**Subject :**Electrical Machines II (AC Machines)

**Unit :**Alternators

## Losses and efficiency of an alternator

**Losses and efficiency of an alternator:**

To calculate the efficiency of a synchronous generator, a procedure is to be followed for establishing the total losses when operating under load. For generators these losses are,

1. Rotational losses such as friction and windage.

2. Eddy current and hysteresis losses in the magnetic circuit

3. Copper losses in the armature winding and in the field coils

4. Load loss due to armature leakage flux causing eddy current and hysteresis losses in the armature-surrounding iron.

With regard to the losses, the following comments may be made,

1. The rotational losses, which include friction and windage losses, are constant, since the speed of a synchronous generator is constant. It may be determined from a no-load test.

2. The core loss includes eddy current and hysteresis losses as a result of normal flux density changes. It can be determined by measuring the power input to an auxiliary motor used to drive the generator at no load, with and without the field excited. The difference in power measured constitutes this loss.

3. The armature and field copper losses are obtained as I^{2}_{a}R_{a} and V* _{f}* I

*Since per phase quantities are dealt with, the armature copper loss for the generator must be multiplied by the number of phases. The field winding loss is as a result of the excitation current flowing through the resistance of the field winding.*

_{f}4. Load loss or stray losses result from eddy currents in the armature conductors and increased core losses due to distorted magnetic fields. Although it is possible to separate this loss by tests, in calculating the efficiency, it may be accounted for by taking the effective armature resistance rather than the dc resistance.

After all the foregoing losses have been determined, the efficiency *η* is calculated as,

where *η* = efficiency,

kvA = load on the generator (output)

PF = power factor of the load

The quantity (kVA*PF) is, of course, the real power delivered to the load (in kW) by the synchronous generator. Thus, it could in general be stated as

The input power P_{in} = P_{out} P_{losses} is the power required from the prime mover to drive the loaded generator.