its coils connected in __?__.

Answer 1

series

Question 2

The primary on a single transformer connected delta on a 2,400-volt system would be connected __?__.

Answer 2

phase-to-phase

Question 3

The terminals on a transformer are called __?__.

Answer 3

bushings

Question 4

Of all transformers installed in North America, __?__ are installed as single-phase transformers to supply a 120/240 volt service.

Answer 4

90%

Question 5

When connecting to a transformer in a wye configuration, the high voltage terminals of a single-phase transformer are connected to the phase and to the __?__.

Answer 5

neutral

Question 6

It is not possible to connect a transformer with two high-voltage terminals phase-to-phase on a wye system.

Answer 6

False

Question 7

The only way to supply 240-volt, phase-to-phase to streetlights is to use a 240/480 volt transformer and tap X1 and X2.

Answer 7

False

Question 8

A transformer has 7,200 volts on the primary, 480 volts on the secondary, and 10.5 amperes are flowing in the primary. The secondary current is __?__.

Answer 8

157.5 A

Question 9

The higher voltage rating of a single-phase transformer’s secondary windings is achieved when the __?__.

Answer 9

secondary coils are in series

Question 10

In order to parallel the coils on a transformer with four secondary bushings, it is necessary to remove the lid.

Answer 10

False

Question 11

According to this lesson, transformers larger than __?__ have four secondary bushings.

Answer 11

100 kVA

Question 12

In order to parallel the coils of a single-phase transformer, the internal leads must be connected __?__.

Answer 12

AC/BD

Question 13

Two transformers rated 120/240 volts on the secondary can be connected to serve a single-phase 480-volt load.

Answer 13

True

Question 14

In a 120/240 volt service, one 120-volt leg is opposite in __?__ to the other.

Answer 14

polarity

Question 15

In a 120/240 volt residential service, the __?__ carries the current difference of the two legs.

Answer 15

neutral

Question 16

To connect the secondary coils of a single-phase transformer in series, __?__ are connected to each other inside the tank.

Answer 16

B and C

Question 17

When paralleling two single-phase transformers, both transformers must be connected to opposite primary phases.

Answer 17

False, they must be on the same phase

Question 18

If two 120/240 volt transformers are to be paralleled for a long period of time, the coils should be paralleled with each transformer feeding one 120-volt circuit.

Answer 18

True