POWER TRANSFORMERS Step up vs. Step down transformer – what’s the difference?

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Step-up and step-down transformers are used in generators, transmission lines, power systems, distribution networks, microwaves, electronic devices, isolators, and many more applications. The article explains step-up and step-down transformers in brief and lists the differences between them.

A power transformer is a static electrical device that works on the principle of electromagnetic induction to convert input AC voltage from one level to another at the output. The power transformer can either increase or decrease the input AC voltage level, without changing the frequency. However, it should be noted that the transformer increases or decreases only AC voltage. Unlike a rectifier, a transformer does not convert AC voltage to DC voltage.

Transformers have primary and secondary windings, each with a different (or same) number of turns to determine the operation. Both windings are embedded in the core but are electrically insulated from each other. The primary winding is at the input and the secondary winding is at the output of the transformer. The primary winding has N1 or Np number of turns and the secondary winding has N2 or Ns number of turns.

There are many types of transformers but two main categories based on the operation, output voltage ratings, and number of turns of windings are- step-up and step-down transformers. Both transformer types are used in a variety of applications such as transmission lines, industries, power electronics, etc.

1. What is a step-up transformer?

A step-up transformer is an electrical device that “steps up” the input AC voltage. As the name suggests, a step-up transformer converts the input low voltage (LV) and high current (HC) AC signal to the high voltage (HV) and low current (LC) AC signal on the output side.

The number of turns “N1” of the primary coil in a step-up transformer is lesser than the number of turns “N2” of the secondary winding.

N1 < N2

The emf-induced e1 in the primary winding is less than the emf-induced e2 in the secondary winding.

e1 < e2

An input AC voltage is fed to the step-up transformer. According to the law of Faraday’s electromagnetic induction, alternating current produces a varying magnetic field that induces an emf in the windings.

The output voltage directly depends upon the winding ratio-

N2 / N1 = Vo / Vi

Vo = Vi. N2 / N1

As a result of variation in the number of winding turns, a higher AC voltage is seen at the output of the step-up transformer.

Vi < Vo

The transformer ratio (K) must be greater than 1 to design a step-up transformer.

N2 / N1 = e2 / e1 = Vo / Vi > 1

K > 1

2. What does a step-up transformer do?

A step-up transformer is used at the input stage of the transmission line to step-up the input voltage. It enables the transmission line to carry high running voltage. Step-up transformer provides a higher output voltage than the input voltage. The primary voltage ratings in a step-up transformer are 220 V, 415 V, 11 kV, etc. The secondary voltage ratings include 33 kV, 66 kV, 220 kV, 400 kV, etc.

For example, the generating station voltage of 11 kV is stepped-up to 220 kV or 380 kV for effective transmission. The transmission line carries a high voltage to prevent any line losses with increased efficiency. However, each country has different values for generator station voltages and transmission lines.

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3. What is a step-down transformer?

A step-down transformer is an electrical device that “steps down” the input AC voltage. In simple words, a step-down transformer converts the high voltage (HV) and low current (LC) input AC signal to the low voltage (LV) and high current (HC) AC signal on the output side.

The number of turns “N1” of the primary coil in a step-down transformer is more than the number of turns “N2” of the secondary winding.

N1 > N2

The emf-induced e1 in the primary winding is more than the emf-induced e2 in the secondary winding.

e1 > e2

An input AC voltage is fed to the step-down transformer. According to the law of Faraday’s electromagnetic induction, alternating current produces a varying magnetic field that induces an emf in the windings.

The output voltage directly depends upon the winding ratio-

N2 / N1 = Vo / Vi

Vo = Vi. N2 / N1

As a result of variation in the number of winding turns, a lower AC voltage is seen at the output of the step-down transformer.

Vi > Vo

To design a step-down transformer, the transformer ratio (K) must be less than 1.

N2 / N1 = e2 / e1 = Vo / Vi < 1

K < 1

4. What does a step-down transformer do?

A step-down transformer provides a lower output voltage than the input voltage. The device is used at the output stage of the transmission line to step-down the input voltage. The high voltage from the transmission line passes through the step-down to safely reach homes and commercial areas.

The primary voltage ratings in a step-down transformer are 33 kV, 66 kV, 132 kV, 220 kV, 400 kV, etc. The secondary voltage ratings include 220 V, 415 V, 11 kV, etc. However, each country has different values for generator station voltages and transmission lines. For example, the transmission line voltage of 220 kV or 380 kV is stepped down to 33 kV or 11 kV or various other required values for safe operation.

5. The difference between a step-up transformer and a step-down transformer at a glance

6. References

• https://www.electrical4u.com/step-up-transformer/

• https://www.electrical4u.com/step-down-transformers/

• https://www.britannica.com/technology/transformer-electronics

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