Rockwell Automation 20G PowerFlex 750-Series AC Drives - Manual (Page 237)

Rockwell Automation Publication 750-RM002B-EN-P - September 2013 237
Motor Control Chapter 4
Polyphase AC Induction Motors
Polyphase squirrel-cage AC motors are basically constant-speed machines, but
some degree of flexibility in operating characteristics results from modifying the
rotor slot design. These variations in AC motors produce changes in torque,
current, and full-load speed. Evolution and standardization have resulted in four
fundamental types of AC motors.
There are five basic NEMA designs for AC motors: A, B, C, D, and F. The speed-
torque curves for all five designs are shown on the following graph.
AC Motors - Designs A and B are general-purpose AC motors with normal
starting torques and currents and low slip. As shown, the characteristics of designs
A and B are quite similar. The primary difference between these two designs is
that the starting current for design B is limited by NEMA standards, but there is
no limitation on the starting current for design A.
AC Motors - Design C have high starting torque with normal starting current
and low slip. NEMA design C motor has a higher starting torque than either the
A or B designs. This torque is in the vicinity of 225% of full-load torque. Design
C AC motors are normally used where breakaway loads are high at starting, but
which normally run at rated full load and are not subject to high overload
demands after running speed has been reached.
AC Motors - Design D exhibit high slip AC motor starting torque, which is
approximately 280% of full-load torque, low starting current, and low full-load
speed. Because of the high slip, speed can drop when fluctuating loads are
encountered. The high starting torque of the design D motor makes it
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Full-Load Torque
Speed - Percent
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Speed - Torque Curves of NEMA A, B, C, D, and F Motors