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WSega.1
WSega.1
Associate II
September 2, 2021
Question

RPM Limit on Sense Resistor Change

  • September 2, 2021
  • 3 replies
  • 1651 views

I have a 2 pole pair BLDC 24V 2A motor capable of 49kRPM. I used the 5.Y.2 Profiler to create a motor profile and the 5.Y.1 to import the motor profile into a SPIN32G4EVAL project. Using the default 5mohm sense resistors, I am able to spin the motor up to 38kRPM without issue in Pilot. When I change the board to 50mohm sense resistors and change the profile to match, I can start the motor in Pilot but when I command speeds above 11.3kRPM the mechanical speed max's out at 11.3kRPM and it doesn't error. I'm seeing less than 400mA of draw on the 24V so it is hard to imagine the current is out of range. Question 1: What could be causing this RPM limit with a larger sense resistor?

I'm not sure if it is related, but I also noticed the default resistor values in the gain for the sense resistors don't match the SPIN32G4EVAL board. I've attached a screenshot of the default but the actual values are 1.5k/11k.

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3 replies

cedric H
cedric H
Technical Moderator
September 3, 2021

Hello,

Could you detail what you actually did when you write : " I change the board to 50mohm sense resistors and change the profile to match".

Thanks

Cedric

WSega.1
WSega.1Author
Associate II
September 3, 2021

Sure. On the SPIN32G4EVAL board, I removed R31-36 and put a 50mohm power resistor in the R31, R33, R35 locations. Then in Motor Control Workbench 5.Y.1, I click on the "Current Sensing" module and change "Shunt resistor(s) value" to 0.05 ohm. I then click on "calculate" to make sure the 0.05 ohm gets updated into the gain calculation. (that where I notice the resistor values in the project do not match the board. Side note: correcting the resistor values only moved the RPM limit slightly and did not resolve the issue). With the 0.05 ohm built into the project, I generate the project, compile with STCubeIDE, and test with Pilot 5.Y.1.

WSega.1
WSega.1Author
Associate II
September 3, 2021

Here is another screen shot of the current sense change in MCW.

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