Over revving a brushless motor beyond no load speed rating

[pedro5189]

Hello,

I am currently building my first ebike and am using a GM 901 direct drive hub motor. The motor is rated at 360rpm no load so I was wondering if you were to run down a hill forcing the RPM above that speed, say to 460rpm, what happens electrically?

Lets please disregard mechanical failure, bearings, centrifugal strain etc...

Regards

Peter

 

[circuit]

It will generate more volts than your battery. Each controller has at least 6 mosfets and those have parasitic diodes, that act as three phase rectifier. So in result it will regenerate with no current limiting.

 

[pedro5189]

Thank you Circuit

Will it try then to push that current into my batteries or will it be stopped at the controller (I dont have a regen controller, just a 15 mosfet golden motor model) so that I dont need to worry about my batteries being forced to charge?

Basically Im just hoping that I cannot cause electrical damage when I go above the motor rev limit down hills

Many thanks and regards

Peter

 

[Punx0r]

It should just go into the batteries. The concern might be voltage or current above what the batteries can safely take. That said, it would have to be a very steep hill or a very heavy vehicle! When trying to spin the motor above no load speed it will operate as an increasingly powerful brake...

 

[tarunsmehta]

@circuit: what do they mean when they say that the controller allows for regeneration? I thought that if the controller does not allow regeneration then no current will be pushed back to the batteries.. :?

 

[circuit]

@circuit: what do they mean when they say that the controller allows for regeneration? I thought that if the controller does not allow regeneration then no current will be pushed back to the batteries.. :?

I have tried to explain this in my previous post.
Here is your typical motor controller schematic:

Note the diodes in parallel to transistors. these diodes are a biproduct of mosfet manufacturing and they are inside of each mosfet.

Now look at conventional 3-phase rectifier schematic:

As you can see, motor controller has a "parasitic" three phase rectifier "built" into it. So if your motor spins fast enough to generate more volts than battery is, it will regenerate into your battery, whatever you do. Controller can be disabled and you will still get regeneration. If you disconnect controller from battery, voltage on it will rise and can blow stuff.

 

[bowlofsalad]

I am so glad I read this. I've observed this concept several times and I assumed it was something like this, but it didn't really make any sense to me.

 

[tarunsmehta]

@circuit: that was very helpful!

i have been logging current data on my bike for some time now and I was noticing loads of negative 'spikes'. From what I understand the negative current should have shown a pretty smooth rise and fall, right? (since it would be directly dependent on change in speed of the wheel).

 

[Kingfish]

Thank you Circuit

Will it try then to push that current into my batteries or will it be stopped at the controller (I dont have a regen controller, just a 15 mosfet golden motor model) so that I dont need to worry about my batteries being forced to charge?

Basically Im just hoping that I cannot cause electrical damage when I go above the motor rev limit down hills

Many thanks and regards

Peter

If your controller cannot regen, then it uses a resistive load and the excess energy produced by the motor is released as heat energy.

See this doc for a good summation: Regeneration Vs Dynamic Braking in DC Drives
~KF