From: firstname.lastname@example.org (chris)
Subject: Re: Stepper motor...applying a reduced phase current
Date: 8 Nov 2002 08:57:49 -0800
NNTP-Posting-Date: 8 Nov 2002 16:57:49 GMT
"MarkP" wrote in message news:...
> 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
> 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 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.
Hi, have you checked the waveforms? your sens resistors seem a bit
high,a ref of around 0.5v-1.0v is more usuall.Circuit layout and
ground returns are critical.