How to suppress back EMF in a BLDC motor?

Thread Starter

kaskkakdni

Joined Jan 24, 2021
2
Hi,
I'm currently in the process of designing an ESC for brushless motors. However, I have no idea how to deal with back EMF when the motor is coasting. The charge would go through my three phase bridge's mosfets' body diodes and into the battery, which I don't want. How can I prevent this current from going in the reverse direction into my power source?
Thanks.
 

Deleted member 440916

Joined Dec 31, 1969
0
With a blocking diode in the battery feed HOWEVER
The EMF may cause a rise in bus voltage that exceeds the voltage rating of components so you may also want to provide a means of absorbing the excess power such as a dump resistor controlled by an overvoltage detector.
 

Thread Starter

kaskkakdni

Joined Jan 24, 2021
2
Why not? Harvesting that energy is usually considered a good thing.
I know, but my BMS has seperate charge and discharge terminals which means I wouldn't be able to recharge my battery pack over the discharge terminal that my ESC (and bridge) is connected to.
 

LowQCab

Joined Nov 6, 2012
538
What is a "BMS", and why do you need one ?
Back EMF is normally used for Real-Time Timing adjustments in the ESC which can
raise efficiency tremendously, reduce noise, and reduce waste heat..
Why are you not using a commercially available ESC ? they're dirt cheap and work really well.
There are some ESCs that will automatically limit Braking Voltage, if that's actually a bonus for you.
Many ESCs have user adjustable software for tuning and special features.
It's very unlikely that you can build a better ESC than you can buy ready made.
What is the application ? Voltages, Current, Schematic ?
.
.
 

Marley

Joined Apr 4, 2016
411
The MOSFET body diodes in the H-bridge will return the energy to the DC supply - which is normally a good thing (as Alec_t said). To prevent the voltage on the DC supply getting too high, for example if braking a high inertia load, have a "braking resistor" set to cut in with a transistor switch controlled by a voltage monitor.
And BTW: - it's not "back EMF" (that would be reversed polarity - like from a relay coil) - it's "generated EMF" (the motor acting as a generator) because it's the SAME polarity! Sorry, one of the misconceptions that keeps coming up.
 
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