Quote:
Originally Posted by Mike Betts
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In my opinion, the PTC added to the FP in recent years is to attempt to save a child's extremity or to mitigate an actual fire in the toy for which the motor was designed. It may not be fast enough to limit damage to the motor.
I would not design a system where the motor could stall.
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PTCs (Postive Temperature Coefficient, a.k.a. resettable fuses) are used all the time in automotive (and other) applications and they can be designed to trip at basically what ever point the engineers pick. That point may be on millisecond before the motor ignites or it may be at 10% loading.
I suppose that FP has picked it to keep the motors alive and well not just safe from fires. I would guess that implies that the motors can probably go to stall for short periods of time without tripping (a few seconds I would guess) at least at room temp.
How long can you run at 40% of stall? Hard to say and I have no data since the PTCs were not in place when I last played FIRST. Mr. Betts is correct in that the fan will play a big (non-linear) role in determining when that PTC will trip.
I am betting that it will run forever at 40% -- note by the way that of the 336W of Electrical Power In (.4*70Amps*12Volts), you get 120W of Shaft Power Out (.4*.45N-m * .4*16,000(Rev/Min) (2 Pi Rad / 1 Rev) * (1 Min / 60 Sec) = 120W. The balance (210W) is turned into heat.
For your reference, a curling iron is about 10-20 W and a blow drier is about 1000-1500W. So... ...this motor is generating heat like 10 to 20 curling irons or 1/5th to 1/7th of a blow drier.
Either way it is a lot of heat! The only way that much heat is going to get out of that motor without a temperature rise that is going to trip that PTC is to have
A LOT of airflow.
Bottom Line: Keep those motors turning!
Joe J.