Motor Troubleshooting

The troubleshooting guide outlines a comprehensive variety of motor problems. Generally the categories are arranged according to symptoms offering brief suggestions concerning what to look for when investigating motor failures (why motor fails?) and often providing advice on how to correct the problem once it has been identified.

Motor Shuts Off Before Full Speed


A. Motor also hums (single phase motor)
  • Run winding may have a loose connection - disassemble motor in order to make the appropriate repair
  • Run winding is burned out - motor must be replaced or rewound

    B. Overloaded
  • Reduce the load.
  • C. Excessively low voltage.
  • A reasonable overload or voltage drop of 10-15% will reduce speed only 1 to2 percent.
  • A report of any greater drop would be questionable.

    D. Inaccurate method of measuring RPM
  • Check meter using another device or method.

    E. Wrong connections
  • F. Bad capacitor

    Motor Shuts Off After Reaching Full Speed


    A. Overload

    B. Bearings

    C. Defective overload protection

    D. Fuses

    Motor Starts Slowly


    10 or more seconds on small motors - 15 or more seconds on large motors

    A. Bad capacitor

    B. Overloaded

    C. Low voltage

    D. Wrong connections

    E. Across the line start.
  • Excessive voltage drop
    (5-10% voltage drop causes 10-20% or more drop in starting torque).
    • Check connections between power supply and motor.
    • Check conductor size between power supply and motor.
    • Consult power company/check system.

  • High Inertia Load.
    • Reduce starting load.
    • Increase motor size.
    F. Reduced voltage starter

  • Loss of starting torque
    a. Y-Delta - starting torque reduced to 33%.
    • Reduce starting load
    • Increase motor size.
    • Choose type of starter with higher starting torque.
    • Reduce time delay between 1st and 2nd step on starter

    b. Part winding starter - starting torque reduced to 50%.
    • Reduce starting load
    • Increase motor size.
    • Choose type of starter with higher starting torque.
    • Reduce time delay between 1st and 2nd step on starter.

    c. Auto transformer starter - starting torque reduced 25% to 64%.
    • Reduce the starting load
    • Increase the motor size.
    • Choose a different model of starter with higher starting torque.
    • Reduce the time delay between the 1st and 2nd starter steps.
    • Get the motor across the line sooner.

    Motor Runs Excessively Hot


    A. Overloaded
  • Reduce load or load peaks
  • Reduce number of load starts/cycles
  • Increase motor size
  • B. Blocked Ventilation:
  • Fan cooled motors
    • Clean external ventilation system
    • Check fan operation

  • Open motors
    • Blow out internal ventilation passages.
    • Eliminate external interference to motor ventilation.

  • C. High ambient temperature over 40°C (104°F)
  • Reduce ambient temperature or provide outside source of cooler air.

  • D. Unbalanced input current.
  • Balance supply voltage.
  • Check motor leads for tightness.

  • E. Single phased
  • Eliminate the single phase condition.

  • F. Drive belt too tight
    G. Bad capacitor

    H. Worn bearings

    I. Wrong connections

    J. Dirt, dust, trash

    Excessive Vibration


    A. Check alignment of motor and driven load - check and realign motor with the driven machine.

    B. Check for Unbalance in Motor and Driven Load



  • Check Motor Mounting
    • Be sure motor mounting/shims are tight and solid.
    • Check grouting for cracks

  • Check Driven Load
    • Disconnect belt or coupling
    • Restart motor
    • If vibration stops, the unbalance is in the driven load.

  • Check Sheaves or Coupling
    • Remove sheave or coupling
    • Securely tape 1/2 key in shaft key way
    • Restart motor
    • If vibration stops, the unbalance is in the sheave or coupling.

  • Check the Motor
    • If the vibration does not stop after checking a, b and c above, the unbalance is in the motor
    • Replace the motor. Make sure the new mounting is proper.
  • C. Tightness of mounting
    D. Bearings?

    E. Load and motor

    F. Grounds or shorts?

    Motor Runs Noisily Under Load


    A. Motor hums excessively
  • The motor may not be connected properly
  • Motor is single phased
  • Improper overload relays
  • There may be a winding fault or a short circuit in the motor
  • Air gap between rotor and field stators may be uneven
  • The rotor is unbalanced or the PTO shaft is bent

  • B. Noisy bearings (listen to bearings)
  • Smooth mid range hum - OK.
  • High whine - internal fit of bearing too tight.
  • Low rumble - internal fit of bearing too loose.
  • Rough clatter
    • Make sure mounting is solid level.
    • The bearing may have been destroyed.

