In a critical, continuous application, where a failure could cost
£10,000 per hour, motor reliability becomes a major issue. Steve Ruddell
discusses just what makes a motor stand up to harsh and demanding duties
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 (155°C) and Class B (130°C). 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 60-80°C. Lower quality motors, on the other
hand, can run in excess of 90°C. Theoretically, a reduction of 10-15°C
should double the life of the windings as well as increase the
re-greasing 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 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
include small winding overhang, high quality grade of copper wire, a 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
1,350/1,400V insulation in motors designed for a 400V delta connection.
The current standard adopted by ABB, for example, is to use 1,600V
insulation material as a minimum. This is especially important to
withstand the voltage spikes produced by some variable speed drives.
While quality windings lead to a more expensive motor, a little extra
spend at the outset could avoid a potentially costly breakdown in the
future.
Steve Ruddell is general manager, electrical machines at ABB