COMMON MOTOR FAILURES AND FAULTS

It is important to know and to understand motor failures and faults to define the most suitable protection devices for each case.

Being non-static machines motors are subjected to electrical and mechanical stress.

Motor failures come in three basic types: electrical, mechanical and mechanical that progresses into electrical.

Common motor failure and faults are:

• Overheating
• Insulation breakdown
• Overloads (electrical and mechanical)
• Phase imbalance and any voltage imbalance will lead to an even higher current unbalance.
• Bearing failure
• Running in reverse
• Shaft misalignment
• Locked rotor
• Vibration

Overheating can occur from undersizing the motor, insufficient cooling at low speed when using variable speed drives (VSD), changes to the load on the motor such as jammed equipment and hot ambient conditions.

Insulation breakdown, leading to burnt windings, imply short-circuit either within the motor or within the power supply circuit for the motor, and may be caused by overheating, overloads and overvoltages.

About 80% of electrical motor failures are a result of winding damage in the motor stator and bearing faults.

Bearing failure on motors can be an indication of the incorrect bearings for the application.

A motor mounted vertically needs different bearings then a motor horizontally. A motor driving a large or multi-belt drive will require bearings that handle big radial loads. A motor bolted to a distorted base plate will twist.

Bearings are usually small compared to other major motor components, making them particularly vulnerable to damage and wear; some studies blame more than half of all motor failures on bearing malfunction, most of which result from too little or too much lubrication. Another significant cause of bearing failure is misalignment.

Shaft misalignment will destroy bearings well before their full working life.

The motor shaft must be directly in-line with the shaft it is driving.

This can only be achieved using precision alignment techniques such as laser.

Other problems that may occur with motors are water ingress and dust ingress into the stator coils or the terminal housing leading to short-circuits, soft foot motor feet are bolted down out of level), wrong motor mounting or housing type and electrical or mechanical unbalance.

Noise indicates motor problems but ordinarily does not cause damage. Noise, however, is usually accompanied by vibration.

Vibration can cause damage in several ways. It tends to shake windings loose and mechanically damages insulation by cracking, flaking or abrading the material. Embrittlement of lead wires from excessive movement and brush sparking at commutators or current collector rings also results from vibration. Finally, vibration can speed bearing failure by causing balls to "brinnell," sleeve bearings to be pounded out of shape or the housings to loosen in the shells.

Whenever noise or vibrations are found in an operating motor, the source should be quickly isolated and corrected. What seems to be an obvious source of the noise or vibration may be a symptom of a hidden problem. Therefore, a thorough investigation is often required.

Noise and vibrations can be caused by a misaligned motor shaft or can be transmitted to the motor from the driven machine or power transmission system. They can also be the result of either electrical or mechanical unbalance in the motor.

Electrical unbalance occurs when the magnetic attraction between stator and rotor is uneven around the periphery of the motor. This causes the shaft to deflect as it rotates creating a mechanical unbalance. Electrical unbalance usually indicates an electrical failure such as an open stator or rotor winding, an open bar or ring in squirrel cage motors or shorted field coils in synchronous motors. An uneven air gap, usually from badly worn sleeve bearings, also produces electrical unbalance.

The chief causes of mechanical unbalance include a distorted mounting, bent shaft, poorly balanced rotor, loose parts on the rotor or bad bearings. Noise can also come from the fan hitting the frame, shroud, or foreign objects inside the shroud. If the bearings are bad, as indicated by excessive bearing noise, determine why the bearings failed.

Another problems motors can face is a long start time

If a motor is subjected to many successive starts, the rotor windings or rotor bars can be heated up to a point where the electrical connections between the rotor bars and the end rings are damaged.