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If a brushless DC (BLDC) motor is moving fast ( the case of ebike going downhill,) what happens if the back EMF (BEMF) generated while going downhill is higher than the supposed applied voltage to the motor terminal by the inverter (for example applying low duty cycle pwm to the motor like 5% or 10%?)

Is the motor going to brake or it will go even faster?
EDIT :

as Charles said the motor will brake. I did a small test on my BLDC: I provided 10% duty cycle to the motor terminal and tried to pedal much faster but pedaling become much harder, so yes the motor will try to brake and operates at the speed that the controller is asking for

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  • \$\begingroup\$ in other words: is the motor going to operate at the speed imposed by the inverter and therefore try to brake or it will run freely( like regular bike when coasting) \$\endgroup\$
    – Mourad
    Aug 17, 2020 at 13:52

2 Answers 2

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The motor will brake, at least initially. The braking energy will be returned to the inverter. If the energy is not dissipated in braking resistors or returned to the battery, it will be stored in the DC bus capacitors. That will increase the capacitor voltage. When the capacitors fail, the energy will be absorbed by their remains until they become an open circuit. At that point, the inverter will probably have failed in some manner. When the situation reaches the point that the motor is attempting to push energy int an open circuit, the motor will stop generating and coast.

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    \$\begingroup\$ So - it'll brake until it breaks ... ? \$\endgroup\$
    – brhans
    Aug 17, 2020 at 11:56
  • \$\begingroup\$ Charles do you mean that the motor will try to reduce the speed untill it reaches the BEMF of the 10% duty cycle \$\endgroup\$
    – Mourad
    Aug 17, 2020 at 12:02
  • \$\begingroup\$ Yes. The motor will try to operate at the speed that the controller is asking for. It is likely to brake until it breaks, but the losses in the motor, controller and mechanical system will be supplied by the energy returned by the moving mass, so the controller may survive. \$\endgroup\$
    – user80875
    Aug 17, 2020 at 12:07
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What happens if the BEMF generated while going downhill is higher than the supposed applied voltage

And

Is the motor going to brake or it will go more faster ?

If the back-emf is greater than the applied voltage then it has to be going faster than the demanded speed. The motor cannot decide to brake on its own - there has to be a circuit that does this. It will likely increase in speed and this has repercussions.

If standard H-bridges are used to control the motor and the motor acts as a generator then, the MOSFETs (normally used) will naturally push the excess voltage onto the H-bridge DC power supply. This can easily raise the DC bus voltage (without much of a braking effect) and cause failure of some components. In some situations, it is necessary to have a protection circuit that shunts away excess current due to back-emf and prevent the DC bus voltage rising too high and endangering other components.

That shunt protection circuit will be regarded as a "brake" to the motor.

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  • \$\begingroup\$ Thanks for the downvote. Anyone wanting to own up? \$\endgroup\$
    – Andy aka
    Aug 17, 2020 at 12:51
  • \$\begingroup\$ Andy thanks for your response, i realy appreciate taking time to answer my question but after i got two different responce here from you and Charles i did a test on BLDC: I provided 10% duty cycle to the motor terminal and tried to pedal much faster but pedaling become much harder, so yes the motor will try to brake and operates at the speed that the controller is asking for \$\endgroup\$
    – Mourad
    Aug 17, 2020 at 13:16
  • \$\begingroup\$ @Mourad you asked what happens if the BEMF got bigger and that is what I answered. On your particular setup you couldn't make the back-emf get bigger (due to a circuit that causes braking) but that's just your setup and I answered in the general case as per what you initially asked. If you did the downvote you should state your reasons why. \$\endgroup\$
    – Andy aka
    Aug 17, 2020 at 13:19
  • \$\begingroup\$ Maybe I didn't explain myself clearly, and for the down vote i didn't mean any offence but since I got TWO completly different answer where you told me it will go faster and Charles said it will brake, i didn't want someone to get confused with the two answers \$\endgroup\$
    – Mourad
    Aug 17, 2020 at 13:27
  • \$\begingroup\$ @Mourad the problem is that for some systems, the motor will go faster so my answer is more generally correct than Charles'. I'm not bitching that you gave him the accept but, I am bitching that you have marked down my answer when it is technically more correct and that means it is more useful to people. To assume that all motor control circuits work the same way as your ebike is complete folly. Your question asked about the BEMF being greater and that happens when the motor is going faster than the set speed. In other words you set the backdrop. It's a well-known problem by the way. \$\endgroup\$
    – Andy aka
    Aug 17, 2020 at 13:36

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