Author Topic: Are fuses okay or a good idea on BLDC phases?  (Read 930 times)

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Offline BoscoeTopic starter

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Are fuses okay or a good idea on BLDC phases?
« on: August 22, 2020, 06:50:38 am »
I'm planning an ambitious ebike build including designing and making my own ESC, system controller, systems bus and all the accessories like speedometer.

I'm in the planning and viability stage (based on budget). Going through my risk assessment my main worries are li ion cell fires and ESC failures while travelling at 100KPH. One particular ESC failure mode I worry about is the output MOSFETs failing short and shorting two phases together essentially locking up the motor until the MOSFETs finally melt or the motor windings do. I'm thinking of adding a fuse on at least two of the windings to disconnect the motor in such a scenario. This can save the motor and the skin on my knees. I can't see why not but could this cause any problems from a control perspective?
 

Offline Phoenix

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Re: Are fuses okay or a good idea on BLDC phases?
« Reply #1 on: August 22, 2020, 10:05:22 am »
There is a joke in power electronics that the MOSFETs are there to protect the fuses. Basically a semiconductor will go open (fail spectacularly) on fault current before the fuse does; unless the semis are massively oversized.

Will you have any series inductance besides the motor?
 

Offline wraper

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Re: Are fuses okay or a good idea on BLDC phases?
« Reply #2 on: August 22, 2020, 10:19:06 am »
If there is any protection that disconnects the motor, it should disconnect all windings simultaneously. Not to say MOSFETs won't melt, they will remain dead short. If any MOSFET fails I doubt very much that any of motor fuses will blow. Another MOSFET in the phase will short too and short the ESC power completely. Not to say winding becoming open is a danger on itself.
 

Offline uer166

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Re: Are fuses okay or a good idea on BLDC phases?
« Reply #3 on: August 23, 2020, 12:02:55 am »
If you are using field weakening techniques to increase max. speed without crazy pack voltages, then the safe state is a 3-phase short across the entire motor. Otherwise in case of a control failure the back-emf voltage will become unlimited, go to a very high value (above pack), back-feed into the pack (through body diodes, or simply shorted FETs), possibly overcharging or destroying the pack.

You'd think a 3-way short on all phases at highway speeds will cause unlimited current, but that is in fact not the case due to inductance of the windings causing current phase lag, preventing the BEMF voltage from causing high currents (the actual maximum depends on specifics of the motor).

This really has to be designed as an entire system, the choice of using field weakening or not will directly define your battery pack voltage requirements, and therefore your motor control strategy, and therefore what kind of safety you need to add for contingency.

The addition of the fuses may make it safer, or it may also make it more dangerous, there is not enough information to make the call.
 


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