- below rated power factor
- below rated voltage
- above rated voltage
Under these conditions, the kVA rating must not be exceeded. If the kVA rating is exceeded, overheating of the field and stator windings could occur.
When the generator is carrying unbalanced loads, overheating of the rotor may result. A load is unbalanced when the phase current and the rated value vary by an amount that is greater than ten percent. The rotor's heating is caused by negative sequence currents. These currents flow in the field pole face. For a general guide, refer to Illustration 1. This chart is based on a 10% equivalent negative sequence current.
The chart is used in the following manner. Find the intersection of the vertical line and the horizontal line. The vertical line is determined by the maximum current in any of the phases. This is expressed in percent of rated current. The horizontal line is determined by the minimum current in any of the phases. This is expressed in percent of rated current. These two lines should intersect within the "Allowable Unbalance Region" for safe operation of the generator.
 | |
| Illustration 1 | g00875907 |
If field excitation is lost, the unit may not be synchronized with the system. This will occur when the unit is operating in parallel. This can cause damage to rotating rectifiers and severe mechanical stresses on the shaft and other parts. Protective devices should open the generator circuit breaker.