Introduction - PowerTec 2000C Instruction Manual

1.1 BRUSHLESS
MOTOR
TECHNOLOGY
© copyright 1995 by
Powertec Industrial Motors
Traditional AC induction motors must "slip" (fall behind their
natural "synchronous" speed) in order to develop torque. The synchro-
nous speed is determined by the frequency of the power at the motor
terminals. At 60 Hertz (the power line frequency in the USA), a four
pole AC induction motor will have a synchronous speed of 1800 RPM.
With a full load on the motor, however, the motor will only be turning
at 1740 RPM (curve A in Figure 2), due to the necessity of slip to
develop torque.
An AC induction motor running near synchronous speed does
not develop any
torque. The amount of
slip varies with the
amount of torque
required from the
motor. Since slip is a
percentage of the
operating speed, and
the amount of slip
varies with the load, it
is difficult to predict
the speed at which an
AC induction motor
will run under any
given set of operating
conditions whether it
is operated across-the-line or on the output of a variable frequency
control. It is very difficult to maintain an exact speed when operating
under varying load conditions. Extraordinary means must be used to
employ AC motors in speed sensitive applications.
Inverters do not help much with AC induction motors, but they
do improve speed regulation a little by increasing the applied frequency
as the load increases (curve B in Figure 2).
Traditional brush-type DC motors are operated using mostly
solid-state AC to DC power converters which have inherent limitations
on their ability to provide power when and as needed. At best they can
supply power only 360 times per second with three phase input power,
or 120 times per second with single phase input power. Worse, the
brush-type DC motor is a self commutating device which uses the
applied power inefficiently, and loses some of the power on the way to
the place where it is applied to work.
The speed of the DC motor is determined by the voltage applied
at the brushes where they come in contact with the commutator, which
is a few windings, and many voltage drops (known as IR losses), away
from the output terminals of the motor control, where the motor's
CEMF (armature voltage) is often used to regulate speed. Since the

INTRODUCTION

Figure 2: Comparison of speed regulation of typical
industrial motors from no load to full load.
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