Important Multiple choice Question Of Transformer 2022

Answer.4. Frame size

Explanation:-

Minimum Nameplate Requirement

Every distribution and power transformer has a metal nameplate attached to the tank that gives vital information on how the transformer is to be connected and operated.

The minimum information to be shown on a transformer nameplate depends on the KVA rating of the transformer as specified in the standards. The standards require the following information for transformers rated above 500 KVA

• Name of manufacturer
• Serial number
• Month/Year of manufacture
• Insulation class (Cooling class)
• Number of phases
• Frequency
• KVA or MVA rating
• Voltage ratings
• Tap voltages
• Rated temperature rise, °C
• Polarity (single-phase transformers)
• Phasor or vector diagram (polyphase transformers  or Three Phase Transformers)
• Percent impedance
• Basic lightning impulse insulation levels
• Connection diagram
• Type of insulating liquid
• Duty

Answer.2. Auto transformer

Explanation:-

An autotransformer is a type of transformer that uses a single tapped winding rather than the two separate and electrically isolated windings used by mutual transformers.

Because autotransformers don’t have separate windings, unlike mutual transformers there is no electrical isolation between the primary and secondary circuits.

Where electrical isolation between the primary and secondary windings is unimportant, the use of an autotransformer has a number of advantages over a mutual transformer.

• With only one winding, the volume of copper required in its manufacture can be lower than for a mutual transformer.
• There are no secondary copper losses, so autotransformers are more efficient than a corresponding mutual transformer.

Answer.4. All of the above

Explanation:-

In case, where electrical isolation is important for safety reasons, then an autotransformer simply cannot be used.

Disadvantages of auto-transformer

• An autotransformer does not provide electrical isolation between its windings as an ordinary transformer does. Hence an autotransformer can not safely be used for stepping down higher voltages too much lower voltages suitable for smaller loads.
• A failure of the insulation of the windings of an autotransformer can result in full input voltage applied to the output.
• If there is a break in the part of the winding, then the transformer acts as an inductor in series with the load.
• Since the neutral connection is common to both the primary and secondary windings, earthing of the secondary winding automatically Earth’s the primary as there is no isolation between the two windings. Double wound transformers are sometimes used to isolate equipment from the earth.

Answer.2. Isolation transformer

Explanation:-

An “isolation transformer” neither steps the voltage up or down, but it simply provides electrical isolation between primary and secondary windings.

Isolation Transformers are used to isolate electronic equipment from 240-volts AC, 50-hertz power while it is being tested. The reason for using a transformer is to prevent shocks. Without the transformer, one side of the power source is connected to the chassis. When the chassis is removed from the cabinet, the “hot”.

Answer.4. All of the above

Explanation

Application of Autotransformer

1. Autotransformer is used as a variac (variable a.c) in the laboratory and other situations that require continuously variable voltage over a broad range.
2. It can be used as a regulating transformer.
3. For giving a small boost to distribution cable to correct voltage drop.
4. For safely starting the machines like an induction motor, synchronous motor i.e., as a starter.

Answer.4. Step down transformer

Explanation:-

• Transformers used in conjunction with measuring instruments for measurement purposes are called instrument transformers. The actual measurements are done by the measuring instruments only.
• The potential transformer is also called a voltage transformer and is a step-down transformer whose secondary is rated at 110 V. The main difference between the voltage transformer and the power transformer is that in the former, the secondary current is of the same order as that of the magnetizing current of the transformer.
• The primary is connected across the high voltage supply and the secondary to the voltmeter or potential coil of the wattmeter.
• Since the voltmeter (or potential coil) impedance is very high, the secondary current is very small and the potential transformer behaves as an ordinary two winding transformer operating on no-load.

Answer.1. Transformer insulation

Explanation:-

The objective of transformer maintenance is to safeguard against breakdowns by detecting potential causes and eliminating them. The transformer undergoes regular maintenance to check the Transformer insulation i.e. oil, dielectric strength, etc.

Transformers are a vital link in the electrical distribution system and should be given proper care and attention. Transformer maintenance schedules should be determined according to the critical or non-critical nature of the transformer and the load that is connected to it.

Transformer Inspection and Maintenance

General inspection

Insulating liquid Test

Solid Insulting winding test

Answer.3. Oil cooling

The coolants used in the transformer are:

1. Air Cooling
2. Oil Cooling

The dry-type transformer uses air as a coolant, whereas the oil-immersed transformer uses oil as a coolant.

The heat produced in dry-type transformers is conducted across the core and windings. Finally, the heat dissipates from the outer surfaces of the windings to the surrounding air through convection.

The cooling methods used in transformers up to 25 kVA size and of dry-type are discussed below:

Natural air: Ambient air is used as the cooling medium in this method, and the natural circulation of the surrounding air is used to carry away the generated heat by natural convection. They are used for transformers having low voltage and output ratings.

Air blast: Natural circulation of air used in cooling becomes insufficient for dissipation of heat from large transformers. Therefore, the circulation of air (air blast) is used to keep the temperature rise within prescribed limits. A continuous blast of cool air is forced through the cores and the windings to cool the transformer in this method. External fans are used to produce air blasts. They are used for machines with ratings higher than 33 kV and 100 MVA.

Oil cooled:- Generally most of the transformers are of oil-immersed types because oil provides better insulation than air due to its better conduction heat. Mineral oil is used for this purpose. The heat generated inside the core and windings of an oil-immersed transformer is conducted across them to their surfaces. This heat produced is transferred by the oil to the walls of the tank through convection. They are used for  large-sized transformers with ratings up to 132 kV and 100 MVA

Answer.2. 80%

Explanation:-

Given

Primary voltage =200 V

Secondary voltage = 250 V

Transformation volatge K = V₂/V₁

K = 200/250 = 0.8

Economy of copper = K = 0.8 = 80%

Answer.3. 2

There are two common transformer structures in use today.

1. Core type transformer
2. Shell type transformer

Core type:- When the magnetic circuit is encircled by two or more windings of the primary and secondary, the transformer is referred to as a core-type transformer. A core-type transformer is one in which there is only one iron path, usually in rectangular form, and the windings are wound on two opposite limbs. The core-type is not often used in small-size transformers as its shape makes mounting difficult, but it offers advantages, considering the insulation requirement when very high voltages have to be generated on small transformers.

Shell type:- When the primary and secondary windings are encircled by the magnetic material, the transformer is referred to as a hell-type transformer. In shell-type construction, there are two parallel magnetic paths into which the flux from the central limb can divide. The primary and secondary windings are wound on the central limb one above the other. This provides a better magnetic coupling between the primary and secondary windings.

Shell-type transformer construction features a short magnetic path and a longer mean length of the electrical turn. The shell-form transformer has a larger core area and a smaller number of winding turns than the core-form of the same output and performance. Additionally, the shell form generally has a larger ratio of steel to copper than an equivalently rated core-form transformer.