Help me with estimating max current for BLDC motor
Tomasz Cybulski wrote 12/23/2020 at 12:56 • 0 pointsHi
I'm using a controller based on L6234D which has max current of 5A. But I can't figure out how to use that information for finding out max voltage I can use for my controller/motors.
The motors I have are gimbal motors with 10ohm per winding. I'm going to drive them with sinusoidal control (or FOC later). Does it mean that 24V should be ok even at full PWM cycle? Should I sum up current going through all 3 windings at any given time?
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There is a good diagram of the electrical and mechanical parameters you can expect over a load range in this post: https://electric-skateboard.builders/t/how-does-the-torque-curve-change-with-voltage/25760/3
It doesn't necessarily directly answer your question, which I think nqtronix already effectively did, but will give you a good idea of what will happen where with your motor as specifics speeds and torques.
As for the thermals, at a maximum 100% duty cycle switching you would have 2.26A^2 * 0.3 Ohms of dissipation, and I think my understanding of a three phase BLDC is that you would have two FETs on at any given time commutating current together so multiply by two, which is about 3W, not insignificant!. But maybe your duty cycle and actual current at a typical application load is so low that it doesn't matter.
EEVBlog has a great video on heatsink sizing if it turns out you need one. https://www.youtube.com/watch?v=8ruFVmxf0zs
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@nqtronix Thanks for the answer. Any suggestions how to cool these chips? Does it make sense to stick a radiator on top of a chip. On the board I'm using there's a cooling pad on the other side of the board connected with vias. Would it make sense to stick radiator there?
This is the exact board I'm using:
http://www.rctimer.com/rctimer-2-axis-brushless-gimbal-controller-amp-imu-v10-p0444.html
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It might be fine as-is. The driver has thermal protection so you can simply try without damaging anything. Note that the RDS_on increases with chip temperature (page 9) so some heatsinking will improve efficiency. If there are no components on the backside my preffered cooling solution it double-sided *thermal* tape + any heatsink you have on hand.
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https://simplefoc.com/
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Because the 0.3Ohm RDS_on is the resistance for each transistor (page 8), I assume the 5A (4A continous) refer to the per channel load. At 24V/10.6 Ohm I expect 2.26A, which should be well within the limits. Power applications are most often limited by the efficiency and thus heat generated, so make sure the cooling is sufficient.
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