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Change in Characteristics

Author: E.E. Kimberly

A generator may be used successfully at much less than rated voltage if separately excited, the desired voltage being obtained by excitation adjustment, by speed adjustment, or by both. If the ventilation were equally good at all speeds, the current that could be taken from the armature without overheating would be the same at all speeds. At reduced speed and correspondingly reduced generated voltage, the voltage regulation is poorer than normal because the IaRa voltage loss per ampere in the armature is as great as before and the armature reaction is also as great as before.

If the generator be driven at rated speed but be under-excited to obtain reduced voltage, the voltage regulation will be poor because the field-distorting ampere-turns of the armature for a given load are as great as ever and have more effect on the weakened field than upon a normally strong field. However, when the generator is so operated, full-load rated current may be drawn from the armature without excessive heating.

As the speed of a self-excited generator is reduced, the maximum terminal voltage to which it will build up is also reduced. A generator cannot be operated at half of its rated voltage when self-excited. In order that a self-excited generator may maintain its voltage within reasonable limits from no load to full load, the magnetic circuit must be excited above the knee of its saturation curve, If a generator is separately excited, its terminal voltage for any fixed current output is proportional to its speed. An example will serve to demonstrate the possibility of altering the characteristics of a self-excited generator to fit a definite need, without actually altering the windings.

Example 10-3. - A compound-wound interpole generator rated 25 kw, 115 volts at 900 rpm is needed. There is on hand a generator rated 60 kw, 230 volts at 1800 rpm. Can the generator at hand be satisfactorily converted without rewinding? Give instructions.

Solution. - The 115-volt requirement will be met when the speed is established at 900 rpm, instead of 1800 rpm. The shunt-field circuit must be broken in the middle, and the two halves must be connected in parallel. Each field coil will then receive full voltage when connected across the armature for self-excitation. The armature cooling will be less effective at the lower speed, and so the permissible continuous current output will be less. The number of ampere-turns in the series field per ampere of load will be the same as before. The compounding percentage will be the same, therefore, and may be satisfactorily altered by means of an adjustable low-resistance diverter connected across the series field. The commutating voltage needed will not be quite so great at the low speed, and the strength of the interpole field should be reduced by means of a diverter.

The generator is therefore capable of delivering somewhat less than its original normal current at 115 volts and 900 rpm, and should be satisfactorily convertible.

Last Update: 2011-02-23