Page 266 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

SYNCHRONOUS GENERATORS

SYNCHRONOUS GENERATORS 12.29

FIGURE 12.34 A capability diagram showing the prime mover power limit.
magnetically able to supply. In general, a typical synchronous generator can supply up to
300 percent of its rated power until its windings burn up.
This ability to supply more power than the rated amount is used for momentary power
surges which occur during motor starting and other load transients. A synchronous gener-
ator can supply more power than the rated value for longer periods of time as long as the
windings do not heat up excessively before the load is removed. For example, a generator
rated for 1 MW is able to supply 1.5 MW for 1 min without causing serious damage to the
windings. This generator can operate for longer periods at lower power levels.
The insulation class of the windings determines the maximum temperature rise in the gen-
erator. The standard insulation classes are A, B, F, and H. In general, these classes correspond
to temperature rises above ambient of 60, 80, 105, and 125°C, respectively. The power sup-
plied by a generator increases with the insulation class without overheating the windings.
In motors and generators, overheating the windings is a serious problem. In general,
when the temperature of the windings increases by 10°C above the rated value, the average
lifetime of the machine is reduced by one-half. Since the increase in the temperature of the
windings above the rated value drastically reduces the lifetime of the machine, a synchronous
generator should not be overloaded unless it is absolutely necessary.
The service factor is the ratio of the actual maximum power of the machine to its name-
plate rating. A 1.15 service factor of a generator indicates that it can operate indefinitely at
115 percent of the rated load without harm. The service factor of a motor or a generator pro-
vides a margin for error in case the rated loads were improperly estimated.

REFERENCE

1. S. J. Chapman, Electric Machinery Fundamentals, 2d ed., McGraw-Hill, New York, 1991.

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