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Electrical Transformer MCQs

Electrical Transformer MCQs

A transformer is electrical that finds its application in modern power systems. It is used for stepping up and stepping down ac electric current from one voltage level to another. Besides voltage stepping, transformers are employed for a variety of applications such as voltage sampling, current sampling, and impedance transformation. Given below are top 10 MCQs on Electrical Transformer.

A transformer works on

A transformer works on:

  1. AC only
  2. DC only
  3. Both of these
  4. None of these

Correct answer: 1. AC only

Explanation: The transformer is an electrical device that changes AC electric power at one voltage level to ac electric power at another voltage level by the action of magnetic field. A transformer comprises of two or more wire coils that are wrapped around common ferromagnetic core.

The primary and secondary of a transformer are coupled

The primary and secondary of a transformer are coupled:

  1. Magnetically
  2. Electrically
  3. Both of these
  4. None of these

Correct answer: 1. Magnetically

Explanation: The electrical transformer is a device that changes AC electric power at one voltage level to AC electric power at another voltage level through the action of a magnetic fie ld. Its basic construction involves two or more coils of wire wrapped around a common ferromagnetic core. For power transformers these coils are (usually) not directly connected. The only connection between the coils is the common magnetic flux that is present within the core.

The maximum flux produced in core of transformer is

The maximum flux produced in core of transformer is:

  1. Inversely proportional to primary voltage
  2. Inversely proportional to supply frequency
  3. Directly proportional to supply frequency
  4. None of these

Correct answer: 2. Inversely proportional to supply frequency

Explanation:

Transformer voltage equation is mathematically written in the form:

V1 = 4.44 * N1 * Φmax * f

Or Alternatively:

Φmax = V1/(4.44 * N1 * f)

Where

V1 = Applied primary voltage

N1 = Number of primary turns

From equation one can understand that the maximum flux produced in core is inversely proportional to supply frequency.

Design Engineers design transformer so that primary and secondary have

Design Engineers design transformer so that primary and secondary have:

  1. Large resistance
  2. Tight magnetic coupling
  3. High leakage reactance
  4. None of above

Correct answer: 2. Tight magnetic coupling

An ideal transformer is the one that has

An ideal transformer is the one that has:

  1. Same number of primary and secondary turns
  2. Same voltage per turns on primary and secondary side
  3. Both 1 & 2
  4. No losses and leakage resistance

Correct answer: 4. No losses and leakage resistance

Explanation:

An ideal transformer possesses following features:

  1. It has no leakage flux
  2. It has no core losses
  3. It has no winding resistance

However, the real-life transformer has some losses, possess some windage resistance and has some leakage reactance as well.

The transformer whose secondary winding should always kept closed

The transformer whose secondary winding should always kept closed:

  1. Current transformer
  2. Potential transformer
  3. Power transformer
  4. Step down transformer

Correct answer: 1. Current transformer

Explanation:

Current Transformers have a very small turns ratio.

In essence a 1200:5 CT has a turns ratio of 1/240, that means the voltage across the secondary windings would be increased tremendously. For the present case it will be 240 times, if there is no load in the circuit.

Therefore, it is vital to assure that the secondary circuit of a CT never becomes open circuited.  Orelse the dangerously high voltage built up across the open-circuited secondary circuit could easily cause an arc and explosion.

To overcome the situation of dangerously high voltages from building up, a low-impedance load must be designed into the secondary circuit, putting a burden on the transformer.

Whenever a CT is not needed, its secondary windings must be short circuited and grounded in order to ensure safety.

Breather is provided in a transformer to

Breather is provided in a transformer to:

  1. Absorb moisture of air during breathing
  2. Provide cold air in the transformer
  3. Adjust mechanical frequency of waves
  4. None of these

Correct answer: 1. Absorb moisture of air during breathing

Explanation: The transformer breather is an accessory of liquid-immersed power transformers attached to the conservator tank of transformer.

The breather comprises silica gel crystals: It is good to note that silica gel crystals have a tremendous capacity of absorbing moisture. As air passes through these crystals in the breather, the moisture in air is absorbed by the silica gel crystals. As a result of this air reaching the conservator is quite dry.

The voltage regulation of a transformer is negative when its load power factor is

The voltage regulation of a transformer is negative when its load power factor is

  1. Unity
  2. Lagging
  3. Leading
  4. Zero

Correct answer: 3. Leading

Explanation:

Voltage regulation is defined as the change in secondary terminal voltage from no load to full load at a specific power factor of load. Voltage regulation is usually expressed in form of percentage.

For a given transformer if:

E2: No-load secondary voltage & V2 = Full load secondary voltage

The voltage regulation = (E2 – V2)/E2

For leading power factor, if magnitude of phase angle is high, V2 becomes greater than E2 thus making a negative voltage regulation.

The temperature rise of transformer is directly proportional to

The temperature rise of transformer is directly proportional to:

  1. Apparent power
  2. Leakage reactance
  3. Reactive power
  4. All of these
  5. None of these

Correct answer: 1. Apparent power

Explanation: Two types of losses occurring in a transformer are:

  1. Iron losses: Depend upon voltage
  2. Copper losses: Depend upon current

The losses create heat thus leading towards temperature rise. Since iron losses depend upon voltage and copper losses depend upon current, the total temperature rise depend upon volt-ampere product only.

Eddy current losses in transformer are directly proportional to

Eddy current losses in transformer are directly proportional to:

  1. Supply frequency
  2. Thickness of core lamination
  3. Square of thickness of core lamination
  4. Flex density in the core

Correct answer: 3. Square of thickness of core lamination

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