Alternator Losses

The following losses are occured in an alternator while its on working condition.

Copper Loss

Copper losses occur only in the armature winding and field winding.

Core Loss

The losses due to eddy current and hysteresis losses is said to be core loss.

Friction and Windage Losses

The loss due to the bearing and brush friction and to the power required to circulate the cooling air is known as friction and windage loss.

Load Loss

The load loss is due to the armature leakage flux which causes eddy currents and hysteresis in the iron surrounding the armature conductors.

Determination of Losses

The losses occured in alternator can be determined by two methods, they are:

• Measurement of losses method.
• Use of a calibrated DC motor to drive the alternator.

Measurement of Losses Method

It is impossible to measure the efficiency of alternators by direct loading and physically it is difficult to get the proper load. Measurement of mechanical power is somewhat difficult. If there is any inaccuracy in measurement, efficiency calculation by measurement of losses method which entails the following advantages.

At any one time, only part of the losses need be provided. Therefore the source of power required for testing has a capacity of less than 5% of the rating of the alternator. It is not required to put an electric load on the alternator. Great accuracy can be obtained since electrical instruments can be used for all measurements. The error occurs only for a small portion of the name plate rating and hence there is no reflection of any inaccuracy during calculation of final frequency.

Use of a calibrated DC motor to drive the alternator

This method is quite accurate and simple to determine losses. In this method, a calibrated DC motor is used to drive the alternator. The meaning of calibration of the motor is that all motor is that all its losses have been determined for varying conditions of operation. Therefore, its output is readily obtained. The following steps are required for the determination of losses in an alternator.

step 1: The alternator is driven at rated synchronous speed without field excitation. The output of the DC motor provides the input of the alternator and windage losses.

step 2: Repeat step 1, but this time with field excitation. The excitation should be that at which the alternator normally operates. If this is unknown, then the open-circuit voltage is adjusted to be equal to the rated voltage plus the internal voltage drops as determined buy one of the methods used to calculate voltage regulation.

step 3: Short-circuit the armature and adjust the field current to obtain rated line current. The difference between this motor output and that of step 1 is the armature copper loss plus load loss. It is assumed that the flux density under short-circuit conditions is so low as to make the core loss negligible.

step 4: Finally, the field copper loss is measured by simple D.C. measurement.

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