We are using the MAE type 17 stepper motor( 6 lead HY200-1713-040-B6
0.4A/phase, 15.6R phase resistance) for development, previously we were
using a unipolar
driver, namely a UCN5804B device from Allegro, but with the documented lack
of power efficiency, we decided to develop a bipolar drive. I used the
L297/L298 combination, as I felt this would give me a better grounding in
how the bipolar chopper functioned, layout requirements, power dissipation
that sort of thing.
The issue...currently when the motor is in a static position, a holding
torque applied, the motor is starting to overheat, hot to touch. However,
when the
motor is moving (typically at 600 steps/second) the motor behaves as it
should and does not
exhibit any overheating. Allied to this, I am trying to reduce the current
consumption at the static position to decrease the demand on the power
supply, yet
provide enough holding torque, this should also reduce the overheating!
If, in a bid to reduce the static position winding current and the demands
on the power supply, I attempt to decrease the Vref (compared with sensed
voltage
to control chopping) to near or lower than the sensed voltage
(sensed winding current), then the Vref and current sense inputs become
'noisy' and although the motor maintains a holding torque, albeit with some
vibration, the
phases are now being turned on and off.
How can I effectively reduce the static position winding current and thus
the overheating?
The datasheets talk about chopper stability and that if spikes on the
sensing inputs exceed the Vref, then the comparator can be fooled into
resetting
the flip-flop; it describes including an RC on the sense input to the
comparator or to use the
inherent set dominance of the flip-flop to mask out the spike. I've tried
increasing the chopper frequency and including an RC at the inputs to the
L297 from the sense
resistors, without any noticeable effect.
Further information...we are using a 15V power supply for the motor. The
controller from start is always enabled, we're in half step mode using
Enable Chopping (not phase
chopping). For the current sense, a pair of 0R5 resistors are employed with
a reference voltage for the comparator set at >0.2V (max phase current =
0.4A). The chopping frequency is about 20KHz.
I hope someone can help.
