Basic Motor Diagnostics

A few posts ago, I presented information on how to diagnose ECM motors. Even though these are becoming more popular, there are still a lot of “standard” motors out there.  So, in this post, I want to give you some of the basic diagnostics of motors.

To do this, I’ll break it down to different problems and what to look for.

    1. Check voltage at the motor line terminals. Voltage should correspond with the motor name plate rating (+/- 10%).  If proper voltage is present:
      1. check all electrical connections at the motor terminal board
    2. If low or no voltage is present:
      1. check fuses, timers, switches
      2. check WIRE SIZE.
  2. FAILURE TO START (motor hums or attempts to start — fuse blows or circuit breaker opens)
    1. Again, check voltage at line voltage terminals.  If voltage is inadequate to start the motor, check for loose connections, undersized wiring, or other causes of voltage drop.
    2. Capacitor (when used) open or shorted (see post on capacitors)
    3. Check motor for open or shorted winding
    4. Check continuity (ohms) through a motor protector — either internal or external.
    5. Turn off the power and turn the motor shaft by hand to get the “feel” of the motor. If the motor feels tight or doesn’t turn  smooth:
      1. check the bearings for smooth operation
      2. check for corrosion or dirt that has gotten into the motor and correct the source.
    1. Compare the running amperage of the motor with the FLA (full load amperage) or Service Factor (SF) amperage of the motor.
      1. If amperage is higher than the FLA or SF amperage listed on the motor with proper voltage applied to the motor, check for the cause of the higher amperage:
        1. belt too tight
        2. high static pressure (see posts on proper air flow)
    2. Check motor windings or capacitor (when used) for “grounds”  or “shorts”,
    3. Check wiring connections at terminal board and compare to the wiring diagram to make sure the motor is wired correctly.
    1. Check motor mounting bracket to assure it is tight and properly supporting the motor
      1. on resilient mount motors, make sure the rubber is not sagging, miss-aligning the motor.
    2. Check the blower wheel and hub for proper tightness
    3. Check the wheel for to make sure it is not out of balance
    4. Check the motor bearings.

Of course, as with any discussion of electrical components, SAFETY IS ALWAYS FIRST! When working on live circuits, caution should always be observed.  Follow proper procedures using LOCK OUT/ TAG OUT.  Make sure you verify the power is off before reaching into and blower compartment or condenser fan. 

If you need more precise procedure, see the post on Compressor Electrical Checks.  Remember, a compressor is just another electrical motor.

I know these seem pretty basic, but most of the problems we see are usually something simple.


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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