Troubleshooting Guide – AC Motor

download print version buttonUse this resource to troubleshoot your AC motor. If the motor problems can’t be resolved with this list, please contact your supplier for assistance.


1. Motor Fails to Start Upon Initial Installation

  • Motor is wired incorrectly
    • Refer to the wiring diagram to verify the motor is wired correctly.
  • Motor damaged and rotor is striking stator 
    • Rotate the motor’s shaft and feel for rubbing.
  • Power supply or line trouble
    • Check the source of power, overload, fuses, controls, etc..

2. Motor Has Been Running, Then Fails to Start

  • Fuse or circuit breaker is tripped 
    • Replace the fuse or reset the breaker.
  • Stator is shorted or went to ground (Motor will make a humming noise and the circuit breaker or fuse will trip)
    • Check for leaks through the coils. If leaks are found, the motor must be replaced.
  • Motor overloaded or jammed
    • Inspect to see that the load is free. Verify the amp draw of motor versus the nameplate rating.
  • Capacitor (on single phase motor) may have failed
    • First discharge the capacitor. To check the capacitor, set the volt-ohm meter to RX100 scale and touch its probes to the capacitor terminals. If the capacitor is OK, the needle will jump to zero ohms, and drift back to high. Steady zero ohms indicates a short circuit; steady high ohms indicates an open circuit.

3. Motor Runs but Dies Down

  • Voltage drop
    • If the voltage is less than 90% of the motor’s rating, contact your power company or check to see that another piece of equipment isn’t taking power away from the motor.
  • Load increased
    • Verify that the load has not changed and the equipment has not gotten tighter. If it is a fan application, verify that the air flow hasn’t changed.

4. Motor Takes Too Long to Accelerate

  • Defective capacitor
    • Test the capacitor per previous instructions.
  • Bad bearings
    • Noisy or rough feeling bearings should be replaced by the motor supplier.
  • Voltage too low
    • Make sure the voltage is within 10% of the motor’s nameplate rating. If not, contact your power company or check if some other equipment is taking power away from the motor.

5. Motor Runs in the Wrong Direction

  • Incorrect wiring
    • Rewire the motor according to the schematic provided with the motor. Groschopp wiring diagrams can be found within the “Wiring Diagrams” page of our resources section or on individual motor pages.

6. Motor Overloaded/Thermal Protector Continuously Drips

  • Load too high
    • Verify that the load is not jammed. If the motor is a replacement, verify that the rating is the same as old motor. If the previous motor was a special design, a stock motor may not be able to duplicate the performance. Remove the load from the motor and inspect the amp draw of the motor unloaded. It should be less than the full load rating stamped on the nameplate (only true for three phase motors).
  • Ambient temperature too high
    • Verify that the motor is getting enough air for proper cooling. Most motors are designed to run in an ambient temperature of or less than 40°C. (Note: A properly operating motor may be hot to the touch.)

7. Motor Overheating

  • Overload. Compare actual amps (measured) with nameplate rating 
    • Locate and remove the source of excessive friction in the motor or load. Reduce the load or replace the motor with one of greater capacity.
  • Single phasing (three phase only) 
    • Check the current at all phases. It should be approximately the same.
  • Improper ventilation
    • Check external cooling fan to be sure air is moving properly through the cooling channels. If there is excessive dirt build-up, clean the motor.
  • Unbalanced voltage (three phase only) 
    • Check the voltage at all phases. It should be approximately the same.
  • Rotor rubbing on stator
    • Tighten the thru bolts.
  • Over voltage or under voltage
    • Check the input voltage at each phase of the motor to make sure the motor is running at voltage specified on the nameplate.
  • Open stator winding (three phase only) 
    • Check the stator resistance at all three phases for balance.
  • Improper connections
    • Inspect all the electrical connections for proper termination, clearance, mechanical strength, and electrical continuity. Refer to the motor lead diagram.

8. Motor Vibrates

  • Motor misaligned to load
    • Realign the load.
  • Load out of balance (direct drive application) 
    • Remove the motor from load and inspect the motor by itself. Verify that the motor shaft is not bent.
  • Defective motor bearings
    • Test the motor by itself. If the bearings are bad, you will hear noises or feel roughness.
  • Load too light (single phase only)
    • Some vibration at a light load is standard. Consider switching to a smaller motor for excessive vibration.
  • Defective winding
    • Test the winding for shorted or open circuits. The amps may also be high. For defective winding, replace the motor.
  • High voltage
    • Check the power supply to make sure voltage is accurate.

9. Bearings Fail

  • Load to motor may be excessive or unbalanced
    • Check the motor load and inspect the drive belt tension to ensure it’s not too tight. An unbalanced load will also cause the bearings to fail.
  • High ambient temperatures
    • If the motor is used in an environment with high ambient temperatures, a different type of bearing grease may be required. You may need to consult the factory.
  • High motor temperatures
    • Check and compare the actual motor loads to the motor’s rated load capabilities.

10. Capacitor Fail

  • Ambient temperature too high
    • Verify that the ambient temperature does not exceed the motor’s temperature rating (found on the nameplate)
  • Possible power surge to the motor (caused by a lightning strike or other high transient voltage) 
    • If this is a common problem, install a surge protector.

How to use the Motor Search Tool

Narrow your search by selecting motor type, gearbox, voltage, and phase options for your desired motor.

Select a dominant variable: choose one of the three parameters to narrow your search. The selected variable determines which slider bar you will be able to manually move.

Use the slider corresponding to your dominant variable to further narrow your motor selection. The other sliders will automatically move to show available ranges based on the range of your selected variable.

Results will upload as your search criteria changes. If you have any questions regarding your results or how to use the search tool, you can chat with us using the green tab on the left-hand side of your screen.

Note: Groschopp Universal motors are custom built to fit your application so no additional options are available to narrow the search. Selecting the Universal motor type will prompt a message taking you to the Universal product page.

Not sure what you need?

One of our team members would be happy to help. Contact us at 800-829-4135 or by email at sales@groschopp.com. You can also chat with us using the green tab on the left side of your screen.

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Universal Motors

Groschopp Universal motors are custom built to fit your application so no additional options are available to narrow the search. Standard frame sizes and motor features can be found on the Universal page.

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