PowerTec 2000C Instruction Manual page 27

The switching of windings is controlled by a three
channel position encoder mounted on the motor shaft. The
hall effect transistors 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 mag-
nets on the rotor. The hall effect switches, which are non-
contact (electrically isolated) devices, are mounted in
positions 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, at all times.
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
Figure 9: The encoder assembly - four magnets
indicate position; outer magnets indicate speed
and direction.
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 reference pulses
than feedback pulses, the accumulation is positive, and the
current to the motor is positive, i.e., motoring current. This
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positive current will try to accelerate the motor to eliminate
the pulse count. If there are more feedback pulses than
there are reference pulses, the count is negative, and the
motor current will be shut off (the motor will coast along)
until the accumulated count becomes positive again.
A motor running at no load will accumulate only a
few pulses of position error, but a fully loaded motor will
accumulate 2/3 of the pulses necessary for current limit.
Loads between no load and full load will accumulate some
number between 0 and the full load value. The maximum
number of error pulses which may accumulate is about 100
(this is an adjustable "gain" function) at which time the
brushless drive will be in current limit. When the maxi-
mum number of pulses are accumulated, the control is in
current limit and further reference pulses will be ignored
until the motor turns far enough to eliminate the excess
pulses in the counter.
As long as the current limiting condition is avoided,
the motor can track the reference frequency pulse for pulse.
IT IS IMPORTANT TO NOTE THAT, SHORT OF
CURRENT LIMIT, THIS PULSE ACCUMULATION
DOES NOT AFFECT SPEED. It only affects the shaft
position relative to the no load shaft position. Most motors
talk about speed regulation in terms of revolutions per
minute difference between set speed and actual speed. A
brushless DC motor set to run at 1750 RPM (with a
frequency reference), will be running at 1750 RPM, pe-
riod.
For motors operating at a steady speed and at a
steady load, the pulse accumulation will not change, stay-
ing at the value necessary to drive that load at that speed.
If the load should change, the pulse accumulation in the up/
down counter will change to a value necessary to drive the
new load. If the speed changes, the pulse accumulation will
not change if the torque required by the load is the same at
the new speed as the torque required at the old speed.
This is a brief, very simplified, introduction to the
operation of brushless D.C. motors. It does not and cannot
cover all the points necessary for a full or complete
understanding. There are books available on the subject of
Brushless DC motors.
POWERTEC manuals contain a great deal of infor-
mation and further understanding will come also with use
of the motors and controls.
Model 2000C / 3000C Manual