How to determine cell count?

[rcjmaxx]

I have a chance to buy a new never used motor from a friend but he does not know the maxximun cell count,is their a formular to determine this? The motor is a Basic XL4200. Thanks

[BrianG]

You generally want to keep the max motor rpm at around 35,000 rpm. So 35,000/4200=8.33v, which is 6 or 7 NiMH cells or 2s lipo.

[wfdfiremedic]

This is a sort of newb question, but it seems that Novak with their HV max line has completely disregarded this formula. For instance, their 4400kv motor with 14.4 volts = 63,360 rpms. Is this why their motors run so darn hot because in fact they are really not appropriate for an e-maxx application? Their new 4.5 is even a higher KV rating than the 6.5. Why would they make such high kv motors for a heavy rc application? Would it have to do with the system being sensored?

Thanks,
Chris

[BrianG]

Well, they might be getting hot because they are spinning kinda fast if that is their true kv rating. That's what happens with sensorless BL motors too; spin 'em too fast and they heat up. Plus, efficiency starts to fall, so more power is being wasted.

The fact it's sensored simply means there is a Hall-Effect sensor to monitor rotor position. Otherwise, it's like a sensorless motor.

I've never had a sensored setup, so I don't know what their "sweet spot" is.

[MetalMan]

Just FYI, the Novak motors were rated without a load. This causes the kv to drop by 1000 or more. Perhaps that is the reason why they rate their motors in turns instead of kv.