Mechanical Consulting Group Inc.
2208 Magnolia Lane
Elgin, IL 60120
Ph: (630) 830-2083
Fax: (630) 830-0630


There are many things which will cause a well pump to fail. Most pumps are submersible pumps and operate under water. If the shaft seal leaks then there is water in the pump motor which will cause the motor to show a low resistance to ground when the motor winding insulation test is run. Many times the pump will pick up sand or small pieces of gravel and prevent the pump from turning. Sometimes the water will damage either the pump or pump motor due to corrosion. The motor is checked in a similar fashion as other electric motors.

On residential pumps the pressure switch that turns the pump on and off should be removed for inspection. On three phase pumps the contactor should be retained for inspection.

1. The pump is removed from the motor and the pump is inspected for objects such as sand, gravel or other foreign objects. The pump shaft is turned to see if the bearings are free and not frozen due to corrosion or lack of lubrication.

2. The resistance of the motor leads to the case of the pump motor is measured with a megohm meter. This measurement is taken to check the quality of the motor winding insulation. A low reading does not necessarily mean that there is lightning or power surge damage. Often a low meg-ohm reading indicates that the shaft seal is bad and there is water in the motor windings.

  • The motor winding resistance of the pump motor is also measured. If the motor windings check good the motor has not been damaged by lightning. The shaft of the motor is checked to see if it is seized. If the shaft is seized the motor windings would expected to be bad because the motor can not turn the shaft due to the frozen or seized shaft. A seized shaft could also cause a low meg-ohm reading .

4. The motor is connected to either 115 volts or 230 volts depending upon the voltage rating of the motor. The current or amperes is read. Since the motor is not connected to the pump the motor will typically pull 70% to 80% of the full load amperes (F.L.A.) or run load amperes (R.L.A.) If the shaft is seized the motor will pull locked rotor amperes, and the motor has not been damaged by lightning. If the current draw of the motor reads the locked rotor ampere rating (LRA) it is a good indication that the motor has not been damaged by lightning. Many motors have both the run load ampere rating and the locked rotor ampere rating listed on the motor name plate.

If the current draw is 80% or less than the FLA or RLA rating on the motor nameplate it is likely that the pump motor is good.

Most large commercial pumps are either 230 volts or 460 volts three phase.

5. A precision tachometer is used to determine the revolutions per minute of the shaft to measure the speed that the shaft is turning. Most pumps are rated to turn at 3450 revolutions per minute (RPM) under full load. Since there is no load on the pump motor, because the motor is not connected to the pump, the motor will typically turn at speeds up to 3500 RPM or slightly more. Theoretically the motor can turn up to 3600 RPM. If the pump shaft turns at a speed of more than 3450 RPM it is likely that the pump motor is good.

Franklin makes many of the residential well pump motors. Franklin uses a solid state or electronic starting device for their two wire motors and a small surge can cause damage to the electronic starting device for the motor . The motor will not get out of the start mode and the motor will at less than full speed which is 3450 rpm.

The Engineering Committee for the Electrical Apparatus Service Association, Inc. has prepared a brochure on typical causes of winding failures in three-phase stators (motor windings) for three phase motors. Most of the members consist of companies that rebuild electric motors. The brochure lists 12 different failures (with large color photographs) of three phase motor windings. One of the failures is winding damage by voltage surge. This guide is used in investigation of all lightning claims for three phase motors.





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