Why Does My Electric Motor Humm And Not Start?

By Dave Rongey - Summary:

How to 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.

Guide to Electric Motor Problems and Motor Capacitors

Electric Question: 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.

Dave’s Reply:
Thanks for your electrical question Charles.

How to Diagnose an Electric Motor Problem

Application: Testing an Electric Motor.
Skill Level: Intermediate to Advanced.
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: 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

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.

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.

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.

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.

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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FAQs Frequently Asked Questions and Comments

14 Responses to “Why Does My Electric Motor Humm And Not Start?”
  1. Dave Rongey says:

    Hi Chris,
    Check the nameplate on the motor to see if it can run on 110 and 220. If so, you will have to adjust the wire configuration of the motor leads for the appropriate voltage. The power cable will need to be sized according to the LRA Lock Rotor Amps or Starting Amps of the motor to prevent voltage drop.
    If the nameplate information states that it the motor will only run on 220 then you cannot run it on 110.
    I hope this helps you,

  2. Chris says:

    I have 1.5 hp Baldor motor on my meat cutting saw. When I plug it in all I get is a hum. When I bought the saw I did see it run so I know the motor is good, but it was on 220. I have put it on 110 because that’s all I get in the home. The power cable is about 100 feet away, could it be I need a bigger wire?

  3. Dave Rongey says:

    Hi Bryan,
    Sometimes using an extension cord is necessary, like when an air compressor is needed out at a job site. Therefore, if an extension cord is needed then the wire size should provide enough amperage for the motor of the equipment.
    Thanks for your comment Bryan.

  4. Bryan Budrovici says:

    Do not use an extension cord, the motor isn’t getting enough power!

  5. Dave Rongey says:

    Hi Doug,
    Air Compressor with Motor Starting Problems

    Electric Motor Centrifugal Start Switch
    Some older electric motors have a internal centrifugal start switch which is part of the starting circuit. The centrifugal start switch contacts may not be closing which will cause the motor to hum and not start or be hard to start.

    Factors that Affect an Electric Motor Centrifugal Start Switch
    Worn contacts on the start switch may need to be cleaned or resurfaced with fine sandpaper.
    Excessive motor shaft end play which is caused by worn out motor shaft fiber washers will prevent the centrifugal switch contacts from closing all the way.

    Air Compressor Pump
    Make sure the lubricating oil is at the full level.
    Change the compressor pump oil as specified by the manufacturer and replace with the recommended oil.
    Make sure the intake air filter is clean, replace as needed.
    Check the pulley belt tension to make sure it is not too tight, adjust as needed.

    I hope this helps,

  6. Doug says:

    I have an older 220v 7hp compressor with motor starting problems. Upon start the motor will buzz most of the time, occasionally engaging and running. I can turn the compressor pulley to help start and sometimes that will work. I replaced the start capacitor with identical item and still have issues with start. The motor seems to work better once it is warm, and will re-start most of the time. The air compressor does seem to have a small amount of friction in the pump. I checked the cylinder walls but they looked OK. Can I use a higher rated start capacitor? I would like to salvage the unit instead of buying new.
    Thanks, Doug

  7. Dave Rongey says:

    Hi Troy,
    It may be that this motor has a centrifugal switch that engages a start winding of the motor. If the motor starts by manually operating the switch then the switch may be faulty and need to be replaced or repaired. I had the same problem with an older lathe where I could start it manually, but it would not start on by itself due to a faulty centrifugal switch.
    Also make sure that the pump is spinning freely and without restrictions as this will effect the operation of the motor as well.
    I hope this helps,

  8. TROY M. says:

    I have a 1 hp 3450rpm Hayward pool pump made by A.O. Smith. The motor will not start on its own, it will only hum. I can take the back off and hit the stationary switch and it works fine. I cant find a start capacitor on it.

  9. Dave Rongey says:

    Hi Brian,
    Be sure the circuit voltage and amperage rating is correct for the unit or motor, and the motor is correct for the application or equipment.

  10. Brian Andrews says:

    I have a 220 volt H P Cleaner which refuses to start under load. I have checked the motor and windings do not look burnt.I have replaced both bearings and the run capacitor but to no avail. It will start and run perfectly until you load the pump then it just hums and trips.Anybody with any help would be greatly appreciated.

  11. benz says:

    Nice reading! I found this post really very helpful and this can be a good project for my high school students. I will definitely discuss it, with my students. We are recently working on electric dryers and I am teaching them how to fix it.

  12. G. Barnhart says:

    I need a drawing of a power tool ( skill saw ) with a too small of a cord,

    showing the ” IR ” for some co-workers, let me know if you know of any web-site.

    Thanks in advance, G. Barnhart

  13. admin says:

    Hi Bob,
    If you can turn the motor shaft by hand easily when the motor is not energized then it may in fact be the starting capacitor that is bad and needs replacing. If there is an electric motor shop in your area it would be great to take the motor or at least the capacitor in to be tested. Be careful because capacitors store electricity and can give quite a jolt. Also make sure that if the motor is 220 volt that half of the circuit breaker is not tripped OFF. Turn the circuit breaker all the way OFF and then back ON again just to make sure.

  14. Bob says:


    My pool motor hums, but I can get it running if I turn the shaft by hand? Is this a capacitor issue?



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