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Building up Voltage of a Self-Excited Generator

Author: E.E. Kimberly

When a generator is driven at constant speed without its shunt circuit being closed, a very small residual voltage (1 to 5 volts) appears at its terminals because of the residual magnetism remaining in the iron. When the field circuit is properly connected, the residual voltage forces a small exciting current through the field circuit and thereby increases the field strength.

Because of the increased field strength , the generated voltage increases. This reciprocal action continues until a point of stability is reached at which the flux produced by the current in the field is just sufficient to generate the voltage required to produce the field current. Most modern generators are so designed that, with no resistance in series with the field, the voltage will rise to about 125 per cent of rated value. This condition is represented by the point a in Fig. 10-15. The straight line Oa is called a field resistance line because its slope V/If is equal to Rf, the resistance of the field circuit including the field rheostat. After the voltage has built up, it may be adjusted to any desired value between a and c by increasing the resistance of the field circuit by means of its rheostat. Generators are usually designed so that the rated voltage is generated at the point b or at somewhat higher field current. For every value of field-circuit resistance, the resistance line will have a particular slope, such as Od or Ob.

Fig. 10-15. Generator Self-Excitation Curve

If the resistance is made so high that the slope of the resistance line, as Oe, is equal to or greater than that of the lower straight part of the magnetization curve, the voltage of the generator will collapse and will drop to the residual value O'. A generator may fail to build up for any of the following reasons:

1. Field-circuit resistance too high; including open circuit.

2. Speed too low.

3. Residual magnetism lost.

4. Direction of rotation incorrect.

5. Generator terminals connected to external circuit of too low resistance.

6. Shunt-field terminals reversed.

A If the generator speed be reduced, the magnetization curve will be reduced in height proportionately, as shown by the dash curve of Fig. 10-15. Whereas at rated speed the generator would build up when the field resistance line is Ob, this line is too steep to permit build-up at the reduced speed. If the direction of rotation is incorrect, the residual voltage will force current through the field coils in a direction to demagnetize the field, and build-up cannot occur. If the generator is connected to a load circuit of too low resistance, the load-circuit current due to residual voltage may, by its magnetizing action in the armature, prevent build-up. Residual magnetism may be restored by connecting the field circuit to any suitable source of direct current.

Last Update: 2010-10-06