Transformer Notes for SSC JE Electrical Engineering

A transformer is a static device that changes the voltage of alternating current from one level to another without changing the frequency. It is one of the most important chapters of Electrical Machines in the SSC JE paper. These transformer notes for SSC JE explain the working, the EMF equation, the losses, the efficiency, the tests and the accessories in simple language with solved examples and practice questions.

Working Principle

• A transformer works on the principle of mutual induction (Faraday's law of electromagnetic induction).
• It has two windings, the primary and the secondary, on a common magnetic core.
• It works only on AC, and it cannot work on DC.
• It changes voltage and current, but not the frequency and (ideally) not the power.

Types of Transformers

Transformation Ratio

Example 1. A transformer has 1000 turns in the primary and works from 2000 V to 200 V. Find the number of turns in the secondary.
N2 = N1 × V2 / V1 = 1000 × 200 / 2000 = 100 turns.

EMF Equation

The RMS EMF induced in a winding is E = 4.44 f N Φm, where f is the frequency, N is the number of turns and Φm is the maximum flux in the core.

Example 2. f = 50 Hz, N = 200 turns and Φm = 0.02 Wb. Find E.
E = 4.44 × 50 × 200 × 0.02 = 888 V.

Losses in a Transformer

• Eddy current loss is reduced by using thin laminations in the core.
• Hysteresis loss is reduced by using silicon steel for the core.
• Transformer has no rotating part, so it has no mechanical losses.

Efficiency and Regulation

Example 3. A transformer delivers an output of 10 kW. Its iron loss is 200 W and copper loss is 300 W. Find the efficiency.
η = 10,000 / (10,000 + 200 + 300) = 10,000 / 10,500 = 95.24 per cent.

Tests on a Transformer

Autotransformer and Instrument Transformers

• Autotransformer: has only one winding, which is common to the primary and the secondary; it saves copper, is smaller and is more efficient, but has no electrical isolation.
• Current transformer (CT): measures high currents; its secondary should never be left open.
• Potential transformer (PT): measures high voltages; its secondary should never be short-circuited.

Accessories

The rating of a transformer is given in kVA, and not in kW, because the copper loss depends on the current and the iron loss depends on the voltage, and neither depends on the power factor.

Quick Tips

• Remember that the frequency does not change in a transformer.
• Learn that maximum efficiency occurs when the copper loss equals the iron loss.
• Remember the tests: the OC test on the LV side gives iron loss, and the SC test on the HV side gives copper loss.
• Remember that a CT secondary must never be opened and a PT secondary must never be shorted.

Quick Revision Points

• A transformer works on mutual induction and only on AC.
• E = 4.44 f N Φm.
• Transformation ratio K = N2 / N1 = V2 / V1 = I1 / I2.
• Maximum efficiency occurs when copper loss = iron loss.
• The rating of a transformer is in kVA.

Practice Questions With Answers

1. On which principle does a transformer work?
   Answer: Mutual induction
2. What is the EMF equation of a transformer?
   Answer: E = 4.44 f N Φm
3. What is the condition for maximum efficiency?
   Answer: Copper loss = iron loss
4. Which test gives the iron loss of a transformer?
   Answer: Open circuit test
5. Which test gives the copper loss of a transformer?
   Answer: Short circuit test
6. Why is a transformer rated in kVA?
   Answer: Because the losses do not depend on the power factor
7. What is used to reduce eddy current loss?
   Answer: Thin laminations of the core
8. Which relay protects a transformer from internal faults?
   Answer: Buchholz relay
9. What should never be done to the secondary of a CT?
   Answer: It should never be open-circuited
10. A transformer has 500 primary turns and 50 secondary turns. If the primary voltage is 2200 V, find the secondary voltage.
    Answer: 220 V

How to Practise This Chapter

Solve one EMF numerical, one efficiency numerical and one turns-ratio numerical daily. Then solve questions from our practice papers, the free SSC quizzes and the previous year papers page. Read the previous chapter, Network Theorems, and see all topics on the SSC JE Electrical Engineering notes page. For other sections, read the Reasoning notes and General Awareness notes.

Frequently Asked Questions

What is a transformer?

A transformer is a static device that changes the level of AC voltage from one value to another without changing the frequency.

On which principle does a transformer work?

It works on the principle of mutual induction between the primary and the secondary windings.

Why can a transformer not work on DC?

A DC supply gives a constant flux, so no EMF is induced in the secondary. EMF is induced only when the flux changes, as in AC.

What is the EMF equation of a transformer?

The RMS EMF is 4.44 times the frequency, times the number of turns, times the maximum flux.

What are the losses in a transformer?

The losses are iron loss (hysteresis and eddy current) and copper loss in the windings.

When is the efficiency of a transformer maximum?

The efficiency is maximum when the copper loss is equal to the iron loss.

What is voltage regulation?

Voltage regulation is the change in the secondary voltage from no-load to full-load, expressed as a percentage of the no-load voltage.

What do the OC and SC tests give?

The open circuit test gives the iron loss, and the short circuit test gives the copper loss and the equivalent impedance.

What is an autotransformer?

An autotransformer has a single winding which is common to the primary and the secondary. It is smaller and cheaper but has no isolation.

What is the function of a Buchholz relay?

A Buchholz relay detects the gas produced by an internal fault in an oil-filled transformer and gives an alarm or trips the supply.

Why is a transformer rated in kVA?

Because its losses depend on the voltage and the current, and not on the power factor of the load.

How many questions come from transformers in SSC JE?

Usually two to four questions come from transformers, but the number changes from year to year.