Why Does My Electric Motor Humm And Not Start?
How can I Diagnose an Electric Motor Problem? Common Problems with Electric Motors and Capacitor Start Motors, How to Check an Electric Motor, Diagnostic Checks You Can Make When an Electric Motor Will Not Start.© By: Dave Rongey
Guide to Electric Motor Problems and Motor Capacitors
Electric Motor Problem: How do I check the electric motor on an air compressor that hums when I turn on the switch.
- The motor and compressor turn free by hand.
- The motor has what looks like two capacitors setting on the top of the motor.
- How do I tell if the motor is bad or it has a bad capacitor?
- The motor just hums when I turn it on.
Background: Charles, a Homeowner from Pittsburg, Missouri.
Additional Comments: This is the first time I have asked a question, but I have been getting your Electrical Newsletter for awhile and enjoy it.
Thanks for your electrical question Charles.
How to Diagnose an Electric Motor Problem
- Application: Testing an Electric Motor.
- Skill Level: Intermediate to Advanced – Best performed by a Licensed or Certified Electrician.
- Tools Required: Basic Electricians Pouch Hand Tools and Voltage Tester or Continuity Tester
- Estimated Time: Depends on experience with capacitor start motors and level of problem solving skills.
Precaution about Motor Capacitors
- Testing capacitors can be dangerous because capacitors store an electric charge.
- Capacitors may be discharged an tested for fault. Electric motors are best serviced by an experienced electrician or knowledgeable technician.
- Electric motors or equipment may also be taken into an electric motor repair shop for extensive diagnosis and servicing.
- Testing the motor and capacitor should only be made after the electric motor circuit has been identified and turned OFF and Tagged.
Common Problems with Electric Motors and Capacitor Start Motors
Diagnostic Checks You Can Make When an Electric Motor Will Not Start
- A start capacitor can go bad, even start to go bad and get weak.
- Capacitors are like a battery and hold quite a charge so be careful.
- Turn off the power to the compressor.
- The capacitor can be tested by disconnecting them from the wiring, then with the shank of a screwdriver short the two terminals while holding on to the insulated handle.
- If this produces a solid discharge then the capacitor then it should be fine, but understand that it may be weak.
- The resistance and discharge of a capacitor may be checked using an Ohm Meter.
Other Factors that will Cause an Air Compressor and Electric Motor to hum and Not Start
#1 Pressure Switch Settings
- Head pressure is too high.
- Make sure the adjustments for the pressure switch are set according to the specifications of the manual.
- Bleed off some of the existing tank pressure.
#2 Pulley and Belt Tension
- Check the belt tension between the motor pulley and the compressor unit. the belt should be firm, but not too tight.
- Check that the alignment between the motor pulley and the compressor pulley is straight and inline.
#3 Electrical Wiring of an Air Compressor
- The electrical circuit should be sized correctly to deliver the necessary starting amperage or inrush current.
- The wiring between the circuit outlet or connection must be adequately sized with the same gauge wire as the required circuit or the electrical current will not be supplied to the motor.
#4 Avoid Using Extension Cords for Air Compressors
- Long Extension cords should be avoided. Only high quality properly sized extension cords should be used.
- An extension cord that is sized too small will choke the power that is trying to get to the unit and cause the extension cord to overheat.
- All motors require an great deal of power at the start cycle which is known as an inrush of power.
#5 The Important Starting Phase of an Electric Motor
- The starting phase of an electric motor occurs for just a few seconds but it requires multiple times the circuit size to deliver this power and start the motor, this is why special motor controls are required for larger motors and prevent the circuit from tripping off during the start cycle.
- The start and running amperage may be tested using a clamp on amp meter, then the readings can be compared to the nameplate information to see if the motor is performing within the stated amperage range.
How to Check an Electric Motor
- If an electric motor has burnt up there will be a burnt smell and the motor housing may be dark from over heating.
- Electric motors have internal windings of special wire which may be tested.
- A continuity tester or meter may be used to check to see if the windings are open or shorted to the ground or frame of the motor housing.
- The amount of continuity readings will depending on the exact motor you have, however basic readings for a direct short or open winding will be very helpful.
Electric Motor and Air Compressor Maintenance
Lastly, check the maintenance or service manual to make sure everything is correct including:
- Compressor lubrication.
- Belt tension.
- Clean air intake filter element.
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