Transformer Rating

A transformer is a commonly used device used to step up or step down voltages in the transmission and distribution of electricity. The transformer rating is decided by the manufacturer. The manufacturer put a name plate on a transformer. This name plate has voltage rating, current rating, VA rating, and frequency rating written on it.

Transformer Rating – Explained

1. Voltage rating

It is the rated voltage that can be applied (on primary) or developed (on secondary) in the transformer windings on the no-load condition. The voltage rating is associated with no-load losses (or iron losses or core losses).

2. Current rating

It is the rated or maximum allowable primary or secondary current that transformer winding can bear without burning itself. The current rating is always decided by copper losses (or winding losses) associated with loading conditions.

It can be calculated as

I_{rated}=\frac{\text{Rated VA}}{V_{rated}}

where Irated = rated current for which winding is designed to operate

Vrated = rated voltage on the corresponding winding

3. VA rating or kVA rating

It is the maximum VA that can be applied to a transformer. The rated VA of the primary winding is taken equal to the secondary winding since losses can be neglected. This is because of transformer operates at very high efficiency.

Let us take an example to understand the transformer rating meaning.

Consider a transformer with the following ratings.

10 kVA, 2500/250 V, 50 Hz

Here

♦ 10 kVA denotes the maximum kVA that can be applied to the transformer.

♦ 2500 V and 250V denote the no-load voltage or rated voltage of the two windings of the transformer. Any of the two windings can serve as a primary or a secondary winding.

♦ 50 Hz denotes the rated frequency for which this transformer is designed to operate.

♦ The rated current (I1rated) in the first winding

I_{1rated}=\frac{\text{Rated VA on first winding}}{V_{1rated}}=\frac{10 \times 10^{3}}{2500}=4 A

where V1rated = rated voltage on the first winding

♦ The rated current (I2rated) in the second winding

I_{2rated}=\frac{\text{Rated VA on second winding}}{V_{2rated}}=\frac{10 \times 10^{3}}{250}=40 A

where V2rated = rated voltage on the second winding


Let us consider this transformer as a step-down transformer then

♦ 10 kVA denotes the maximum kVA that can be applied to the transformer.

♦ 2500 V denotes the no-load voltage on the primary side (or rated voltage on the primary side) of the transformer.

♦ 250 V denotes the no-load voltage on the secondary side (or rated voltage on the secondary side) of the transformer.

♦ 50 Hz denotes the rated frequency for which this transformer is designed to operate.

♦ The rated current (Iprated) in the primary winding

I_{prated}=\frac{\text{Rated VA on primary}}{V_{prated}}=\frac{10 \times 10^{3}}{2500}=4 A

where Vprated = rated voltage on the primary winding

♦ The rated current (Israted) in the secondary winding

I_{srated}=\frac{\text{Rated VA on secondary}}{V_{srated}}=\frac{10 \times 10^{3}}{250}=40 A

where Vsrated = rated voltage on the secondary winding


Let us consider this transformer as a step-up transformer then

♦ 10 kVA denotes the maximum kVA that can be applied to the transformer.

♦ 250 V denotes the no-load voltage on the primary side (or rated voltage on the primary side) of the transformer.

♦ 2500 V denotes the no-load voltage on the secondary side (or rated voltage on the secondary side) of the transformer.

♦ 50 Hz denotes the rated frequency for which this transformer is designed to operate.

♦ The rated current (Iprated) in the primary winding

I_{prated}=\frac{\text{Rated VA on primary}}{V_{prated}}=\frac{10 \times 10^{3}}{250}=40 A

where Vprated = rated voltage on the primary winding

♦ The rated current (Israted) in the secondary winding

I_{srated}=\frac{\text{Rated VA on secondary}}{V_{srated}}=\frac{10 \times 10^{3}}{2500}=4 A

where Vsrated = rated voltage on the secondary winding

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