TMC2100 driving dual Z - thermal shutdown?

I have a Tevo Tarantula-style printer with dual Z (two identical motors connected to a single TMC2100 in parallel) with MKS Gen 1.4 board.

With Vref set to 1.2V, making RMS 1.2V*0.7A/V = 0.84A, which does not seem excessive.

Symptom: about 20-30 minutes into the print, Z axis stops moving. It's pretty easy to rotate by hand, you can feel the steps, but not much resistance.

Hypothesis: thermal shutdown?

Head scratchers:
1. Cancel the print, then immediately go to Control and move Z axis, it moves as if nothing was happening!
2. I run X and Y closer to 1.4V, and their motors get to 45C with no ill effects. Z motors are barely 28C (but there are two in parallel, remember!)
3. Measuring temperature optically through the cooling fins reports ~27C on Z driver, 28-29C on X and Y. Does not look anywhere close to panic levels.

Any ideas?

Two motors parallel might produce more noise (EMC-noise ) on the motor wires than the TMC2100 can take?
You can try to wire them in series or use a different driver like A4988.

But I'm just wild guessing....

I almost convinced myself it was thermal. Vref was at 1.6V - over the "1.5 max / 1.3 max recommended", so I was running in the red zone. Backed to Vref=1.3V, works fine for now. The motors are dead cold, but there is almost no torque required to move that Z carriage anyway. I was not solving any real problem by increasing that Vref.

Steppers will get hot, no matter if they stand still or move. Yours are cold, because they share the current by running them parallel.

You haven't told us what Z motors you are using; but the types of motor typically used in 3D printers are better connected in series instead of in parallel, so that you can more easily drive them at a suitable current.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].