# Braking resistor 1750W motor

Hi,

We’re to use these sort of braking resistors:
Arcol 2Ω Non-Inductive Resistor 50W ±1% AP851 2R F 300PPM | RS Components (rs-online.com)

I was wondering since the odrive itself came with a 50W resistor and i have not found any values much higher, maybe a 100W, on this forum… is 50/100W a safe option for our motor?

We run the motor at 48V and take for the max current about 20 Amps, but it is rated for a max of 1750W. How does this 1750W relate to the braking resistor. Especially in the case that this resistor is only rated for 50 or 100W depending on the one we choose. I’ve found that this can be (a lot) higher for just peak moments but still.

Can one explain the theory behind this a bit and maybe a little bit of advise / tips too look into the correct value for a braking resistor.
And, does an ‘overpowered’ braking resistor also exists? I mean, I’m not too afraid to just implement enough margin in terms of power rating and resistance if that’s what it takes.

It’s like you said. You put a certain amount of energy into the motor. That energy must go somewhere, so we burn it in the resistor. The watt rating of the resistor is usually continuous duty in some moving air, so short power spikes can be much larger. I would do an experiment running your motor, and just measure the temperature rise of the resistor.

Sure, you can get heater coils or nichrome heaters, etc. A stovetop element is a big ol’ resistor, in reality.

Forgive me if this is too basic on the theory end:
The way I think about the theory is that motors are also generators, just spinning the motor by hand generates current in the wires. The same thing is true while the motor is running, but its’ generating current in the opposite direction to the current needed to spin it. When you want to stop a motor that’s spinning you add current in the same direction as current the motor is already generating, which means the power the motor was using to resist spinning is now being dumped back into the Odrive. To avoid over-volting the Odrive, it now dumps that power into the break resistor.

one kinda niche problem you may encounter:
There’s a little bit of a delay before the Odrive starts using the break resistor. If you’re trying to break very hard, and your power supply is near the Odrive’s maximum voltage, you may shut down the Odrive by over-volting it before the Odrive even reacts.
To fix this you can use the overvoltage ramp, which will start dumping extra power into the resistor at whatever voltage you set (let’s say 50V). Just be careful: now if you plug your Odrive into a 54V power supply it’s going to dump all that extra power into the resistor, and the power supply will try and increase power to reach 54V, until the power supply is dumping all its’ power into the resistor which will then melt.

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