AC MACHINES (TRANSFORMER) REVIEW PROBLEMS

1. A 25 Hz, 120/30 volts, 500VA transformer is to be used on a 60Hz source. If the flux remain

unchanged, determine the maximum permissible primary voltage.

A. 230 V C. 292 V

B. 288 V D. 240

2. A transformer having a 360 turns primary winding takes 150 W in power and 1.4 A in current at an

input voltage of 230 volts. Determine the no- load power factor.

A. 0.606 C. 0.466

B. 0.866 D. 0.528

3. A transformer has a primary winding of 2000 turns and of 2400 volts and current of 8.66-j5 amp with

an impedance Z2 across the secondary winding. If the secondary winding has 500 turns, what is the

value of secondary current?

A. 34.64-j20 A C. 34.64+j20 A

B. 20-j34.64 A D. 20+j34.64 A

4. A transformer has a turn ratio of 5. If a 150 ohms resistor is connected across the primary, what is the

resistance referred to secondary?

A. 2 ohms C. 6 ohms

B. 4 ohms D. 5 ohms

5. An ideal transformer has a voltage rating of 2400/240 volts, a motor is connected to the secondary

draws 50 A at 200 volts, what resistance must be connected to maintain this voltage.

A. 0.6 C. 0.5

B. 0.3 D. 0.8

6. A transformer has primary and secondary turns of 120 and 240 respectively. If the 120 turns winding

has a ±2.5% tap, calculate the primary voltage applied to 97.5 percent of the winding that will permit

the rated output voltage of 480 volts?

A. 492 volts C. 468 volts

B. 117 volts D. 234 volt

7. A 1φ, 100 kVA, 2400/240 volt, 60 Hz transformer has a primary resistance and secondary winding

resistance of 0.828 ohms and 0.009 ohm respectively. Calculate the equivalent resistance referred to

primary.

A. 1.728 ohms C. 1.6 ohms

B. 0.837 0hms D. 1.46 ohms

8. A transformer rated 2000 kva, 20,000/240 volts has 5.75% impedance. What is the impedance

referred to primary.

A. 0.0635 C. 0.0656

B. 11.5 D. 34.2

9. A 1φ -250 kVA, 7967/240 volt, 60 Hz transformer has 1.5% resistance. What is the full-load copper

loss?

A. 626.25 watts C. 2505 watts

B. 1252.5 watts D. 3750 watt

10. The following information is given in connection with test performed on a 50 kva, 4600/230 volts 60 Hz.

Transformer:

Short circuit test: Esc= 150 volts Psc= 615 watts

What is the percent resistance & percent impedance?

A. 1.26 & 3.62 C. 1.16 & 3.58

B. 1.23 & 3.26 D. 1.32 & 5.8

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A 25 Hz, 120/30 volts, 500VA transformer is to be used on a 60Hz source. If the flux remain unchanged, determine the maximum permissible primary voltage. A. 230 V C. 292 V B. 288 V D. 240
A transformer having a 360 turns primary winding takes 150 W in power and 1.4 A in current at an input voltage of 230 volts. Determine the no- load power factor. A. 0.606 C. 0. B. 0.866 D. 0.
A transformer has a primary winding of 2000 turns and of 2400 volts and current of 8.66-j5 amp with an impedance Z2 across the secondary winding. If the secondary winding has 500 turns, what is the value of secondary current? A. 34.64-j20 A C. 34.64+j20 A B. 20-j34.64 A D. 20+j34.64 A
A transformer has a turn ratio of 5. If a 150 ohms resistor is connected across the primary, what is the resistance referred to secondary? A. 2 ohms C. 6 ohms B. 4 ohms D. 5 ohms
An ideal transformer has a voltage rating of 2400/240 volts, a motor is connected to the secondary draws 50 A at 200 volts, what resistance must be connected to maintain this voltage. A. 0.6 C. 0. B. 0.3 D. 0.
A transformer has primary and secondary turns of 120 and 240 respectively. If the 120 turns winding has a ±2.5% tap, calculate the primary voltage applied to 97.5 percent of the winding that will permit the rated output voltage of 480 volts? A. 492 volts C. 468 volts B. 117 volts D. 234 volt
A 1φ, 100 kVA, 2400/240 volt, 60 Hz transformer has a primary resistance and secondary winding resistance of 0.828 ohms and 0.009 ohm respectively. Calculate the equivalent resistance referred to primary. A. 1.728 ohms C. 1.6 ohms B. 0.837 0hms D. 1.46 ohms
A transformer rated 2000 kva, 20,000/240 volts has 5.75% impedance. What is the impedance referred to primary. A. 0.0635 C. 0. B. 11.5 D. 34.
A 1φ -250 kVA, 7967/240 volt, 60 Hz transformer has 1.5% resistance. What is the full-load copper loss? A. 626.25 watts C. 2505 watts

