From: "John Jardine"
Subject: Re: Stepper motor...applying a reduced phase current
Date: Fri, 8 Nov 2002 15:08:14 -0000
NNTP-Posting-Date: 8 Nov 2002 14:57:19 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
MarkP wrote in message
> 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
> 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
> exhibit any overheating. The L297 uses a comparator to determine applied
> winding current, based on sense resistors and a reference voltage. If, in
> bid to reduce the static position phase current and the demands on the
> supply, I attempt to decrease the reference voltage to lower than the
> 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
> 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
> 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
> the comparator or to use the inherent set dominance of the flip-flop to
> out the spike. I've tried increasing the chopper frequency and including
> 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.
When the L298 is running correctly the motor *will* be hot to touch. This is
The 15V supply is *way* too low for this particular motor and is the reason
the motor is running underpowered (cool) at higher speeds (L/R time
constants). Take the supply up to the chip maximum of 40V. At the moment,
the chopping supply voltage is so low, as to make the cost and complexity
of the bipolar chopping not even worthwhile.
The vibration and noise are down to a process known as 'sub-cycling' where
the coil current 'skips' an occasional charge cycle (or more). Results in a
plethora of sub-harmonically linked switching frequencies. This problem is a
function of ... winding inductance, supply voltage, coil series resistance,
operating frequency, phase/enable chopping. This *is* a common problem and
many people put up with it, because it does not in most cases cause
operational problems. It "looks" like noise on the threshold comparitor but
*isn't*. There is no easy cure other than to try changing the variables one
(or more) at a time. You will find little info in the data-sheets.