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Andy,
A failed brush assembly provides a dead short across the output of the controller. This results in several hundred amps as opposed to the 129 amps at stall from the motor alone. Since the short is there constantly, even running a relatively slow speeds causes high currents to flow. High currents results in high temperatures at the transistors, enough to melt the silicon inside. This causes high internal pressures, blowing the case (of the transistor) open and causing the arcing and smoke you saw. I remember seeing a spec sheet that the controller was able to handle short duration loads of 600 amps but with a sustained short, the components just can't handle the stress.
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Good Luck All. Learn something new, everyday!
Al
WB9UVJ
www.wildstang.org
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Storming the Tower since 1996.
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