How to interpret stepper driver current specification?

zapta

Hi everybody, I am trying to undestand stepper motor driver specifications. For example the Duet Mini specification says:

"5 Trinamic 2209 stepper drivers running at up to 1.4A RMS, 2A peak with firmware control of the full feature set including up to 256 microstepping, StallGuard 2 and StealthChop2"

If the motor stops at a half step, the current is 2A constant (DC) and so it the RMS. Will this over spec RMS damage that driver channel? (and if not, why to specify RMS in the first place?)
The stepper below has max current specification of 2A. Does this mean peak or RMS? (I presume peak).

https://www.omc-stepperonline.com/nema-17-bipolar-59ncm-84oz-in-2a-42x48mm-4-wires-w-1m-cable-and-connector.html?search=17HS19-2004S1

dc42

@zapta it's complicated. Stepper motors are usually (but not always) quoted with the rated current applied to both phases. But microstepping drivers only ever apply the peak current to one phase at a time.

If your motor is rated for 2A in both phases, then it's definitely OK to run it at 2A peak. One phase will get 2A while the other gets nothing, or both phases will get 1.414A, or somewhere in between.

When the motor it running at a reasonable speed, it's also safe to run it at 2.8A peak (2A RMS). This is because the total power dissipation will be the same as with 2A in each phase. The current won't always be split evenly between the phases, but this doesn't matter as long as the motor is turning, because the current will switch from one phase to the other.

What I would not do it run the motor at 2.8A in standstill. That could result in 2.8A in one phase, and nothing in the other. Even though the total power dissipation would be the same, I do not regard it as safe to dissipate all the power in one phase. However, he TMC2209 drivers can automatically reduce the current when the motor is standing still or turning slowly.

Note, if you do run the motor at its full current, it will get very hot. For 3D printer use, a rule of thumb is to set the peak current to around 85% of the rated current.

As for peak vs RMS, the convention in 3D printing always used to be that motor currents were expressed as peak current, because that is what the stepper driver chip manufacturers always quoted until Trinamic arrived. So that is how M906 was defined for the Duet 06 and 085, and we followed that convention on subsequent Duets.

HTH David

zapta

Thanks @dc42. If I got it correctly, there are several constraint on the current, both per channel and combined and momentary and aggregated and all need to met. That makes sense.

In my case, it seems that the Min5+ and TMC2209 should be able to drive the 2A (peak) motor I plan to use (will set the gcode to peak current of ~80% of 2A).

As for TMC adding the RMS specification, I wonder if this just adds more confusion from user' perspective, since motors and gcode use peak specification and the per channel max RMS leaves open the half step issue.