A Timely Topic – Using a Generator with a Franklin Sub

As a result of the recent weather events on the east coast, Franklin Electric’s Water Systems Technical Hotline has been receiving a high number of calls concerning the use of generators with submersible installations. In order to provide an easy reference for all, it makes sense to review generator sizing here in Franklin AID.

Note: The use of generators must follow all local, state, and national electrical codes. ALWAYS consult these codes before installing a generator. In addition, make sure the generator is properly ventilated and that you are familiar with its operating instructions before putting it into use.

Guidelines specific to using an engine driven generator with a Franklin submersible motor can be found on page 5 of the Franklin Electric AIM (Application, Installation, and Maintenance) Manual.

To determine proper sizing, refer to Table 5 on page 5 of the AIM Manual. (Click the illustration above for a close-up view.)Note that the numbers in the sizing chart apply to both 3-wire and 3-phase motors. If it’s a 2-wire installation, the minimum generator sizing is 50% higher than listed in the chart. This is because of the higher starting current required for 2-wire motors.

Also note that the sizing chart only applies to one submersible motor. If other devices are being powered, they must be identified along with their power consumption. Even though some of these items may not run continuously, they still need to be taken into account, per the generator manufacturer’s recommendations.

The frequency of the voltage delivered by the generator will be a function of the engine’s RPM. Motor speed varies with the frequency of the output voltage, and since pump affinity laws relate power to performance, generator sizing can have significant impact on pump output. For example, if the generator is putting out a voltage at a frequency that is below 60 hertz, the pump will not meet its performance curve. Likewise, if the frequency is above 60 hertz, it may overwork the motor and trip its overloads. The generator manufacturer’s instructions will contain guidance on how to adjust the generator’s frequency. Of course, you’ll also need a voltmeter that measures frequency. Most of today’s digital voltmeters contain this function.

The thrust bearing in a submersible motor requires a minimum speed of 30 hertz (about 1800 RPM), so it is important to start the generator before starting the motor. Likewise, it is equally important to stop the motor before the generator is shut down. Failure to do so may result in damage to the motor’s thrust bearing during start-up and coast down. The installation of a simple transfer switch will allow the motor to be turned on and off independently of the generator. (Note: circuit breakers should NOT be used for this function.)

More critically, a transfer switch also functions as a safety device to isolate the utility electrical supply from the generator. Without a transfer switch, the generator can back feed into the utility lines and, in a worst case scenario, cause serious injury or death. Unfortunately, the transfer switch is one of the more commonly overlooked safety devices required by the National Electrical Code (NEC).

Code also requires that the generator be properly grounded in order to protect against electrical shock in the event of a fault. Like all electrical conductors, the ground wire must be correctly sized for the load it is designed to carry.

Hopefully, you won’t find yourself in a no-power situation that necessitates using an engine-driven generator. In the event that you do, taking appropriate precautions and following this protocol will help make sure you can get your Franklin sub back online.