How to Troubleshoot 3 Phase AC Motors.

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Three phase induction motors are one of the most popular electric motors commonly found in processing plants or any manufacturing concern. They are used in situations where large power is required. The squirrel cage brand is the most popular and they perform various tasks wherever they are applied.

Because of the critical roles these motors play in any plant, a failure of the motor, inability to start, noisy operation and sundry other problems need to be remedied as soon as possible to avoid costly production downtime. The table below gives the commonly encountered problems in 3 phase squirrel cage induction motors, cause of problems and what remedy to apply to bring the motor back to production. This troubleshooting guide can also be applied to other types of three phase induction motors:

Motor Problem Cause Remedy
Motor fails to start Blown fuses Replace fuse with proper type and rating
Overload Trips Check and reset overload in starter
Improper power supply Check to see that power supplied agrees with nameplate specifications  and load factor
Improper line connections Check connections with wiring diagram supplied with motor
Open circuit in winding or control switch This is normally indicated by a humming sound when switch is closed. Check for loose wiring connections. Confirm that all control contacts are closing.
Mechanical failure Check to see that motor and drive turns freely. Check bearings and lubrication
Short circuited stator Indicated by blown fuses.  Motor must be rewound
Poor stator coil connections Remove end belts. Locate poor connections with test lamp.
Rotor defective Check for broken bars or end rings
Motor may be overloaded Reduce motor load
Motor stalls One phase may be open Check supply lines for open phase
Wrong application Change type or size. Consult motor manufacturer
Overload Reduce load
Low voltage Check that nameplate voltage is maintained. Check connection.
Open circuit Fuses blown. Check overload relay, stator and push buttons
Motor runs and then dies down Power failure Check for loose connections to line, to fuses and to control
Motor does not come up to speed Motor is applied for the wrong application Consult manufacturer for right application of motor
Voltage too low at motor terminals because of line drop Use higher voltage on transformer terminals or reduce load. Check connections. Check conductors for proper size.
Starting load too high Check load motor is supposed to carry at start.
Broken rotor bars or loose rotor Look for cracks near the rings. A new rotor may be required as repairs are usually temporary not permanent
Open primary circuit Locate fault with testing device and repair.
Motor takes too long to accelerate and/or draws high current (Amps) Excessive load Reduce load
Low voltage during start up Check for high resistance. Adequate wire size.
Defective squirrel cage rotor Replace with new rotor
Applied voltage too low Improve voltage at terminals of transformer by tap changing.
Wrong rotation Wrong sequence of phases Reverse connections at motor or at switchboard.
Motor overheats while running under load Overload Reduce load
Frame or bracket vents may be clogged with dirt and prevent proper ventilation of motor. Open vent holes and check for a continuous stream of air from the motor.
Motor may have one phase open Check to make sure that all leads are well connected.
Grounded coil Locate and repair
Unbalanced terminal voltage Check for faulty leads, connections and transformers.
Motor vibrates Motor  misaligned Realign
Weak support Strengthen base
Coupling out of balance Balance coupling
Driven equipment unbalanced Re-balance driven equipment
Defective bearings Replace bearing
Bearings not in line Line bearings up properly
Balancing weights shifted Re-balance motor
Poly-phase motor running single phase Check for open circuit
Excessive end play Adjust bearing
Unbalanced line current on poly-phase motors during normal operation Unequal terminal volts Check leads and connections
Single phase operation Check for open contacts
Unbalanced voltage Correct unbalanced power supply
Noisy Operation Airgap not uniform Check and correct bracket fits or bearing.
Rotor unbalance Rebalance
Hot bearings general Bent or sprung shaft Straighten or replace shaft
Excessive belt pull Decrease belt tension
Pulley too far away Move pulley closer to motor bearing
Pulley diameter too small Use larger pulleys
Misalignment Correct by realignment of drive
Hot bearings ball Insufficient grease Maintain proper quantity of grease in bearing
Deterioration of grease or lubricant contaminated Remove old grease, wash bearings thoroughly in kerosene and replace with new grease.
Excessive lubricant Reduce quantity of grease, bearing should not be more than 1/2 filled
Overloaded bearing Check alignment, side and end thrust.
Broken ball or rough races Replace bearing, first clean housing thoroughly

This troubleshooting guide is by no means exhaustive but can provide guidance for the resolution of commonly encountered problems as far as poly-phase induction motors are concerned.

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