Electrical Machines

Transformer MCQ

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1. In a transformer zero voltage regulation is achieved at a load power factor which is
  1. leading
  2. lagging
  3. unity
  4. zero

Answer: 1
Explanation : At leading power factor the voltage regulation can be negative or zero. This can be found from this equation % regulation = εxcosθ – εrsinθ

2. What is the magnitude of mutually induced emf, E2 in a transformer?
  1. directly proportional to rate of change of flux and number of secondary turns
  2. inversely proportional to rate of change of flux and number of secondary turns
  3. proportional to rate of change of flux and inversely proportional to number of secondary turns
  4. inversely proportional to rate of change of flux and proportional to number of secondary turns

Answer: 1
Explanation: Due to the supply voltage, a secondary induced emf is produced. V1 is in antiphase with E2 and the magnitude of E2 is proportional to rate of change of flux and number of secondary turns.

3. Which of these will happen in a transformer when the number of secondary turns is less than the number of primary turns?
  1. The voltage gets stepped up
  2. The voltage gets stepped down
  3. The power gets stepped up
  4. The power gets stepped down

Answer: 2
Explanation: According to the fundamentals of electrical engineering, the voltage transformation ratio is the ratio of the number of secondary turns to that of the number of primary turns. When the transformation ratio is less than 1, the step-down operation occurs.

4. Which of the following Transformer can have tap changer?
  1. Power transformer.
  2. Earthing transformer.
  3. Current transformer
  4. Potential transformer

Answer: 1

5. The constant losses in transformer is/are
  1. eddy current loss.
  2. hysteresis loss.
  3. copper loss.
  4. both 1 & 2

Answer: 4
Explanation: Eddy current loss and hysteresis loss are almost independent of load, significantly depending on supply voltage and frequency. As the flux density or flux is constant for a given voltage and frequency, eddy current loss and hysteresis loss remain constant at any load. Therefore, these losses are called constant losses. Copper loss varies as the square of load current and called variable loss.

6. The efficiency of two identical transformers under loaded conditions can be determined by
  1.  open circuit test.
  2. short-circuit test.
  3. back-to-back test.
  4. any of the above.

Answer: 3

7. What is the number of primary turns in a 200/1000 V transformer if the emf per turn is 10V?
  1.  5
  2. 10
  3. 20
  4. 40

Answer: 3
Explanation: According to the electrical engineering fundamentals, the number of primary turns is the ratio of induced emf in the primary to the emf induced per turn. Thus, the number of primary turns in a 200/1000 V transformer with emf per turn of 10V is 200/10 = 20.

8. What is the equivalent resistance of the primary of a transformer with a voltage transformation ratio of 10 and primary resistance of 0.05Ω when referred to secondary?
  1.  0.005
  2. 0.5
  3. 5
  4. 50

Answer: 3
Explanation: According to fundamentals of electrical engineering, the equivalent resistance of the primary of a transformer with a voltage transformation ratio of K and primary resistance of RΩ when referred to secondary is equal to K2*R. Thus, the answer will be 102*0.05 = 5.

9. In a transformer, hysteresis and eddy current losses depend upon
  1.  load current
  2. maximum flux density
  3. supply frequency
  4. both 2 and 3

Answer: 4

10. In a transformer operating at constant voltage if the input frequency increases, the core
  1.  increase
  2. decrease
  3. remains constant
  4. none of the above

Answer: 2

Transformer Overview

Electrical Machines / By

What is Transformer?

Transformer is an AC machine that transfers electrical energy from one electrical circuit to another. Since there is no moving part in Transformer, it is also called Static device. Due to its static nature it requires minimum amount of repair and maintenance.


Types of Transformer

(1) Step-up Transformer:- When Output Voltage of a  Transformer is higher than its input Voltage, it is called step up transformer. Such transformer’s are used at Power stations where generating voltage i.e. 11 KV is raised to higher levels i.e. 66/132/220/400 KV for transmission purpose to Sub-stations.

(2) Step Down Transformer:- When Output Voltage of a Transformer is lower than its input Voltage, it is called step down transformer. Such Transformer’s are used at Sub-stations to lower down voltage from 66/132/220/400 KV to 33KV and 11 KV for distribution to various Industrial Consumer’s and also Distribution Transformer’s to fed house hold consumers. 

(3) Distribution Transformer:- These type of Transformer is used to convert 11KV voltage level to 440V which is used by House-hold various commercial consumers like shops, Panchayat ghar’s etc. Distribution Transformers are mostly seen at vicinity of house-hold consumers installed over H type structure. Since it converts HT voltage to LT voltage so these are used at nearby consumer’s to eliminate voltage drops