  • C. Mechanical noise
  • Driven machine or motor noise
  • Motor noise amplified by resonant mounting
  • Driven machine noise transmitted to motor through drive
  • Misalignment on close coupled application - improve alignment.

  • D. Clicking sound is regularly heard
  • Check for foreign matter in the air gap
  • The cooling fan may be hitting the end bell or housing

  • E. Rapid knocking sound heard on all types of motors - motor is out alignment causing excessive end play.
    F. Check for brush chatter.


    Never Reaches Normal Operating Speed (RPM)


    A. Motor also hums (single phase motor)
  • Run winding may have loose connection
  • Run winding may have burned out

  • B. Possible overload
    C. Excessively low voltage

    D. Inaccurate method of measuring RPM

    E. Wrong connections

    F. Bad capacitor

    Smells Funny / Smokes


    A. Smell something burning or overheating or see smoke
  • The motor is overheating.


  • The cooling air flow may be restricted.
    • Clean motor by wiping off excess dirt
    • Use compressed air to blow dust particles out of passageways

  • Bearing problem may be causing an overload
    • Bearings are overheated
    • Too much lubrication of the bearing
    • Bearings are bad

  • Short in the windings
  • B. Smell of ozone - burning windings

    High Input Current


    A. Accuracy of ammeter readings

    B. Motor running idle

    C. Motor running loaded
  • Motor overloaded
  • Motor voltage rating does not match power system voltage.

  • Unbalanced Input Current


    5% or more deviation from the average input current

    A. Unbalanced line voltage due to:
  • Power supply
  • Unbalanced system loading
  • High resistance connection
  • Undersized supply lines
  • B. Defective motor
      If there is doubt as to whether the trouble lies with the power supply or the motor, perform the following test:
      Rotate all three input power lines to the motor by one position. (i.e. move line #1 to #2 motor lead, line #2 to #3 motor lead and line #3 to #1 motor lead).

    If the unbalanced current pattern follows the input power lines, the problem is in the power supply.
    If the unbalanced current pattern follows the motor leads, the problem is in the motor.

    {NOTE: A small voltage unbalance will produce a large current unbalance.}

    Excessive Voltage Drop


    More than 2 or 3% of nominal supply voltage

    A. Excessive starting or running load
    • Reduce load.
    B. Inadequate power supply
    • Consult power company.
    C. Undersized supply conductors
    • Increase supply conductor line sizes.
    D. High resistance connections
    • Check motor leads and eliminate poor connections.
    E. Each phase lead run in separate conduits
    • All 3 phase leads shall be in a single conduit, per National Electrical Code. (This applies only to metal conduit with magnetic properties.)

    Overload Relays Tripping


    A. During motor starting

  • Slow starting (10 - 15 seconds or more) due to high inertia load.)
    • Reduce starting load.
    • Increase motor size if necessary.

  • Low voltage at motor terminals
    • Improve power supply.
    • Increase motor size if necessary.
  • B. When running loaded
  • Overloaded
  • Unbalanced input current
  • Single phasing
  • Excessive voltage drop
  • Starting or overloading too frequent.
  • Ambient starter temperatures too high.
  • Wrong size or type of relays

  • Sparks


    A. Sparking and flashing coming from the brushes
  • Roughened commutator
  • B. The commutator is blackened
  • Commutator may have a low bar
  • Commutator may have a high bar
  • May have a high mica
  • Worn or short brushes
  • Not enough tension on the brushes
  • Brush springs may be weak
  • Brushes stick in holders, do not fully surface on commutator
  • Commutator has dirt or oil on it
  • Water dripping onto the commutator
  • Short in the armature or field windings

  • Wound Rotor Motor Problems


    A. Sparking and flashing from brushes

    B. Brushes are worn, stuck, not set

    C. Commutator is dirty, rough, has high bars, or is out of round.

    D. Voltage is either too high or too low.

    E. Governor is either stuck or out of adjustment.
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