Subject: Stepper motor...applying a reduced phase current
Date: Fri, 8 Nov 2002 09:05:57 +0000 (UTC)
Organization: BT Openworld
NNTP-Posting-Date: Fri, 8 Nov 2002 09:05:57 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 5.50.4807.1700
We are using a type 17 stepper motor( 6 lead, 0.4A/phase, 15.6R phase
resistance) with the L297/L298 combination controller and driver.
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. The L297 uses a comparator to determine applied
winding current, based on sense resistors and a reference voltage. If, in a
bid to reduce the static position phase current and the demands on the power
supply, I attempt to decrease the reference voltage to lower than the sensed
voltage (sensed phase current), then the Vref and current sense inputs
become 'noisy' , indeed the circuitry as a whole is 'noisy', the motor
although it maintains its static position, is vibrating (unstable) and the
phases appear to be being turned on and off.
THE QUESTION...under normal conditions, is it actually possible to provide a
minimal holding torque by reducing the applied phase current to below that
of the nominal winding current, in this case 0.4A/phase, without the 'noisy'
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
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 5R
resistors are employed with
a reference voltage for the comparator set at >2V (max phase current =
0.4A). The chopping frequency is about 20KHz.
I hope someone can help.