Going downhill - Watts

[Ronrrm]

So, I was out for a ride the other day, and went down a fairly long steep hill...ended up with a bunch of questions about my Magic Pie Edge.

I wasn't pedaling, and I hit the brakes just at the top to turn off the cruise control.

I noticed about mid way down, the watts (on the display) were at 376, what's that all about?

I have disabled re-gen via BT, and I thought re-gen only worked with when hitting the brakes...is that true?

Under any circumstance, is it possible to damage the controller when riding without throttle applied or without a battery at all connected to the controller?

Ron

[Bikemad]

Hi Ron,

Even without activating regen, the motor still generates voltage, and the voltage being generated will be directly proportional to the motor's rpm.
If the motor's rpm exceeds its maximum unloaded rpm, then the voltage being generated by the motor will be greater than the battery voltage, and this will cause current to flow from the motor into the battery.

The regenerative braking current is usually a lot higher than the residual generated current that naturally occurs when the motor's rpm exceeds the maximum unloaded motor rpm.

As an example, if the maximum unloaded speed of the motor at full throttle is 35mph, then the motor will automatically begin to generate current if the bike exceeds 35mph down a steep hill.

This is why I do not recommend using the bike at high speed without a battery, as there would be nowhere for the excess voltage to go, and if the voltage climbs too high, it could easily exceed the maximum safe voltage limit of some of the electrical components in the controller.

Alan
 

[Sam.Vanratt]

Hi Ron and Alan
you should also mention that this unwanted feature is there to prevent the electronic (ESC) to get damaged., A lot of MP controller died that way, because
a) the BMC shut down and cut the currentflow => overvoltage=> dead ESC
b) the battery fuse burned and cut the currentflow => overvoltage=> dead ESC
c) cycling downhill with no battery attached=> no currentflow => overvoltage=> dead ESC

For such a case mechanical freewheels are practical. a) and b) could be used in combination of a Z-diode which limited the voltage to say 60V or less=> ESC+battery is protected.

Cheers
Sam

[Ronrrm]

Thanks guys that makes a ton of sense.

Just to confirm...If I stay under a specific wheel RPM I shouldn't have problems (with the battery connected).  Battery or not, exceeding some particular RPM blows up the controller.

Where are all the specs for the MP Edge, so I can run some calculations?

Ron

[Sam.Vanratt]

Hi Ron
it's the voltage (60V is max for the controller; any more and it blows the FETs and that's it), which is connected to the RPM.
When you have a fully charged 48V battery (~58V LiIon/LiPo real), you accelerate and use regen braking with high amperage, the voltage climbs very high (battery couldn't take much energy in such a short time) and the ESC is blown.
YOu do not even have to use regen with a full battery and go downhill (with no braking at all), as soon as the generated voltage of the Pie reaches higher than 60V it done.
Directdrive motors have a few features which are critical when the ESC is "living at the edge" (no pune intended). A good method to downsize the possibility is to use a power Z-Diode (e.g. 58V) with a shunt and have braking energy wasted instead of ruining the battery (LiPo burns hot...) or the ESC.
Cheers
Sam