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Driving PMSM with ESC
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Circlotron:
I have a 3-phase 6 pole permanent magnet synchronous motor that I want to drive from a cheap ebay electronic speed control. Not to do anything useful, just to see what happens. The motor has 3 hall effect sensors but I might try a sensorless controller. Motor is about 105m long and 60mm diameter. Came out of an automatic sliding door.

I rotated it in a drill and it generated 17.85Hz and 5 volts. Say I want to go to much higher speeds. If I have a supply of 20VDC to the ESC, does this mean I can't or shouldn't rev the motor faster than the rpm at which it would generate 20 V peak AC?
Siwastaja:
With the standard "optimum" 90 degree advancement, which most inverters, in most operating modes, try to tightly follow in a closed loop, the motor's back-EMF limits the current you can push into it - this basically means that available torque falls to zero when you hit the RPM you measured the "open circuit generator" voltage with. Including both mechanical and electrical losses, the actual RPM will be less. This is analogous to a circuit where you are charging a capacitor with a 20V voltage source - the current you can push through the parasitic resistances drops and finally is zero at 20V, so you can't charge it to any higher voltage (~ rpm).

There is a remedy, namely field-weakening mode, which means deviating from the optimum 90 degree advancement and going over it, causing large reactive currents, which cancel out the permanent magnet field, reducing the RPM/V constant of the motor temporarily. This is only relevant when you need high speeds with small torque. It's likely the ESC won't perform in this mode, as their typical intended usage is driving propellers, meaning the mechanical load increases with speed, necessitating increasing torque as well, so compromising the torque would be a dead end.

Try it out - I'd guess your RPM will be somewhat limited, and the best way to overcome this is increasing DC bus voltage, not driving in field weakening mode.
Circlotron:

--- Quote from: Siwastaja on December 20, 2018, 07:29:06 am ---the motor's back-EMF limits the current you can push into it - this basically means that available torque falls to zero when you hit the RPM you measured the "open circuit generator" voltage with.

--- End quote ---
Cool. Thanks.
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