Deck 9: Single-Phase Loads for Three-Phase Transformers

Question 1

When a transformer bank must supply both three-phase and single-phase loads, the utility company will often provide an open-delta connection with one transformer ____.

A) center-tapped
B) double-tapped
C) tapped
D) double-loaded

Answer 1

A

Question 2

High-leg voltage can be computed by increasing the single-phase center-tapped voltage value by ____.

A) 1.732
B) 3
C) 5.196
D) 9

Answer 2

A

Question 3

Connecting the center tap of a primary to the center tap of a secondary will solve an unbalance problem if a wye primary is used on a three-wire system.

Answer 3

False

Question 4

The type of three-phase transformer connection to use is generally determined by the ____.

A) amount of power available
B) amount of power needed
C) transformer manufacturer's specifications
D) none of the above

Question 5

Typical voltages for a three-phase transformer connection with a delta-connected primary and wye-connected secondary are 208/120 and ____.

A) 104/180
B) 416/240
C) 440/220
D) 480/277

Question 6

In a given three-phase transformer connection with a delta-connected primary and wye-connected secondary, the secondary has been center-tapped to form a neutral conductor. The neutral conductor will ____.

A) carry the vector sum of the balanced current
B) not carry the vector sum of the balanced current
C) not carry the vector sum of the unbalanced current
D) carry the vector sum of the unbalanced current

Question 7

Three-phase transformer connections are often used to supply single-phase loads, which tend to unbalance the system.

Question 8

In a delta connection where one transformer is center-tapped to form a neutral conductor, the two lines are 1800 out of phase when compared with the center tap. In the wye connection, the lines will be ____ out of phase.

A) 600
B) 900
C) 1200
D) 1800

Question 9

In a three-phase transformer connection with a delta-connected primary and wye-connected secondary, the neutral conductor will not carry the vector sum of the unbalanced current.

Question 10

If incoming power is a three-phase four-wire system, a wye-connected primary can be used without problems.

Question 11

The amount of current flow in a neutral conductor will be the sum of the unbalanced load between lines L2 and L3, with the addition of the current flow in the high leg, L1. To determine the amount of neutral current, use the formula ____.

A) AN = A1 + (A2 x A3)
B) AN = A1(A2 - A3)
C) AN = A1 + (A2 - A3)
D) AN = A1(A2 + A3)

Question 12

In a three-phase transformer connection with a delta-connected primary and wye-connected secondary, a wye-connected secondary with center tap can, under the right conditions, experience extreme unbalance problems.

Question 13

In a given transformer bank, if the three loads have been connected directly across the three-phase lines, they form a delta-connected load.

Question 14

In a given circuit, the three-phase load has a power requirement of 75 kVA, and the single-phase load requires an additional 50 kVA. Three 25-kVA transformers could be used to supply the three-phase power needed. The addition of the single-phase load, however, requires ____.

A) one of the transformers to be larger
B) two of the transformers to be larger
C) all three of the transformers to be larger
D) a fourth transformer

Question 15

It is possible to use a high leg to supply power to a single-phase load.

Question 16

Assume line L1 has a current flow of 100 amps, line L2 has a current flow of 75 amps, and line L3 has a current flow of 50 amps. The amount of current flow in the neutral conductor would be ____ amps.

A) 25
B) 50
C) 75
D) 125

Question 17

In many circuits, it is possible for the neutral conductor to carry more current than any of the three-phase lines.

Question 18

When true three-phase loads are connected to a three-phase transformer bank, there are problems in balancing the currents and voltages of the individual phases.

Question 19

In a given transformer bank, the three loads form a delta-connected load. If these three loads are intended to be used as single-phase loads, they will in all likelihood have ____.

A) a low leg
B) changing resistance values
C) unchanging vector relationships
D) constant resistance values

Question 20

When a line is known as a high leg, it is because the voltage between this line and the neutral conductor will be higher than the voltage between the neutral and either of the other two conductors.