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

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 3

An open delta transformer is assembled from two 150 kVA transformers. The output power of the open delta is _____.

Accepted Answer

A)   20 kVA
B)   113.1 kVA
C)   150 kVA
D)   259.8 kVA D

Question 4

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 5

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 6

An open delta transformer is assembled from two 50 kVA transformers. The output power of the open delta is _____.

Accepted Answer

A)   100 kVA
B)   86.6 kVA
C)   150 kVA
D)   157.7 kVA

Question 7

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 _____.

Accepted Answer

A)   three-phase bank
B)   wye-delta bank
C)   wye bank
D)   delta bank

Question 8

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 9

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 10

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 11

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

Accepted Answer

A)   secondary; primary
B)   primary; secondary

Question 12

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 13

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

Question 14

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 15

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 16

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 17

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

Accepted Answer

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

Question 18

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 19

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 20

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 current flow is in each load resistor?

Accepted Answer

A)   1.6 A
B)   2.8 A
C)   4.77 A
D)   5.54 A

Exam 29: Three Phase Transformers