Stepper motor current, peak or RMS?
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I was reading the specification of 6HC and am trying to understand the peak vs RMS stepper current rating.
"6 High Current Trinamic 5160 stepper drivers running at up to 4.45A RMS, 6.3A peak with firmware control of the full feature set including up to 256 microstepping, StallGuard 2 and StealthChop2"
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If we use M906 to set a peak current of 6.3A and the motor stops at a peak current microstep, will this result in a 6.3A DC current (=6.3A RMS) which is beyond the driver's spec?
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When a stepper vendor specifes the max motor current, is this typically peak or RMS? E.g. this is what StepperOnline say
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The failure of stepper companies to specify whether the are measuring peak or RMS current is always annoying.
The good news about your scenario of stopping right on the peak current in one phase is that the current in the other phase is at zero. The total power dissipated in a microstepped motor is constant, since the currents go as sine and cosine, and the sum of their squares never changes. There should be plenty of heat conduction between the windings, so it should be no problem to land and stop right on a peak. Also, typically one sets the driver board to turn the current down after a few seconds when you are parked, anyways, so any excess power just disappears into the thermal mass of the motor, causing a small step in the temperature of the one winding until the driver goes into park mode.
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As @mendenmh says, if you set the peak current to 6.3A then the standstill current will be reduced by the standstill current reduction factor set. The default to 75% but you can reduce it using M917. If you set the current close to the maximum supported by RRF, then RRF will enforce a maximum standstill current percentage.
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The rated current in stepper motor datasheets is normally per phase with both phases energised. That's roughly equivalent to a RMS current rating when the stepper motor is moving.
If you want to run a stepper motor at maximum current, then you can set the peak current in M906 equal to sqrt(2) of the rated current, but IMO you should also set the standstill current to 70% or lower so as not to exceed the per-winding rating at standstill, because I do not consider the heat conduction between the two windings to be good enough..
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@mendenmh, your analysis seems correct to me but it focuses on extra power on the motor, not the driver, which was my concern. @dc42 addressed the driver side concern using the automatic current reduction on 'standstill' (not to be confused with 'idle') which should avoid the long time peak current through the driver. That's a good thing.
I wonder if the standstill detection will kick in in the case of continious fast transitioning between two adjacent close-to-peak microsteps, e.g. 7, 8 with x16 microsteping, but this is probably not a real life scenario and if happens the driver will still protect itself from thermal runaway.
