Brushless Motor Technology - PowerTec 2000C Instruction Manual

1.1 BRUSHLESS MOTOR TECHNOLOGY

Traditional AC induction motors must "slip" (fall
behind their natural "synchronous" speed) in order to
develop torque. The synchronous 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 1), 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
Figure 1: Comparison of speed regulation of typical
industrial motors from no load to full load.
means must be used to employ AC motors in speed sensi-
tive applications.
Inverters do not help much with AC induction mo-
tors, but they do improve speed regulation a little by
increasing the applied frequency as the load increases
(curve B in Figure 1).
Traditional brush-type DC motors are operated us-
ing 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 really counts.
© copyright 1992, 1996 by Powertec Industrial
Motors
1.0 Introduction
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 voltage differences between the brushes and the motor
control output vary with load, speed varies with load.
Extraordinary means must be used to employ DC brush-
type motors and solid-state SCR controls in speed sensitive
applications.
Even when a DC tachometer is used with a brush-
type DC motor to regulate speed (curve C in Figure 1), the
tachometer only reads an AVERAGE speed, and the reso-
lution between speeds limits the response.
The Brushless DC motor and control system over-
comes these problems to provide smooth and efficient
power and speed control. Its speed regulation is not mea-
sured in percentage of RPM, as is the case with other types
of motor and control systems, but in physical shaft position
within a single revolution. With a brush-type DC motor and
solid state SCR control, speed regulation of +/-0.5% might
be obtained with a very expensive tachometer. This means
that a motor set for 1750 RPM may be operating anywhere
between 1741 RPM and 1759 RPM and still be within
specifications. A brushless DC motor set for 1750 RPM
must run at 1750 RPM, period. The only alternative is to be
in current limit. Short of current limit, the brushless D.C.
motor may not be more than 240 degrees behind its no load
shaft position with the standard control set at its minimum
gain. The Brushless DC motor control does not look at its
speed, it looks at where the motor shaft is in relation to
where it should be.
The Brushless DC motor control also can supply
power when and as needed at a rate which more than five
times the solid-state SCR control's line limited rate.
All of this comes in a compact package which will
exceed the performance of across the line operated AC
motors, AC adjustable frequency operated motors, or the
conventional DC brush-type motors.
POWERTEC's Brushless DC motors have the high
reliability and low maintenance requirements which have
always been associated with AC induction motors. Brush-
less D.C. motors have speed and torque control which are
superior to the traditional brush-type DC motor, plus a very
high level of efficiency.
The Brushless DC motor and control give your
industrial applications outstanding performance, long life,
and very efficient service in the the most demanding
industrial conditions.
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