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VESC 6 motor setup: Resistance measurement jumps

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thomasjfox
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VESC 6 motor setup: Resistance measurement jumps

Hi,

when doing the motor setup on a VESC 6.41 using firmware 3.29, the detected resistance sometimes jumps up and down.

For my "250W" ebike motor the resistance is detected most of the time as 126 mOhm. On some invocations of the detection, the measured value is 107 mOhm. On the next invocation it's 127 mOhm again.

 

Ist that to be expected? Does it make a difference at all?

The motor is currently mounted with the complete wheel, though it can spin freely. Control using 126 mOhm is just fine.

Cheers,
Thomas

benjamin
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Joined: 2016-12-26 15:20
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Do you use different current limits and/or a different motor temperature when you measure the resistance? One problem with a small ebike motor is that when you set the current limit to e.g. 15A only 5A are used to measure the resistance. 5A compared to the 160A the current measurement is designed for is quite small, so if the accuracy and offset at 160A is e.g. 1% this will be amplified to 30%. One thing you can do to measure the resistance is using the terminal function and specify the current to use, e.g. 10A.

However, if your motor runs fine this is not a problem at all. Above 10% or so of full speed the tracking of the observer should be almost spot on even if the parameters are a bit off, so you only have to investigate this if you get bad startup and low speed performwance.

thomasjfox
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I've set the motor current to 45A and did the measurement again. Most of the time it gave me a resistance of 114 mOhm. One time it was 106 mOhm, but I ignored that value.

The new value of 114 mOhm gives much better startup in FOC mode without sensors at a small hill. With sensors it's smooth.

Thanks for your help, matter solved.