Circuit Negma

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Fault Finding – Three Phase Induction Motors

Posted by Circuit Negma on March 28, 2006


Fault Finding – Three Phase Induction Motors

Motor will not start.
Supply or Starter trips out at start.
Motor Starts but has not torque.
Motor does not reach full speed.
Motor takes a long time to reach full speed.
Motor overheating.
No load amps in excess of Full load amps.
Mechanical noise or Vibration.
Noisy bearings.
Bearing Overheating
Motor amps in excess of nameplate full load amps on load.
Excessive electrical noise.
Unbalanced amps in different phases when motor on load.
Motor runs wrong direction.

Problem Possible Cause Tests Solution
Motor will not start.

  1. Fault with supply.
  2. Motor or load locked up.
  3. Wrong connections in control circuit.

  1. Check for correct voltage at motor terminals.
  2. Make sure motor and load are free to turn.
  3. Check to ensure contactors operate.

  1. Fit new fuses, reset curcuit breakers, etc.
  2. Remove clamps, locks, etc.
  3. Sort out control circuit.
Supply or Started trips out at start.

  1. Wrong or loose connections.
  2. Motor overloaded.
  3. Intertia of load to high.
  4. Low Voltage due to volt drop in cables
  5. Overload or circuit breaker incorrectly set or sized.

  1. Check all lugs are properly crimped or soldered, and connections are tight.
  2. Check load performance data against motor performance data.
  3. Measure voltage at motor terminals while motor starting.
  4. Check settings of overload and circuit breaker and allow for starting current

  1. Fix up connections.
  2. Change motor for correct size.
  3. Change cables for correct size.
  4. Correct setting of overload or breaker or change.
Motor starts but has no torque. Motor does not reach full speed or takes a long time to accelerate.

  1. Incorrect connection.
  2. Delta wound motor connect in star.
  3. Star/Delta starter staying in Star.
  4. Inertia of load to high.
  5. Motor overloaded.
  6. Low voltage due to volt drop in cables.

  1. Check connection diagram and nameplate data.
  2. Check load performance data against motor performance data.
  3. Measure voltage at motor terminals while motor starting

  1. Sort out and correct connections.
  2. Check timer and starter control circuit.
  3. Change motor for correct size.
  4. Change cables for correct size.
Motor Overheating.

  1. Motor overloaded.
  2. Ineffective cooling.
  3. Excessive ambient.
  4. Wrong connection.
  5. Delta wound motor in star.
  6. Motor “Single Phasing”.
  7. Wrong voltage or frequency.
  8. Supply voltage unbalanced.

  1. Check load performance data.
  2. Check fan and air flow and temperature of air. Look for build up of dirt.
  3. Check connection diagram and nameplate data.
  4. Check volts and amps in all three phases.
  5. Check nameplate
  6. Measure phase to phase voltage accurately

  1. Fix problem with load or fit larger motor.
  2. Clean motor. Sort out cooling of air temp. and flow.
  3. Sort out connections.
  4. Restore supply to all phases.
  5. Correct voltage or frequency.
  6. Balance supply or accept unbalance.
No load amps in excess of Full load amps

  1. Incorrect connection
  2. Star wound motor connection Delta.
  3. Voltage in excess of nameplate.
  4. Motor supplied for different voltage or frequency.

    1 & 2. Check connection diagram and nameplate data.

    3. Measure voltage at motor terminals.
    4. Compare supply voltage and frequency to nameplate.

    1 & 2. Sort out and correct connections at motor terminals.
    3. Correct supply voltage
    4. Change motor for correct voltage and frequency

Mechanical Noise or Vibration. Noisy bearings. Bearings overheating.

  1. Thrust from load or misalignment.
  2. Damaged bearings, too much grease, no grease, or foreign matter in grease.
  3. Rotor pulling or foreign matter in air gap.
  4. Out of balance load, coupling or pulley.
  5. Excessive belt pull.
  6. Motor foundations not rigid.

    1. Check gap between coupling halves and alignment.
    2 & 3. Turn shaft slowly by hand and feel for roughness or stiffness. Check for bent shaft or fan rubbing.
    4. Run motor disconnected from load and then with pulley or coupling removed.
    5. Run motor without belts.
    6. Check design and construction foundations

    1. Re-align couplings
    2 & 3. Clean bearing housing, change bearings and repack with fresh grease.
    4. Fix up out of balance items
    5. Loosen belt tension
    6. Increase strength of foundations

Motor amps in excess of nameplate full load amps on load

  1. Motor overloaded.
  2. Low supply voltage.
  3. Wrong voltage or frequency.
  4. Wrong Connections.
  5. Motor ‘Single-Phasing’.
  6. Supply voltage unbalanced.
  7. Motor Speed not matched to load.

