Gain And Stability - PowerTec 2000C Instruction Manual

which will be close to the best value for that size unit. The
Trip and Horsepower Calibration Resistor values may
vary for the various sizes for the following reasons:
It is the nature of Brushless DC motors that the
amount of current drawn by any given Horsepower and
Voltage rating will vary somewhat with the RPM's of the
motor. For this reason, the Horsepower Calibration Resis-
tors of units shipped from the factory may vary.
The hall effect sensors on the Capacitor Board may
vary, not so much from unit to unit, as from batch to batch.
Each motor control is calibrated at the factory and its
calibration resistor is installed under actual load test.
Trip and current feedback calibration adjustments
on the Capacitor Board are also set at the factory.
DO NOT CHANGE THE VALUE OF THE HORSEPOWER
CALIBRATION RESISTOR WITHOUT CONSULTING THE FACTORY.
THERE ARE MANY FACTORS INVOLVED IN THE CHOICE OF THE
CALIBRATION RESISTOR.
The current limit potentiometer may be adjusted
anywhere from 0% to 100% of its rotation. When the pot
is at 0% (full counter-clockwise), the current limit is 0%.
Full clockwise rotation gives 150% current. The current
setting is linear through the rotation (i.e., 50% rotation is
about 75% of full load current).
Current limit in the motoring mode will result in the
motor finding a speed (which may be zero) at which it can
maintain the load at the torque level allowed by the current
limit. It will continue to find whatever speed it can main-
tain as long as it is in current limit, and that speed may vary.
When current limit is reached, the control is putting
out all of the current allowed. If current limit is reached
frequently, other than during periods of rapid acceleration,
the motor and/or the control size should be increased.
DO NOT ATTEMPT TO SOLVE THIS PROBLEM BY
CHANGING THE CALIBRATION RESISTOR. THIS MAY
CAUSE DAMAGE TO THE MOTOR AND/OR THE CONTROL.
A condition which gives a current limit indication,
but is not current limit, occurs when the motor is running
as fast as its counter-EMF (CEMF is the voltage generated
by the motor) will allow. This condition normally occurs
with no load or a light load, and, most of the time, when the
motor is cold. As the speed command increases, the motor
will not speed up any more and the current limit light
comes on, though little current is being drawn.
This condition is referred to as running against the
bus because the motor speed is being limited by the
available bus voltage. A cold motor may do this below
base speed, because a cold motor produces more CEMF
(and more torque per amp also). After a cold motor warms
up, its speed will increase.
If a warmed up motor appears to be up against the
bus, turn down the MAX SPEED until the current limit
light goes out and recheck the speed.
© copyright 1992, 1996 by Powertec Industrial
Motors

5.4 GAIN AND STABILITY

A certain level of current must be drawn from the
control by the motor to develop the torque required to turn
the load at any given speed. The POWERTEC Brushless
DC motor control works on the basis of balancing the
number of reference pulses against the number of feedback
pulses from the motor. The output current from the control
is directly proportional to the number of pulses in an up/
down counter which accumulates the difference between
the number of reference and feedback pulses. This differ-
ence is between the NUMBER of pulses, NOT the fre-
quency of the pulse train. The counter will accumulate
pulses until it has developed the current which the motor
is demanding to satisfy the speed loop. The accumulated
pulses will remain in the counter until the load changes.
Each feedback pulse from the motor represents 3 degrees
of shaft rotation on a four pole motor.
Once the pulses necessary to develop the current are
accumulated, each one of the reference pulses must be
answered by a feedback pulse from the motor. If the load
is decreased, torque required is decreased, and the drive
responds by shedding some of the accumulated pulses.
Increasing the GAIN pot reduces the number of
pulses needed to reach the required current level. This
shows up as a more responsive control since the effect of
each pulse is more pronounced. When the GAIN pot is set
fully counter-clockwise, the number of pulses needed to
reach current limit is about 90. Each pulse accumulated
will increase the current output of the control by 1/60th of
the full load current level. If the GAIN pot is turned fully
clockwise, only about 10 pulses will have to accumulate
before the control reaches current limit. Then each pulse
will increase the current output of the drive by 1/10th of the
current limit level. Only six or seven pulses will be
required to reach full load.
While this may seem desirable from the standpoint
of having a tight control, it produces a lot of cogging in the
motor shaft at low speeds, particularly on unloaded or
lightly loaded motors. The large change in current level
causes the motor to overshoot (it only has to go a few
degrees to overshoot). This overshoot causes the counter
to count down to zero. Then it has to wait for more
reference pulses before it can accumulate enough pulses to
put out current again.
Experience has shown that in single motor applica-
tions a setting of about 50% on the GAIN pot gives good
results in ANALOG mode. In DIGITAL mode higher
settings may be needed so that the input pulses may be
more losely tracked.
The STABILITY adjustment is a dynamic adjust-
ment. In general, as the GAIN adjustment is turned higher,
the STABILITY adjustment must be increased. Lower
settings of the STABILITY will produce a jittery opera-
tion while higher (CW) adjustments will produce sluggish
operation.
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