Deck 29: Three-Phase Transformers

Question 1

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

A) secondary; primary
B) primary; secondary

Answer 1

B

Question 2

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

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

Answer 2

D

Question 3

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

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

Answer 3

A

Question 4

A wye-connected secondary has a phase voltage of 277.14 V. What is the line voltage?

A) 160 V
B) 277.14 V
C) 480 V
D) 554.28 V

Question 5

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

Question 6

What function is performed by dielectric oil?

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

Question 7

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?

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

Question 8

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?

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

Question 9

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?

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

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

Question 11

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?

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

Question 12

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

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

Question 13

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

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

Question 14

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?

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

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?

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

Question 16

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

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

Question 17

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

A) secondary; primary
B) primary; secondary

Question 18

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

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

Question 19

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 _____.

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

Question 20

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 _____.

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 21

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 _____.

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

Question 22

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

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

Question 23

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.

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

Question 24

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

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

Question 25

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

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