Page 362 - Improving Machinery Reliability

328 Improving Machinery Reliability

Motor Life Insurance Terms

To maximize the life and performance of AC squirrel-cage induction motors in
process industry applications, one must have:

A motor with the proper design (A, B, C, D, or E) for the application
An enclosure properly configured to protect the motor from environmental conta-
mination
An adequately sized motor to handle the load requirements
A conservatively designed (or applied) motor that will allow it to work well within
its designed thermal capacity
A proper installation that assures good alignment, eliminates vibration, and does
not transfer unwanted external loads to the motor
An adequately designed and maintained (if required) bearing lubrication system
A motor environment that provides adequate air circulation and maintains the
effectiveness of motor heat removal or ventilation systems
A regulated power supply that supplies the correct voltage, maintains voltage bal-
ance between phases and provides protection against over currents
A load cycle designed to avoid frequent starts and other causes of motor overheating.

Notes
1. “Slip” refers to the difference between the synchronous speed (determined by the
frequency of the alternating current and number of motor poles) and the actual
full-load speed of an AC induction motor. Slip is required to induce current in the
rotor. A high-slip motor (Design D) produces higher torque at lower speeds than
other types of motors.
2. NEMA medium motors (open-motor Designs A, B and C; 1 HP to 200 HP) are
required to have a “service factor” [SF] of 1.15. This means that such a motor is
able to operate at 115% of rated load for a prolonged period while remaining with-
in the allowable temperature limits of the insulation system.