Downhill and Coasting

[Brendon1128]

I have a Smart Pie motor and I just got a question about going downhill and coasting. My commute to work on my bike is pretty uphill going to, but coming back home, its a lot of downhill more than half of the time. I'm generally staying above 21 mph (35 kph) and even hit 25+ mph (40+ kph) on some descents. Does this do any damage to the motor? Does the motor have a max speed it can spin at?

Also, a couple of days back I went out for a bike ride without the battery or controller plugged in (I have an external controller). I was looking to get some exercise so I thought I'd go for a ride without e-power. After about 6 miles, my ride became incredibly difficult. It felt as though the regen braking had suddenly engaged, even though I didn't even have the motor plugged in. I noticed one of the three main plugs (the blue, green, yellow ones) looked partially burnt. Can this happen, and does the controller+battery always have to be plugged in even if you are riding on without using e-power?

Thanks!

[Sam.Vanratt]

Hi Brendon1128
oh my dear.
When you have regen braking enabled all the energy will be delivered to the battery or burned inside via the FET's reverse diode.
That's what I know from Regen capable BLDCs. When you decouple the Controller everything would have been fine (from my point of KnowHow), but when I constructed my battery the overcurrent system needed a reverse diode for the incurrent so the system is never without load.
Maybe your controller has some self protection or very good cooling?
Cheers
Sam

[Bikemad]

Hi Brendon andto the forum.

Coasting at high speed without a controller should not cause any problems as long as you ensure that the three phase wires remain safely isolated from each other.
At high speeds, the motor will try to act as a generator, but if the ends of the phase cables are not connected to anything (i.e. not connected to the controller or each other) there is no way that any voltage/current can flow and therefore no load will be put on the motor.

Touching two or more phase wires together will cause the motor (working as a generator) to be heavily loaded and could easily result in arcing at the terminal if they intermittently touch against each other at high speed.
If you were to short all three phase wire together, and then try turning the wheel by hand, you would see just how powerful the effects of electromagnetic dynamic braking can be.

With the phase wires disconnected from the controller and properly isolated, you could spin the wheel at over 100mph (assuming the wheel and tyre were balanced well enough) and it would not cause any damage whatsoever to the motor, but touching the phase wires together at that speed would probably melt the phase wires.
The voltage across the disconnected phase wires (at 100mph) could be in excess of 200V, so it might not be a good idea to touch the connectors with bare hands either.

Having said that, I do not recommend running the bike at high speed with the controller fitted if the battery is disconnected or turned off, as the high voltage produced by the motor at high speed has nowhere to go and would probably damage the controller.

Alan
 

[GoldenMotor]

The MagicPie/SmartPie controller check regen braking voltage every milisecond, if the voltage exceeds 60V, it will switch off regen braking feature automatically to protect controller from damage.  So it is safe whether you install the battery or battery is full.

[GoldenMotor]

Recommended maximum downhill and coasting speed is not exceed 150% rated ebike speed. For example, rated ebike (motor) speed is 40km/h, then maximum downhill speed should not exceed 60km/h. Otherwise, the regen braking voltage may go higher than 80Vdc which can damage the MOSFETs inside controller. That is out of control.