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Originally Posted by David Guzman
Ok now im confused because firt John V says that the motors will burn before the 30 amp breaker trips. However Al says that their stall current is over 90 amps. I don't know if im understanding correctly but if the motor burns before it trips the 30 amp circut breaker then that would mean that it would burn before it gets to its stall current?
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Dave,
Please refer to the chart labeled "time vs. current" on page 2 of this document
Snap Action Specification then read on...
Any fuse or circuit breaker uses heat to trip. Heat is related to current by ohms law. When you plot the time versus current required to trip the device, you get a characteristic curve called an inverse time curve.
All current interrupting devices are made to pass the specified current (what is called 100% rated current) forever. Therefore, a 30 amp circuit breaker will not trip at 30A.
Referring to the curve for Snap Action, at 90 amps (300% rated current), the device will take between 1.5 and 4 seconds to trip. The range of values is due to variations in manufacturing.
Two factors further mitigate this operation point. (1) You never get to 90A because of voltage drops in the speed controller and wiring. (2) The internal resistance on the Exide battery will drop your voltage under load. As such, the voltage seen at the motor is closer to 8 or 9 volts during a stall.
It is likely that it could take 5 to 10 seconds for a FP motor st stall conditions to trip the breaker.
Now let's give the system a second or two to cool, the breaker resets and it starts again... During a two minute match, this can happen many times... The thermal mass of the FP is large compared to the breaker. Heat builds up in the motor and, after a while, the motor wires smoke.
The breaker tried to do it's job. If we had used a fuse or a non-self resetting circuit breaker, the motor would have been stopped after the first stall and the interrupting device would have protected the motor.
Note here that the FP is not unique. I have seen many smoking drill motors in my 10+ years of doing this...
Engineering is all about trade offs. Most teams would prefer a self resetting breaker which allows you to continue playing the match. The down side is that you run the risk of frying a motor.
You can try and educate your drive team to avoid motor stalls. Even better, you can try and design systems which do not stall...
Bottom line... Read and understand the spec sheets.
Peace.