Depending on the number of turns in primary and secondary windings, the transformers may be classified into the following three types −

• Step-up transformer

• Step-down transformer

• One-to-one transformer

Depending on the application, we may classify the transformers in the following three main types

• Power transformers

• Distribution transformers

• Instrument transformers

Step-Up Transformer

A transformer in which the number of turns in the secondary winding are greater than the number of turns in the primary winding, as a result its output voltage is greater than the input voltage is known as a step-up transformer. In power systems, the step-up transformer is used to increase the low voltages to a higher value for transmission purposes.

Step-Down Transformer

A transformer in which the number of turns in the secondary winding are less than that in the primary winding as a result the output voltage is less than the input voltage is known as a step-down transformer. In power systems, the step-down transformers are used to decrease the high-voltages to a lower value for distribution and utilization purposes.

One-to-One (1:1) Transformer

A transformer in which the number of turns in both primary winding and secondary winding are same so that it produces an output voltage equal to the input voltage is known as a one-to-one transformer. It is also known as isolation transformer. It finds application in such areas where two electrical circuits are required to be isolated electrically, but coupled magnetically for power transfer.

Power Transformers

A transformer with high volt-ampere (VA) rating, commonly of the order of Mega or Giga, is referred as a power transformer. The power transformers are designed to operate with an almost constant load which is equal to their rating. These transformers are used in generating stations, receiving stations and substations at ends of the power transmission lines for stepping-up or stepping-down the voltage.

In practice, power transformers are put into operation during load periods while they are disconnected during light load periods. These transformers are designed to have maximum efficiency at or near full-load. However, power transformers are so designed that they have considerably high leakage reactance. Thus, for power transformers the current limiting effect of high leakage reactance is more important than the voltage regulation.

Distribution Transformers

The transformer which is used to reduce the high voltage to a low value for distribution purpose is known as a distribution transformer. Distribution transformers are designed to operate with variable load which is considerably less than their rating. Therefore, these transformers are designed to have maximum efficiency at the load between ½ and ¾ of the full-load. Distribution transformers are kept in operation all the 24 hours of a day whether they are carrying any load or not. Distribution transformers have a good voltage regulation, and are designed to have a small value of leakage reactance.

Instrument Transformers

It is very difficult to measure high alternating currents and voltages with simple measuring devices. Thus, to make the measurement of high alternating currents and voltages simple, we use specially designed transformers, called instrument transformers. By employing instrument transformers, we can measure high alternating quantities with low-range AC measurement devices.

Depending on the type of quantity transformed, instrument transformers are of the following two types −

• Current Transformer (C.T.)

• Potential Transformer (P.T.)

Current Transformer

A current transformer is a type of instrument transformer which is used to decrease the high alternating current of power line to a measurable low value. Basically, a current transformer is a voltage step-up and current step-down transformer. It has a primary winding of a few turns of thick wire, and a secondary winding having more turns of fine wire. The primary winding of CT is connected in series with the line whose current is to be measured, and the secondary winding is connected across a low-range AC ammeter to measure and indicate the current.

Potential Transformer

A potential transformer is a voltage step-down transformer which is used to reduce the high line voltages to a measurable value. The primary winding of a PT has many turns while the secondary winding has few turns. The primary winding is connected across the power line whose voltage is to be measured and the secondary winding is connected across a low-range AC voltmeter to indicate the measured voltage value.