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Electric Motor Testing Explained

By R. W. Hurst, The Electricity Forum

The Essentials of Electric Motor Testing

Electric Motor Testing is usuallly the first thing to be sacrificed when cutting back on operational expenses. But smart companies understands is that what this means for maintenance programs is billions of dollars of lost revenue through increased motor repair costs, downtime, and waste in industrial and commercial companies. This is because modern electrical maintenance practices often do not take into account the importance of electric motor testing for proper equipment uptime and plant competitiveness.

Electrical maintenance and management programs are designed to improve equipment readiness and uptime while reducing capital overhead. This program consists of particular maintenance and management tools designed to aid the maintenance engineer in electric motor systems and their care.

The following are Recommended Electric Motor Testing Tasks:

Electric Motor Impulse Testing
Electric Motor impulse testing is an integral part of predictive maintenance of electrical motors. Through the following questions the influence that extensive impulse testing has on a motor is investigated. Can impulse testing damage healthy or deteriorated insulation? Can DC Resistance, Inductance, Megger or HiPot tests diagnose weak turn-to-turn insulation? After failing an impulse test, are motor with weak insulation able to operate? Are motors with a turn-turn short capable of continued operation? This was accomplished by putting a low voltage motor through extensive testing rigors, until inducing a failure. Following the failure, additional testing investigated the possible deteriorating effects on turn-turn insulation due to impulse testing beyond the motor’s dielectric breakdown. NOTE: This paper was edited from the original version of the IEEE paper published in 2003.



Electric Motor Rotation Testing
Check for fan or pump motor rotation when testing offline with the MCE. Fans may continue to slowly rotate due to drafting in the Plenum. Pumps that are connected to a common header may continue to rotate if other pumps connected to the header are operating. This will adversely affect the Standard Test results, possibly creating higher than normal resistive and inductive imbalances.

Wound Rotor Electric Motor Testing
Wound rotor motors have a three-phase winding wound on the rotor which is connected to three phases of start-up resistors in order to provide current and speed control on start-up. Failed components in the resistor bank are common and often overlooked when troubleshooting. These faults can have a significant impact on the overall operation of the motor and should be given considerable focus when troubleshooting these motors.

Electric Motor Insulation Resistance Testing
Electric motor insulation exhibits a negative temperature coefficient, meaning as temperature increases, resistance decreases. This would lead you to believe that insulation resistance of a de-energized motor will decrease after starting the motor. However, most often the resistance will initially increase after running due to moisture being evaporated by the increasing temperature of the windings. The governing standard (IEEE43) on insulation resistance testing requires a temperature correction to 40 degrees Celsius, which could quickly turn acceptable measured resistance readings into unacceptably low corrected resistance readings. Before sending a motor to be refurbished, consider space heaters.

The recommended off-line in-service electric motor tests are:

  • Stator winding resistive imbalance
  • Stator winding insulation resistance (Meg-Ohm checks)
  • Polarization Index (PI)
  • Step Voltage test
  • Surge test

The recommended spare electric motor tests are:

  • Stator winding resistive imbalance
  • Stator winding insulation resistance (Meg-Ohm checks)
  • Polarization Index (PI)
  • Step Voltage test
  • Surge test

The recommended new/refurbished electric motor tests are:

  • Stator winding resistive imbalance
  • Stator winding insulation resistance (Meg-Ohm checks)
  • Polarization Index (PI)
  • Step Voltage test
  • Surge test.

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