What is No Load Current in Transformer?Explained

In simple words, when a transformer is not connected to any load but consumes some electric current then the amount of current consumed by the transformer is called No Load Current. A transformer is in no-load means the secondary winding of the transformer is open-circuited. So in the no-load condition, no current will flow in the secondary winding of the transformer. But if it is connected to a power supply then a small current will flow in the primary winding which is known as no-load current. Generally, the value of no-load current varies between 2% to 8% of the full load current of a transformer.

Why No Load Current is consumed by a Transformer?

There are mainly two important reasons for no-load current consumption by a transformer,

1. When the primary winding of the transformer is connected to a power supply or we can say when a voltage is applied across the primary winding it creates magnetic flux which links both primary and secondary winding. So this flux-producing purpose a transformer consumes current even if its secondary is open-circuited. But remember that the current consumed by a transformer to produce magnetic flux is called the magnetizing current which is a part of the no-load current. In an ideal transformer, there is no loss so no power is consumed for the magnetizing purpose or producing flux. But in a practical transformer hysteresis loss occurs so it consumes some power or we can say it wastes the electrical power.

2. Another part of the no-load current is used to supply the loss in the transformer or we can say a transformer also consumes no-load current due to iron loss and copper loss. Although the transformer winding has a very low resistance, but we can not neglect that resistance. So due to this resistance, some copper loss also happens and it consumes a current.

If we come to the point of Iron Loss, Iron Loss occurs in the transformer core due to variations in the magnetic flux. The variation happens in the magnetic flux due to two reasons - 1. Variation in the polarity and magnitude of AC supply 2. Due to load Variation

This iron loss also has two types - 1. Eddy Current Loss 2. Hysteresis Loss

So the conclusion is a transformer consumes No Load Current due to Magnetization and Iron Loss.

Effects of No Load Current of Transformer

1. Due to no load current, the transformer wasted electrical energy.

2. Due to the no-load current, the transformer causes a very low power factor at no load, it varies between 0.1 to 0.15

Transformer No Load Current Phasor Diagram

Now, let's see the phasor diagram of the load Current of the Transformer. We know that no-load current has two components - 1. Magnetizing Component(denoted by Im) 2. Active Component(denoted by Iw)

The voltage applied to the primary winding is V1, and EMF induced in the primary winding is E1

The flux produced in the transformer is ϕ, and the no-load power factor angle is denoted by ϕ0

In the above diagram, you can see the V1 and E1 in the opposite direction because emf is always induced in opposite to the applied voltage. The working component Iw is on the same axis with voltage.

From the above phasor diagram, we can find the below equations,

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