How windings affect motor life

An ABB Automation Tech (Drives and Motors) product story

Edited by the Engineeringtalk editorial team Jan 17, 2005

After the bearings, the second most common cause of motor failure is the winding.

Motor reliability means different things to different users.

A motor used only occasionally on a noncritical application will not clock up the running hours and will potentially have a long life, whatever the quality of materials used in its manufacture.

But a critical continuous application, where a failure could cost GBP 10,000 per hour, calls for a motor made of sterner stuff; a motor built of high quality materials that are designed for continuous heavy-duty operation.

After the bearings, the second most common cause of motor failure is the winding.

Internal running temperature is critical to the life expectancy of the winding and factors contributing to increased temperature can include poor design or manufacture, the use of lower quality materials, incorrect dimensioning, poor application and use on unsuitable supplies.

The winding can be insulated to different standards: the most common are Class F (155C) and Class B (130C).

The majority of motors are manufactured with Class F insulation material but designed to run no hotter than with Class B temperature rise.

The cooler the motor runs, the longer the insulation system will last and the longer the motor will operate.

High quality motors running at full load can have a normal running temperature as low as 60-80C, whereas lower quality motors can run in excess of 90C.

Theoretically, a reduction of 10-15C should double the life of the windings as well as increase the regreasing intervals on the bearings.

Over the course of a long and busy operating life cycle the insulation system will start to deteriorate, and it is then that a short circuit may occur.

In most cases a high quality motor can then be rewound with a minimal reduction in efficiency.

By contrast, rewinding a low quality motor may result in much greater reductions in efficiency, due to the lower quality grade of electrical steel used in the construction of the stator laminations becoming unstable during the controlled burnout phase of the rewinding procedure.

Production techniques also affect the life of the windings.

The best performance is obtained from a compact winding with a good slot fill - ie a high density of copper in the stator slot thus aiding heat dissipation from the winding itself.

Automated winding and insertion equipment can result in a higher density slot fill than hand winding and a repeatable quality time after time.

Characteristics of a good winding are: small winding overhang; high quality grade of copper wire; good slot fill ratio; and high quality slot insulation, impregnation systems and phase insulation systems.

The integrity of the winding is measured in withstand voltage.

No reputable European manufacturer uses materials with less than 1350/1400V insulation in motors designed for a 400V delta connection.

The current standard adopted by ABB is to use 1600V insulation material as a minimum.

This is especially important to withstand the voltage spikes produced by some variable speed drives.

Although quality windings lead to a more expensive motor, a little extra outlay up front could avoid a potentially costly breakdown in the future.