Electric Motor Standards As Defined By The Iec And The Harmonized European Standard

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 merk 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.