Deck 8: Three-Phase Transformers

Question 1

If three-phase transformation is needed and a three-phase transformer of the proper size and turns ratio is not available, three single-phase transformers can be connected to form a ____ bank.

A) consolidated
B) temporary
C) three-phase
D) triple consolidated

Answer 1

C

Question 2

The primary windings of three single-phase transformers have been connected to form a wye connection. In a wye connection, the phase voltage is less than the line voltage by a factor of ____.

A) 0.577
B) 1.732
C) 3
D) 5.196

Answer 2

B

Question 3

A ____ diagram, which illustrates a delta-wye connection, is generally used to show the main power distribution system of a large industrial plant.

A) one-line
B) three-line
C) one-phase
D) three-phase

Answer 3

A

Question 4

In a delta connection, the phase voltage and line voltage are the same, so it would be appropriate to say that EP(Secondary) = EL(Secondary).

Question 5

Delta connections should only be checked for proper polarity after making the final connection and applying power.

Question 6

One of the most common three-phase to six-phase connections is the ____ connection.

A) diametrical
B) zig-zag
C) interconnected-wye
D) interconnected-delta

Question 7

Only phase values of ____ can be used when computing transformer values.

A) voltage
B) current
C) voltage and current
D) wattage

Question 8

Three single-phase transformers are connected to form a wye-delta bank. Once the voltage across each of the load resistors is known, the current flow through the phase of the load can be computed using the formula ____.

A) IP(Load) = R / EP(Load)
B) IP(Load) = 1.732 / EL(Load)
C) EP(Load) = IP(Load) / R
D) IP(Load) = EP(Load) / R

Question 9

A three-phase transformer is connected in a delta-delta configuration. The load is connected as a wye, and each phase has an impedance of 7 ohm. The primary is connected to a line voltage of 4160 volts, and the secondary line voltage is 440 volts. Therefore, the turns ratio is 9.45:1.

Question 10

A three-phase transformer is constructed by winding three single-phase transformers on a single core.

Question 11

If power is applied to the transformer bank before the delta connection is closed, the voltmeter should indicate ____ volts.

A) zero
B) at least 10
C) fewer than 50
D) about 100

Question 12

A voltmeter is a low-impedance device.

Question 13

A wye-delta transformer has its primary winding connected in a delta and its secondary winding connected in a wye.

Question 14

The open-delta transformer connection is often used when the amount of three-phase power needed is not excessive, such as in a small business.

Question 15

When three single-phase transformers are used to make a three-phase transformer bank, their primary windings are always connected in a wye connection.

Question 16

When three-phase transformers are connected as a wye or delta, the primary and secondary windings themselves become the phases of a three-phase connection. This is true whether a three-phase transformer is used or whether three single-phase transformers are employed to form a three-phase bank.

Question 17

In a three-phase transformer, the phase value of the primary voltage can be computed using the formula EP(Primary) = ____.

A) EL / 1.732
B) 1.732 / EL
C) EL x 1.732
D) 3(EL) / 1.732

Question 18

The Scott connection is used to convert three-phase power into one-phase power using three single-phase transformers.

Question 19

A connection involving the use of two transformers to supply three-phase power is the T-connection. In this connection, one transformer is generally referred to as the main transformer and the other is called the ____ transformer.

A) alternate
B) teaser
C) secondary
D) extra

Question 20

A dielectric oil provides electrical insulation between the windings and the ____.

A) three-phase bank
B) leads
C) transformer
D) case