Smart Pie current risk question

[Airman2]

I have a Smart Pie 4. I run it on 12s, sometimes on 14s lipo.
Yesterday I set the battery current up to 25A and 30A for a short test. I left the phase current at 56A.
The acceleration was amazing. (I know the efficiency is not linear and get worse as I increase the current.)

My questions:
Can higher amps ruin the controller/motor or is there controller/motor overheating protection?
Over a certain temperature will it cut off, and wait till the temperature drops to normal level?

[Bikemad]

Hi andto the forum.

The higher current will not damage the controller, as it has a built in temperature sensor that will cut the current if it becomes too hot, but I'm not sure how well the stator windings will cope with the additional heat under sustained high load.

As long as you don't strain the motor up steep hills at very slow speeds, it may be OK.

The Maximum recommended current setting for the Smart Pie is 18 Amps, but when loaded down to 30 rpm on a 48V supply it still only draws ~15 Amps.
At low rpm and high current, the original Smart Pie (with the noisier Square Wave controller) is very inefficient, at the above 30rpm/15A/48V it was consuming over 700 Watts of power, but the motor is only producing an output of ~125 Watts! This means that ~575 Watts of power is being converted into  heat within the motor, controller and wiring. 

If  the motor is made to pull 25-30 Amps at even lower rpm, the efficiency is going to be a lot lower than the 17.8% achieved in the above scenario, therefore  an even larger proportion of the additional power (>83%) will also be producing more unwanted heat.

If the stator windings heat up quicker much than the controller, there's a possibility that the excessive heat could permanently damage the insulation on the windings and/or the magnets inside the motor, before the controller becomes hot enough to start reducing the power.

If the high current is only used for short bursts, then it may well be fine, but if the motor is loaded below 30 rpm for longer periods, then you will probably "cook" the motor.

When I first installed my original 26" Smart Pie (six years ago) I tested it on the maximum permissible settings with a 14S LiPo pack:



And here are the results obtained with the wattmeter:

Maximum Battery Current: 31.72 Amps
Minimum Voltage:  51.98 Volts
Maximum Power:   1648.8 Watts
Maximum Speed:   25.3mph (measured with my phone's GPS)


It would be interesting to see the results from a Smart Pie 4 or 5 (with the nice and quiet Sine Wave controllers) to see how they compare.

Alan
 

[Airman2]

Thank you for the detailed reply.

My friend has a Smart Pie 5 26” which has the same parameters as my Smart Pie 4 26”.
So we want to compare the accelerations at different current settings side by side with the same weight. I think it will answer if it is worth increasing the current limit or not.

I noticed the maximum speed has not increased on flat surface on different current settings. It depends only on the voltage. We have measured always 46km/h (28.6mph) in no wind on 14s lipo with different phones GPS. On 12s the max. speed was 40km/h (24.9mph) on flat surface as well. Probably we could reach higher speed uphill on higher current limit, but I think on flat surface the speed limit depends on the motor KV. But I usually met the same speed or lower you were writing about.
Is the sine wave better on this? I always thougt that others use battery with higher inner resistence. As I see your battery had also higher IR if the cell voltage droped to 3.7V (52/14).

[Airman2]

Intentionally I have overcharged the lipo (4.25V) to be closer to 60V.
At the beginnig it has reached 1764W. Its too far from 400W.
The current was 31,57A.