Page 348 - Improving Machinery Reliability

314 Improving Machinery Reliability

TYPICAL TORQUE VS. SPEED CURVES
NEMA DESIGNS A, I), C, D, E
‘050%

300%

250%
3
u 200%
E
2
k! 150%
r;
K
8 100%
8
50%

Figure 6-1. Typical torque vs. speed curves for NEMA designs A, B, C, D, E.

Squirrel Cage Motors Are Most Prevalent

The stator of a squirrel cage polyphase motor is comprised of a group of coils of
insulated wire that create a rotating magnetic field as the polyphase (three-phase)
alternating current rises and falls within the coils. The rotating magnetic field also
induces currents and magnetic fields in the rotor, which is mounted on bearings and
is free to turn. The attraction and repulsion of the magnetic fields in the rotor and
stator create the torque that causes the rotor to turn.
While most of the current is converted to mechanical energy needed to drive the
connected equipment, some of it is converted to heat and referred to as “watt losses.”
This heat must be removed by the motor ventilation system. Heat is the enemy of the
motor insulation and can also curtail bearing life. The condition of both insulation
and bearings essentially governs the life of the motor.
Motor life can be maximized by protecting the insulation through reducing watt
losses and/or improving the ventilation system. Similarly, bearing life can be extend-
ed by maintaining an adequate amount of uncontaminated lubrication.

Motor Insulation Systems

In addition to the polymer coating that insulates the coiled magnet wire turns from
each other, insulation materials are applied to prevent the current from flowing into