PowerTec 2000C Instruction Manual page 18

© copyright 1995 by
Powertec Industrial Motors
Frequencies from 2 kilohertz to 20 kilohertz or more are common.
There is also no need to wait for the power line conditions to shut off
the transistor; it may be cut off at any time.
As the field continues to move around the stator, the rotor
follows it. Unlike the induction AC motor in which all windings are
continuously excited, the Brushless DC stator is a DC excited field,
and moves because the windings are continuously switched in re-
sponse to the movement of the rotor. The non-excited windings are
carrying no current, since the windings in a Brushless DC motor are
either on or off. They do not go through the slow transitions that the
AC motor windings go through. If winding switching were to stop, the
motor would come to a halt.
The switching of windings is controlled by a three channel
position encoder mounted on the motor shaft. The hall effect transis-
tors which are mounted on the feedback assembly are turned on and
off by magnets in the encoder's "xolox wheel". These magnets are
aligned with the magnets on the rotor. The hall effect switches, which
are non-contact (electrically isolated) devices, are mounted in posi-
tions 60 degrees apart ( on a four-pole motor, they are 30 degrees
apart on an eight pole motor). It is not possible for the three switches
to be all on or all off at the same time. There will always be two on
and one off, or one on and two off.
The speed of POWERTEC's four pole Brushless DC motor is
regulated by a two channel, 30 pulse (60 pole) digital tachometer on
the motor shaft, which indicates both speed and direction of rotation.
Eight pole motors (frames 280 and larger, except over 2500 RPM)
have 120 magnetic poles around the outside of the xolox wheel. These
poles alternately turn on and off two hall effect switches which are
connected to the control for speed control. There are two speed
feedback channels which are offset by 90 electrical degrees, in
quadrature.
These 30 pulse-per-revolution signals from the motor are
electronically multiplied by 4 to supply a pulse every 3 degrees of
shaft rotation (larger motors use a 60 pulse tachometer, yielding 240
pulses per revolution, a pulse every 1.5 degrees). Reference pulses are
compared to pulses coming back from the motor's encoder. If the
number of pulses from the motor (the absolute number, not the
frequency) do not equal the number of reference pulses applied, a
position error count accumulates in an up/down counter on the Speed
Controller board. The number of accumulated pulses determines how
much current is made available to the motor. If there are more refer-
ence pulses than feedback pulses, the accumulation is positive, and
the current to the motor is positive, i.e., motoring current. This
positive current will try to accelerate the motor to eliminate the pulse
count. If there are more feedback pulses than there are reference
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