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I've designed a 3 phase inverter for driving a BLDC hub motor. The design (if anyone wants to look at schematics, is here). This works well, the motor spins up and down fine, no worries.

However, if I try to hold the wheel/motor (the wheel is 65cm in diameter, if that helps), applying torque, when it's not spinning, and I attempt to increase the throttle, the motor makes an odd screeching noise, and doesn't spin. Likewise, if the motor is spinning and I load the wheel, I see the current on the power supply increase (as you would imagine) but I can keep applying a load to the wheel and stop the wheel.

I bought the motor from a conhis motor company about 5 years ago, it's no longer on their site that I can see, but I did take screenshots when I bought it - images of torque curves, etc

My question is, have I designed the three phase controller wrong (perhaps the code I've written needs to slowly increase the duty cycle to the inverter, or needs to do it faster? Currently, I've implemented a limit controller such that the duty cycle can't increase more than 1% per interrupt of a timer). Is there something fancy that normal ebike controllers due when starting a motor?

Or, and I fear this maybe the problem, is the motor that I have chosen not the correct choice? Looking at the torque curve, at 0 RPM, there's no torque available. This isn't a showstopper (given that on the bicycle, I will be peddling first before using the motor).

Thanks very much for reading through, I appreciate the help. If there's any more information I can provide, please let me know.

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  • \$\begingroup\$ There is no evidence here that anything is wrong. Torque at 0 rpm shouldn't be 0 and obviously isn't because you can start the motor unloaded, but there is no torque curve in the data. And we have no idea of the current when you stall it. \$\endgroup\$
    – user16324
    Sep 16, 2021 at 11:46
  • \$\begingroup\$ I'm not sure what you mean - in the "images of torque curve" there's a graph of RPM vs Torque? The current when I held it stalled, with 20V supply voltage, was <1A, maybe even around 0.5A. Current limit was set to 2A, so there was amble current available. \$\endgroup\$ Sep 16, 2021 at 13:11
  • \$\begingroup\$ You need to revise the question to include the voltage of the power supply and the current that you observed, particularly when you are holding the motor from turning. If you look at the red speed vs torque curve, you can see that the speed should be nearly 490 RPM at no-load, zero torque, and drop linearly as torque increases to below 350 RPM at 25 N-m torque at the edge of the chart. Torque and current would need to be very high if you extend the speed vs. torque curve linearly off the chart. Limiting the current would make the curve drop vertically at the limit value of torque. \$\endgroup\$
    – user80875
    Sep 16, 2021 at 13:16
  • \$\begingroup\$ If the motor controller current is set to 2A, perhaps the power supply has a current limiter. 2A is a very low setting for a motor designed to draw 24 amps. The motor is operating so low on the curves that limiting at 1A rather than 2 may be in the normal range of accuracy. \$\endgroup\$
    – user80875
    Sep 16, 2021 at 13:23
  • \$\begingroup\$ That would have to be the first time I've the use of current mirrors as the I2C pullup! Got to ask why? 2k2 resistors would do just fine. \$\endgroup\$
    – Kartman
    Sep 16, 2021 at 13:28

1 Answer 1

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If you look at the red speed vs torque curve, you can see that the speed should be nearly 490 RPM at no-load, zero torque, and drop linearly as torque increases to below 350 RPM at 25 N-m torque at the edge of the chart. Torque and current would need to be very high if you extend the speed vs. torque curve linearly off the chart. Limiting the current would make the curve drop vertically at the limit value of torque.

There may be something unexplained between the power supply current limit performance and the motor controller performance. However your test has the power supply voltage less than half of rated motor voltage and current limited to less than 10% of rated current. The results seem to indicate that the motor and controller are working as well as can be expected.

You could attach a torque arm the to motor and measure the stall torque with a scale, perhaps a bathroom scale or a luggage scale. The next step should probably be to test with a higher current.

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