Rather than braking, some vehicles rely on plugging, which involves reversing the motor voltage to power to a stop. In the Design New Calamities column, The Case of the Culpable Contactor, Forensic Engineer Myron Boyajian recounts his investigations into a fork lift that was plugging rather too quickly -- almost dumping unsuspecting drivers to the floor.
"Because two drive motors were used, our designers used a dual-drive contactor system with separate contactors for each motor. In sharp turns, power to the inside motor would be cut to prevent wheel spin...During my tests, the motors correctly stopped and started as the wheels were moved through their steering arc and I saw the plugging contactor pick up and drop out during plugging. Motor control should have been normal. With a bright light and close examination, I found a heavy deposit of corrosion on the plugging contactor tips, possibly due to salt-air contamination, that prevented an electrical tie between the armature circuits during plugging."
Boyajian cleaned the contactor tips and, not surprisingly, plugging returned to normal. But it raised some interesting questions about the choice of dual motor control in the first place.
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