Three-Phase Transformers

Two single-phase transformers having a capacity of 50 kVA each are connected to form an open delta connection. What is the total kVA capacity of this connection?

86.6 kVA

A delta-wye transformer has a turns ration of 9:1. The primary line voltage is 3600 V. What is the secondary line voltage? a. 32,400 V b. 692.8 V c. 230.95 V d. 400 V

b

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

b

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. 1:11.55 c. 20:1 d. 1:20

c

A wye-delta transformer has a turns ration 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. How much is in the primary phase current? a. 9.6 A b. 5.54 A c. 3.2 A d. 4.8 A

c

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

False

What is the high leg identified by?

Orange Tape

A wye-delta transformer has a turns ration 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. How much is the primary phase current? a. 1.6 A b. 0.924 A c. 2.77 A d. 3.2 A

a

If a single three-phase transformer of the proper ratings is not available, three single-phase transformers can be connected for the job. When this is done, the connection is called a __ a. three-phase bank b. wye-delta bank c. wye bank d. delta bank

a

A wye-delta transformer has its __ connected as a wye and its __ connected as a delta a. secondary, primary b. primary, secondary

b

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

c

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. 20:1 c. 1:11.55 d. 11.55:1

d

A wye-delta transformer has a turns ration 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. How much current flow is there in each resistor? a. 9.6 A b. 4.8 A c. 2.77 A d. 5.54 A

d

An open delta transformer is assembled from two 150 kVA transformers. The output power of the open delta is? a. 300 kVA b. 173.1 kVA c. 150 kVA d. 259.8 kVA

d

Dielectric oil performs what function? a. acts as an insulator b. prevents moisture formation c. provides cooling d. all the above

d

Three single-phase transformers have a rating of 60 kVA each. If the transformers are connected to form a three-phase bank, what will be the kVA rating of the bank?

180 kVA

A simplified schematic diagram of three-phase transformer connections is called a __ diagram a. one-line b. wiring c. three-line d. pictorial

a

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. 1:8.66 c. 1:15 d. 15:1

a

A wye-delta transformer has a turns ration 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. How much is the primary line current? a. 1.6 A b. 0.924 A c. 2.77 A d. 3.2 A

a

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 measured to ground b. has the lowest voltage measured to ground c. has the most amperage d. is mounted highest on the panel

a

A delta-wye transformer has its __ connected as a delta and its __ connected as its wye a. secondary, primary b. primary, secondary

b

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. 13.33 A b. 40 A c. 23.1 A d. 69.28 A

b

An open delta connection is made using __ single-phase transformers a. one b. two c. three d. four

b

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. 50 kVA d. 57.7 kVA

b

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

c

For installations that will have both three-phase and single-phase loads on a three-phase transformer, the utility company often provides a __ three-phase transformer a. three wire delta b. three wire wye c. open delta with one transformer center tapped

c