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sizing generators for use with submersible water well pumps

Table 5 lists minimum generator sizes based on typical 80D C rise continuous duty generators with 35% maximum voltage dip during starting, for Franklin’s 3-wire motors, single or three phase.

This is a general chart. The generator manufacturer should be consulted whenever possible, especially on larger sizes.

There are two types of generators available: externally and internally regulated. Most are externally regulated. They use an external voltage regulator that senses the output voltage. As the voltage dips at motor start-up, the regulator increases the output voltage of the generator.

Internally regulated (self-excited) generators have an extra winding in the generator stator. The extra winding senses the output current to automatically adjust the output voltage.

Generators must be sized to deliver at least 65% of the rated voltage during starting to ensure adequate starting torque. Besides sizing, generator frequency is important as the motor speed varies with the frequency (HZ). Due to pump affinity laws, a pump running at 1 to 2 HZ below motor nameplate frequency design will not meet its performance curve. Conversely, a pump running 1 to 2 HZ above may trip overloads.

Always start the generator before the motor is started and always stop the motor before the generator is shut down. The motor thrust bearing may be damaged if the generator is allowed to coast down with the motor running. The same condition occurs when the generator is allowed to run out of fuel.

Follow generator manufacturer’s recommendations for de-rating at higher elevations or using natural gas.