Deck 29: Three Phase Transformers

Question 1

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

Answer 1

D

Question 2

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

Answer 2

B

Question 3

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

Answer 3

A

Question 4

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 5

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 6

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

Question 7

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

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

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 secondary phase current?

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

Question 9

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 10

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

Question 11

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

A) secondary; primary
B) primary; secondary

Question 12

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 13

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 14

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

Question 15

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 16

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

A) secondary; primary
B) primary; secondary

Question 17

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 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

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 20

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 21

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 22

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 23

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 24

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 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