# current limiting controller question



## Qer (May 7, 2008)

az_tom said:


> Lets say that we have a PWM controller that is current limiting. What is the purpose of the current limiting? Is it to protect the MOSFETs in the controller or the motor?


Yes, yes and also the batteries.

If you have a pack that's above 100 Volt and you let PWM go all the way to 100% duty cycle you can easily reach killing currents for all three. Most batteries don't like currents above 500-750 Amps (depending on chemistry and capacity) so pulling currents way north of the maximum rating can easily blow up the batteries. The transistors in the controller (MOSFET, IGBT whatever) will fail very fast at over current and either melt together to a constant on (not very likely at massive overcurrent) or act as a fuse (more likely). Finally the motor won't take brutal currents for a long time and if you slam 100% duty cycle into a motor that's standing still can very well result in an unknown absurdly high current with 4 digits and no motor will take that for more than a few seconds.

So current limit is essential. The most likely result for smaller controllers is that the controller will blow first and thus protect the motor and batteries, but with high power controllers the controller should have a max current user setting (preferably different for batteries and motor) since it's likely that the controller will outlast both motor and batteries at extreme currents.


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