- 30 January 2005 -

AC induction motor specifications for larger AC motors

Increasing plant capacity can also increase headaches for the system specifier and motors present their own particular minefield. Robin C H Cowley of Baldor Motors promotes some thought into the specifying of larger AC motors. While the term larger is relative, here it covers some motors below 75 kw and certainly those above.

Newer materials and processes have been incorporated into motor manufacturing that have made some older specifications obsolete, while newer components and devices offer better motor protection and service to the end user. The goal is to discuss the more common features of electric motors that require the thought and recognition of the OEM and end user alike, in order to specify the correct solution. It should also follow that in specifying the correct product and options, the best value and shortest manufacture/delivery time should be achieved.

Bearing system

The number one failure mode with electric motors is the bearing system (over 60%). Bearing failure reasons include improper lubrication, too much or too little grease and using greases that are not compatible with each other. Contamination is another leading cause for bearing failures, promoting the importance of bearing seals.

When developing bearing specifi-cations for electric motors, care must be exercised in specifying actual operating conditions so motor manufacturers can select the best bearing system.

For most motors, it makes a difference whether the load is direct coupled or belted. Special high strength shaft steel may be required for heavy overhung loads. Applications that place high axial thrust loads on motor shafts require a different bearing system. Mounting orientation of the motor should be specified, as this needs to be considered on large motors. Vertical pump motors often have a unique bearing system to handle severe axial loads. Some specifications allow a bottom thrust bearing and others only allow a top-mounted system. Shaft thrust in both directions should be noted for these applications. Many of these motors are oil-bath lubricated. Larger sleeve bearing motors have their own set of requirements depending on the design of the motor and its speed.

Bearing seals

Contamination of bearing systems by liquids and solids can be prevented by the use of slingers, labyrinth and contact seals. Simple, flat "rubber" slingers are being replaced by newer designs that incorporate a v-ring seal. This type of slinger has a lip that seals against the motor end bracket when at rest. Centrifugal force raises the lip when the motor comes up to speed, resulting in a non-contact slinger. These newer devices are rapidly replacing older slingers.

Bearing isolators (rotaing labyrinth seals) are often used on motors where additional protection is needed from dust, liquids and slurries. As these isolators rotate, they expel liquids and keep them from entering the bearing cavities. A labyrinth device normally affords optimal protection whether the motor is stationary or running. Most motor manufacturers install labyrinth seals on the output and fan-end shafts of IEEE 841-2001 (IEEE standard for petroleum and chemical industry — severe duty totally enclosed fan-cooled (TEFC squirrel cage induction motors — up to and Including 370 kW (500 hp) compliant motors.The IEEE 841 motor standard is designed for motors built to NEMA standards and does not translate well to IEC style motors.

A common practice on IEC motors is to use contact seals to provide a solid barrier to prevent anything from entering the bearing system. These have a lip that is in continuous contact with the rotating shaft. The lip can be damaged if allowed to run without lubrication. Abrasive contaminants can also cause the lip to fail prematurely. Lip failure leads to contaminants entering the motor and results in bearing failure. There are some newer designed contact seals available that minimize these problems. For severe applications, these lip seals are not a good solution as the rotating labyrinth sea.

When motor specifications are written, more complete details of application data and environmental issues will be most useful to the motor manufacturer. Knowing pulley and belt data, special thrust requirements, or the existence of severely contaminated environments provides essential data allowing the motor manufacturer to design the best motor for the job.

Anti reverse rotation

Some pumping applications may require the specifying of an anti-reverse ratchet on the motor to prevent reverse rotation in the stopped condition.

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