    1. Check load and performance data.
    2. Measure voltage at motor terminals
    3. Check nameplate.
    4. Check nameplate
    5 & 6. Check volts and amps in all three phases.
    7. Measure motor speed and check load speed requirements.

    1. Fix problem with load or fit larger motor.
    2. Fix problem, maybe with larger cables.
    3. Correct voltage or frequency.
    4. Sort out and correct.
    5 & 6. Restore balanced supply to all three phases.
    7. Change motor for correct motor speed.

Excessive electric noise

  1. Wrong connections.
  2. Wrong voltage.
  3. Motor ‘Single-Phasing’.

    1. Check connections
    2. Check voltage with nameplate
    3. Check volts with amps in all three phases.

    1. Fix up connections
    2. Correct voltage.
    3. Restore supply to all three phases.

Unbalanced amps in different phases when motor loaded

  1. Unbalanced supply voltage

  1. Measure phase to phase voltage accurately

  1. Balance supply or accept unbalance
Motor runs in wrong direction

  1. Wrong connections.

  1. Watch shaft rotation

  1. Swop and two phases of supply.
 
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19 Responses to “Fault Finding – Three Phase Induction Motors”

  1. vicctoriorus76 said

    Here are some links that I believe will be interested

  2. Peter Flanagan said

    Very Very helpfull, many thanks.

  3. naga lavaKumar said

    hai.
    i have seen ur site. i nice presently i have a dough how to give connections to the coils after replacing the old i.e re-winding the motor when the coils are burned please specify the single phase motor winding and three phase motor winding details.Is their any difference between winding in 720,960,1440,2800 R.P.M

  4. Dan Martinez said

    When checking amps on three phase motor do you add all three reading (L-1,L-2 and L-3)for exampal L-1=4.0, L-2=3.8, and L-3= 4.2 so total amps =12amps
    Dan

  5. To Dan Martinez:

    You can not add all 3 readings to get the total amps drawn by the motor.

    depending on what kind of calculations you are trying to perform, each leg has a different phase from the other leg but equal in magnitude for a balanced system only.

    lets see if i could remember my machine courses back from old days.

    you need to determine the following first:

    1. is the motor AC or DC?

    2. is the motor a balanced or non balanced one?

    3. is the motor loaded or unloaded at the time of taking the measurements?

    4. Motor configuration: is it Y or delta configuration?

    Total Power (3-p) = sqrt(3) x PF x E x I x cos(theta)
    PF = Power Factor
    E = line to line Voltage [V]
    I = phase or line Current [A] ( current in each leg :: http://mysite.du.edu/~jcalvert/tech/threeph.htm)
    theta = phase angle between voltage and current
    cos(theta) = 1 when the voltage and current are in phase

    links:
    1. 3-p induction motor :: http://myweb.dal.ca/gillm/Assets/inductionmotor_lab.pdf
    2. 3-p motor :: http://mysite.du.edu/~jcalvert/tech/threeph.htm

  6. Johnny said

    can u give me a site that can indicate a full maintenance on a three phase motor(fault finding)????

  7. Jay said

    See Website:

  8. three phase induction motor ,transformer,generator,and other electrical falt please give me question&answar

  9. Nitesh Chaugule said

    Give me the current status of preventing the 3 phase induction motors from eccessive voltage, current and temperature. Plz.

  10. Nitesh Chaugule said

    Plz give me,current status of prevention of 3 phase induction motor from eccessive current, voltage, temperature.

    • It has been a while since I worked on motors related stuff. What I am trying to understand from your question is that you are trying to prevent the motor from loading. for example, the load changed and hence impacting the motor output, which may result in drawing more current or voltage than the motor can output/handle.

      I think that you need to determine the maximum output current and voltage of the motor and then use the thumb rule of operating at 10% less, or something like that I cannot remember exactly.

      I found the following document online Differential Protection of 3 phase Induction Motors by Ninad Deo.

      Also I used the following keyword search on google : 3 phase induction motors excessive voltage

      Sorry I could not be much of help on this one.

  11. Mutekanga Tonny said

    thumbs up, helpful big time

  12. supperrrrrrrrrrrrrrrrrrrrrrr then i want one information from u,HOW CAN FIND OUT MOTOR WINDING FOR MELT USE TESTING LAMP (terminals name give)

  13. Louis said

    how do u do fault finding on a DC motor? u test the brushes and what else?

  14. Nagarajan said

    This use full tips

  15. Motors said

    Motors…

    [...]Fault Finding – Three Phase Induction Motors « Circuit Negma[...]…

  16. ashiq said

    its very useful 2 me thnkz a lot…………

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