Transformers have high efficiencies, especially the larger sizes operating at 50 to 100% of full load. However, they are not perfect. Some losses occur in all transformers. Transformer losses are divided into two types
- Copper losses
- Core losses
Copper loss is the loss of power caused by the resistance of the copper windings. As current the passes through the windings, it generates heat. The greater the current in the windings, the more power is lost in producing heat. The more current required by the load, the greater the copper loss. For this reason, copper losses are sometimes called load losses. You can calculate the copper loss (P), for any given load current if you know the resistance of both windings. You calculate the current in the primary winding by multiplying the current in the secondary by either the ratio of turns or the ratio of potential differences. Then, you square each current, multiply it by the resistance of the winding, and add the two values together.
H= High Voltage side, X= Low Voltage side
Core loss occurs in two ways. One is the eddy-current loss, caused by the formation of eddy currents in the core material. The other is hysteresis loss, caused by the reversal in direction of the magnetic field in the core. Both losses produce heat in the transformer.
How to reduce Eddy currents
Laminating core reduces the eddy current losses. Laminations break up the path of currents. The laminations strip insulated from one another by oxidation on the surface of the plates, or by a thin coat of varnish.
How to reduce Hysteresis loss
A proper material of core reduces hysteresis losses. The material must be one that can reverse its magnetic polarity easily. Silicon steel or high-grade steel use for the formation of a transformer core. 4% Silicon value is sufficient to reduce hysteresis losses. Core losses are usually constant for all loads. The total losses measured by connecting a wattmeter to the primary winding while the secondary winding is open-circuited. The power consumed by the primary winding equals the power lost in the transformer.
The efficiency of a transformer defines as the same way as the efficiency of any other device.
Efficiency = (work output) / (work input)
The ratio of work output to work input equals the ratio of the power output to the power input because work is the product of power and time.
Efficiency= (work output) / (work input)
Efficiency= (power output)×time / (power input)×time
Efficiency= power output / power input
Transformer efficiency also depends upon copper & core/iron losses, if the copper and iron losses are equal to each other than the transformer is maximum. Maximum Efficiency of transformer possible if,
Copper losses = Core losses