# Brake Resistor Value

I’m trying to get max acceleration out of an oDrive D6374 150 KV motor with a 42 V server PSU (setup to deliver a maximum of 70 A at present). I’m using the oDrive CUI 8192 cpr encoder, the supplied 2R 50 W braking resistor and a 56 V oDrive v3.6 board.

As I understand it, the D6374 limits are approximately 40 A continuous (70 A peak) at 48 V.

I’ve been stepping up the current_lim and reached 25 A with vel_limit = 325000, and I’m now getting ERROR_DC_BUS_OVER_VOLTAGE. As I understand it, this is because the braking resistor can’t dump enough power. I’ve done some quick calcs:

42 V @ 90% efficiency = 37.8 V
To dump 40 A, gives 37.8 / 40 = 1R
To dump 70 A, gives 0.54R (2x 1R in parallel)

I’m also a little concerned because the 2R braking resistor doesn’t get even slightly warm, even after several runs.

I’d be grateful for some feedback before I go order the resistors.

Thanks,
Richard

Hi! Those numbers look right. I think the only thing you’re missing is that the ODrive will only drive the motor to 70% of the bus voltage.

So 0.7*42V*70A = 2058W
P = V^2 / R -> R = (0.9*42)^2 / 2058 = 0.698 ohms. So the resistance should be lower than that.

Also, we just made some significant changes to how the brake resistor behaves on the RazorsFrozenTesting branch (specifically to fix this error). I’d appreciate it if you can try it out!

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Thanks - most helpful!

Wetmelon: do you think a 50 W resistor will be ok?

That’s much harder to predict. Resistors state their continuous wattage and have derating curves and pulse-current ratings. Unfortunately I recommend Google or “Try it and don’t burn your house down”

With regard to this post, does it mean that for a 100Kv motor with 48V battery, the maximum speed is about 0.7*100*48 = 3360 RPM?

Yessir, that is correct.

I was testing some allied motion frameless motors on ODrive and now I understand why I can’t spin the rotors close to their no-load speed values. So ODrive drive the motors at 70% of the bus voltage (48V input x 70% ? ). Can we change this 70% value through the firmware or if I want to provide exact 48V to the motors do I have to supply 48/0.7 = 68.5V to the ODrive? (ODrive max voltage 56V?)

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You can tweak it in firmware, but it’s a hardware limitation due to only having 2 current shunts. v4 will have 3 shunts and should be able to overcome this limitation.

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Thanks @Wetmelon. Yes, I understand that it should be due to a hardware limitation. BTW, can you please explain how can I tweak the firmware to supply voltage close to 48V for the motors?