Page 342 - Compression Machinery for Oil and Gas

Drivers Chapter 7 327

TABLE 7.1 Summary of the Performance of NEMA Motor Classes

Class A Class B Class C Class D
Type General General High starting Very high
purpose purpose torque starting
torque
Start torque 100% rated for 100% rated for Approximately >275% rated
larger motors, larger motors, 250% rated
200% rated, 200% rated,
smaller motors smaller motors
Start current 800% rated 500%–600%
rated
Pullout 200%–300% 200% rated Slightly lower
torque rated than class A
Pullout slip <0.2 <0.2 High, can be
as much as
1.0
Rated slip <0.05, lower Must be <0.05, <0.05, higher High,
than similar usually <0.03 than class B typically
sized class B 0.07–0.11,
can be up to
0.17
Applications Fans, blowers, As for Class A Compressors, High inertia
pumps, pumps, applications,
machine tools conveyors for example,
mechanical
punches
Notes High starting Replacement Applications Very high
inrush current for Class A due that require high inertia
causes power to lower start start torques. applications.
system current. The Note that the For example,
problems, it can standard off-the pull-up and in a punch or
cause the shelf pullout torque reciprocal
supply voltage commodity can both be pump where
to sag and motor lower than the the slip may
requires special start torque. vary between
starting Less efficient 0 and 0.50.
techniques. than class B Much less
More efficient efficient than
than same sized other designs
Class B