Question 1

A wye-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?

Accepted Answer

A)   1:20
B)   2:1
C)   10:11.55
D)   11.55:1 D

Question 2

A delta-wye transformer has its _____ connected as a delta and its _____ connected as a wye.

Accepted Answer

A)   secondary; primary
B)   primary; secondary B

Question 3

When using an open delta with a center tapped transformer, one wire is the high leg. The high leg is the one that _____.

Accepted Answer

A)   has the highest voltage
B)   has the lowest voltage
C)   has the most amperage
D)   is mounted highest on the panel A

Question 4

A wye-delta transformer has a primary line voltage of 7200 V and a secondary line voltage of 480 V. What is the turns ratio?

Accepted Answer

A)   8.66:1
B)   9: 66
C)   10:15
D)   15:1

Question 5

In an open delta with a center-tapped transformer, label the two ends of the center-tapped transformer as L2 and L3 and label the high leg as L1. The current flow in the neutral (center tap) is equal to _____.

Accepted Answer

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

Question 6

In three-phase transformer connections, the primary leads are labeled with the letter _____ and the secondary leads are labeled with the letter _____.

Accepted Answer

A)   X; H
B)   H; X
C)   L; X
D)   X; L

Question 7

The main power distribution system of a large industrial plant  is generally shown using a(n) _____ diagram.

Accepted Answer

A)   one-line
B)   wiring
C)   three-line
D)   pictorial

Question 8

A wye-delta transformer has its _____ connected as a wye and its _____ connected as a delta.

Accepted Answer

A)   secondary; primary
B)   primary; secondary

Question 9

An open delta connection is made using _____ single-phase transformers.

Accepted Answer

A)   one
B)   two
C)   three
D)   four

Question 10

When single-phase loads are tied to a three-phase transformer, the phase currents of the transformer will be automatically balanced.

Accepted Answer

A) True 
 B)False

Question 11

A delta-delta transformer has a primary voltage of 7200 V and a secondary voltage of 360 V. What is the turns ratio?

Accepted Answer

A)   11.55:1
B)   15:11.55
C)   20:1
D)   51:20

Question 12

A wye-delta transformer has a turns ratio of 2:1. It has a secondary line voltage of 480 V. The secondary is connected to a wye-connected three-phase resistive load of 50 Ω in each phase. What is the primary phase current?

Accepted Answer

A)   1.6 A
B)   1.924 A
C)   2.77 A
D)   3.2 A

Question 13

For installations that will have both three-phase and single phase loads on a three-phase transformer, the utility company often provides a(n) _____ three-phase transformer.

Accepted Answer

A)   three wire delta
B)   three wire wye
C)   open delta with one transformer center tapped

Question 14

When connecting three transformers for three-phase use with a delta secondary, a voltage test should be done before closing the secondary. The correct voltage reading is _____.

Accepted Answer

A)   the rated secondary voltage value
B)   twice the rated secondary voltage value
C)   zero volts
D)   the sum of the three transformer primaries

Question 15

A wye-delta transformer has a turns ratio of 2:1. It has a secondary line voltage of 480 V. The secondary is connected to a wye-connected three-phase resistive load of 50 Ω in each phase. What is the primary line current?

Accepted Answer

A)   1.6 A
B)   1.924 A
C)   2.77 A
D)   3.2 A

Question 16

A wye-delta transformer has a secondary phase voltage of 480 V. A delta-connected resistive load has a resistance of 12 Ω in each leg. What is the current in each load leg?

Accepted Answer

A)   30.33 A
B)   40 A
C)   53.1 A
D)   69.28 A

Question 17

A delta-wye transformer has a turns ratio of 9:1. The primary line voltage is 3600 V. What is the secondary line voltage?

Accepted Answer

A)   324.02 V
B)   692.8 V
C)   730.95 V
D)   800 V

Question 18

A wye-delta transformer has a turns ratio of 2:1. It has a secondary line voltage of 480 V. The secondary is connected to a wye-connected three-phase resistive load of 50 Ω in each phase. What is the secondary phase current?

Accepted Answer

A)   1.6 A
B)   2.54 A
C)   3.2 A
D)   4.8 A

Question 19

In an open delta with single-phase loads, the high leg must be identified by _____.

Accepted Answer

A)   using orange wire
B)   tagging
C)   both a and b
D)   neither a nor b

Question 20

What function is performed by dielectric oil?

Accepted Answer

A)   It acts as an insulator.
B)   It prevents moisture formation.
C)   It provides cooling.
D)   All of the above

Exam 29: Three-Phase Transformers