Troubleshooting Electric Motors – Part 2

This post is a continue from the troubleshooting electric motors part 1.As I mentioned on my last post, I will share information and guideline about electric motors breakdown.

Once it has determined that the motor is at fault, you can proceed to locate the problem with the motor. A 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 and often providing advice on how to correct the problem once it has been identified.

Troubleshooting Guides

This topic is important to all Electrician and Maintenance person that always involve with electric motors troubleshooting.This guide is basic troubleshooting guides and can be a baseline when handling about electric motors.The following is an example of a troubleshooting guide that outlines fault symptoms common to most types of motors.

1) The motor fails to start. Possible causes :-

a) Blown fuse or open circuit breaker.

Check the voltage at the input and output of the over current protection device. If voltage is measured at the input but not at the output, the fuse is blown or the circuit breaker is open.Check the rating of the fuse or circuit breaker. It should be at least 125 percent of the motor’s full-load current.

b) Motor overload relay on starter tripped.

Allow overload relay to cool and reset it. If the motor causes the overload relay to open after a short period, check for motor short circuits and grounds. Check the full load current of the motor and compare it to the setting of the overload relay.

c) Low voltage or no voltage applied to the motor. 

Check the voltage at the motor terminals. The voltage must be within 10 percent of the motor nameplate voltage. Determine the cause of the low voltage. Loose fuse clips and connections at the terminals of the disconnect switch or circuit breaker can result in low voltage at the motor.

d) Defective motor windings.

Make resistance checks of the motor windings for opens and shorts in coil windings and coils shorted to ground faults. An ohmmeter reading of infinity across a set of coil windings means that there is an open somewhere—sometimes it is at one end of the coil and accessible for repair.

A short circuit in only a few turns of a coil, while difficult to detect, will still result in a motor overheating. One way to test for a short circuited coil winding is to compare its resistance reading with that of a known good identical coil.

e) Burnt-out motor.

If one or more of the motor windings looks blackened and smells burnt, it is most likely burnt out and needs to be replaced with rewinding process.

f) Mechanical overload.

Rotate the motor shaft to see if a binding load is the problem. Check for frozen bearings. Check the air gap between the stator and rotor. Reduce the load or try operating the motor with no load applied.

 

2) Excessive motor noise and vibration. Possible causes:

a) Coupling mechanism.

Check for bent shaft on motor or load. Straighten if necessary. Measure the alignment of the couplings. Realign if necessary.

b) Bearings.

With the motor stopped, try gently moving the shaft up and down to detect bearing wear. Use a stethoscope to check the bearings for noise. When the handle of a screwdriver is placed to the ear and the blade to the bearing housing, the screwdriver will amplify the noise, like a stethoscope. Replace worn or loose bearings. Replace dirty or worn-out oil or grease.

c) Loose hardware.

Tighten all loose components on the motor and load. Check fasteners on the motor and load mounts. Motors with centrifugal mechanisms,brushes, slip rings, and commutators can cause noise due to wear and looseness of the mechanisms.

 

3) The motor overheats. Possible causes:

a) Ambient temperature.

Higher-than-normal ambient temperatures. Take steps to improve the motor’s ventilation and/or lower the ambient temperature.

b) Load.

A basic rule is that your motor should not get too hot to touch. Check ammeter reading against full load current rating of motor. For a higher-than-normal current reading, reduce the load or replace motor with a larger sized one.

c) Insufficient cooling.

Remove any buildup of debris in or around the motor.

d) Source voltage .

If the operating voltage is too high or too low, the motor will operate at a higher temperature. Correct voltage to within 10 percent of the motor’s rating.

 

4) Symptom: Motor overload protector continually trips. Possible cause:

a) Excessive Load.

Load too high. Verify that the load is not jammed. Remove the load from the motor and  measure the no-load current. It notably should be less than the full-load rating stamped on the nameplate.

b) Winding short-circuited or grounded.

Inspect windings for defects using insulation tester meter and loose or cut wires that may cause a path to ground.Normally insulation winding is weak due to dust or lifespan.

c)  Ambient temperature too high.

Verify that the motor is high temperature using temperature meter and if need more ventilation,getting air for proper cooling condition.