To Restart a motor

Restarting of a motor that has tripped should only be considered after these faults have been factored out.

There are six areas of interest known as Fault Zones that must be looked at during the troubleshooting effort. Missing any of these zones could result in missing the problem and losing credibility in our skills.

Six electric Fault Zones:

1. Power Quality
2. Power Circuit
3. Insulation
4. Stator
5. Rotor
6. Air Gap

TROUBLESHOOTING an electric motor that is suspected to have an electrical problem should not result in the statement, “ The motor is fine.”

Power Quality problems - Voltage and current harmonic distortion, voltage spikes, voltage unbalance and power factor are a few of the many concerns when discussing power quality.

Harmonic Distortion or Total Harmonic Distortion (THD) is [the ratio of the root-mean-square of the harmonic content to the root-mean-square value of the fundamental quantity, expressed as a percent of the fundamental.] or in Jed terms, each phase should have a perfect 60 Hz sine wave would have 0% THD.  So anything other then the fundamental line frequency (60 Hz) would be considered a harmonic distortion.  In other words, out of phase.

The presence of harmonics in a distribution system results in excessive heat from the increased current demands. A load designed to pull 100 amps at full load may draw now 120 amps if the harmonic distortion is high. This additional current can lead to insulation damage and possibly a catastrophic failure.

Power Circuit: What is a Power Circuit?  The power circuit refers to all the conductors and connections that exist from the point at which the testing starts through to the connections at the motor.  This can include circuit breakers, fuses, contactors, overloads, disconnects, and lug connections.  Many times a motor, although initially in perfect health, is installed into a faulty power circuit. This causes problems like harmonics, voltage imbalances, current imbalances, etc.  As these problems become more severe, the horsepower rating of a motor drops, causing temperatures to increase and insulation damage to occur.  High resistance connections resulting in voltage imbalances will reduce the horsepower rating significantly.

Insulation Condition: Refers to the insulation between the windings and ground.  High temperatures, age, moisture, and dirt contamination all lead to shortened insulation life.

Insulation systems today are better than ever and are able to handle higher and higher temperatures without significant reduction in life.  Although insulation is many times involved in a failure, this fault zone is heavily influenced by other problems.  The power circuit for one can heavily influence the insulation.  If a high resistance connection exists upstream of the motor, which develops better than a 5% voltage imbalance, and we continue to run the motor at its normal Hp rating, we will see a shortened insulation life. Reverse sequence currents developing rotating magnetic fields in the opposite direction will not only reduce the torque capability, but can allow the temperature to rise out of control and exceed even the 150 0C limit on your class F insulation systems.  Was the insulation system the real cause of the motor failure or was it just a symptom?  It is easy to diagnose the evident insulation failure as the fault mechanism but it will happen again with a different motor if the problem is not fixed.  Then what will the explanation be?

Stator Condition: When we mention the stator, we are referencing the DC or 3 phase AC windings, insulation between the turns of the winding, solder joints between the coils, and the stator core or laminations.

One of the common faults occurring with motor windings is a turn to turn fault. This occurs when the insulation between two turns in the same coil breaks down and reduces the coil’s ability to produce a balanced magnetic field. Unbalanced magnetic fields result in vibration, which can then cause degradation of the insulation as well as bearing failures. Localized heating around the short can also spread to other coils, resulting in a coil to coil short. Excessive heating will eventually not only destroy the motor windings, but will also damage the insulation between the laminations of the stator core.

Another fault that can occur with motor windings is a phase to phase fault. This results from the insulation breaking down between two separate phases, usually lying adjacent to each other in the same slot. A higher difference in voltage potential tends to make this fault accelerate very quickly.

A turn to turn or a phase to phase short can occur many times without resulting in an immediate ground fault. Because of this, testing with just a megger for preventive maintenance or following a motor trip may not identify the fault.

Rotor Condition: Refers to the rotor bars, the rotor laminations, and the end rings of the rotor.  The rotor, although a small percentage of the motor problems, can influence other fault zones to fail.  When a motor is started with a broken or cracked rotor bar, intense heat is generated around the vicinity of the break.  This can spread to other rotor bars and destroy the insulation around the nearby laminations.  It can also effect other parts of the motor.  What is just a few millimeters away from the rotor?  The stator!  Stator insulation can not hold up to the intense heat developed by the broken rotor bar and will eventually fail.  Unfortunately, many times broken rotor bars are not easily seen without technology and it may be missed as the root cause of failure.  This will result in a motor rewind, and replacement of bearings, but not a rotor repair.  When the motor returns to service, it has the same problem all over again, just with new insulation to destroy.

Rotor / Stator Relationship: This relationship references the air gap between the rotor and stator.  If this air gap is not evenly distributed around the 360 degrees of the motor, uneven magnetic fields can be produced.  These magnetic imbalances can cause movement of the stator windings, resulting in winding failure, and electrically induced vibration, resulting in bearing failure.  A faulty relationship between the rotor and stator is also called an eccentricity.