B. 1252.5 watts D. 3750 watt

The following information is given in connection with test performed on a 50 kva, 4600/230 volts 60 Hz. Transformer: Short circuit test: Esc= 150 volts Psc= 615 watts What is the percent resistance & percent impedance? A. 1.26 & 3.62 C. 1.16 & 3. B. 1.23 & 3.26 D. 1.32 & 5.

The full load Cu loss of a 75kVA single phase transformer is thrice the iron loss. At half load unity p.f. the efficiency is 97%. Determine the full load efficiency at 0.8 p.f. A. 95.77% C. 96.55% B. 94.55% D. 97.77%
A certain transformer has a core loss of 100 watts at 30 Hz core loss of 300 watts at 60 Hz. What is the hysteresis loss at 60 Hz? A. 100 C. 200 B. 150 D. 250
Two (2)-100 kVA, 1φ transformers are connected open-delta, calculate the maximum kVA of a 3- phase balanced load without overloading any of the transformers. A. 200 C. 300 B. 150 D. 173
Three single phase, 10 kva transformer connected delta-delta supply a balanced 3- phase wye- connected load of 26 kva. One transformer is removes for repair. Find the kva load of each remaining transformers. A. 15 C. 30 B. 25 D. 35
A 750 hp, 2400 V, 3- phase motor of 90 percent power factor and 95 percent efficiency is to be operated from a 13.2 kV utility service through a wye-delta connected transformer. What is the KVA rating of each transformers? A. 218 C. 175 B. 250 D. 150
The eddy current loss in a 2300 volts 60Hz transformer is 280 watts. What is the eddy current loss at 2400 volts 50 Hz source? A. 280 watts C. 305 watts B. 300 watts D. 275 watts
The hysteresis loss in a 6,000 volts, 60 Hz transformer is 480 watts. What will be the loss when the tansformer is connected to 6,900 volts, 40 Hz source? A. 558 watts C. 520 watts B. 657 watts D. 417 watts
Energy is transferred from the primary to the secondary coils without a change in _________ a. current b. voltage c. frequency d. ampere – turns
Transformer is associated with ________________current? a. Direct b. Alternating c. Neither a or b d. Either a or b
The rating of transformer is expressed in. a. Ampere-hours b. watts c. kilowatt-hours d. volt-amperes
A transformer Core is made of? I. Copper II. Steel III. Aluminum a. I only b. II only c. III only d. I , II or III
When the primary winding has more turns than the secondary, the voltage in the secondary winding is … a. increased b. doubled c. decreased d. halved

b. outdoor- and indoor cooled d. dry and liquid cooled

Insulation of transformers may be classified in two groups: a. double-wound and autotransformers c. core and shell types b. dry and oil filled types d. core and cross types
Parallel operation of single-phase transformers can be accomplished when the a. voltage and percentage impedance ratings are identical b. voltage and current ratings are equal c. cooling methods are identical d. primary and secondary voltage ratings are equal
A transformer has subtractive polarity when the a. two primary coil voltages oppose each other c. X 1 lead is opposite the H 1 lead b. two secondary coils have opposite polarities d. X 2 lead is opposite the H 1 lead
A transformer has additive polarity when the a. two primary coils are in series c. X 1 lead is opposite the H 1 lead b. two secondary coils have aiding polarities d. X 2 lead is opposite the H 1 lead
Potentail transformer secondaries are grounded to a. stabilize meter readings b. ensure readings with an accuracy of 0.5 percent c. complete a system with the primaries d. eliminate high voltage hazards

AC MACHINES (TRANSFORMER) REVIEW PROBLEMS, Quizzes of Electrical Engineering

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AC MACHINES (TRANSFORMER) REVIEW PROBLEMS

B. 1252.5 watts D. 3750 watt