An AC electric reach truck was experiencing violent, terrifying phantom braking. The operator would be driving down an aisle at full speed, and without touching the brake pedal or the deadman switch, the truck would violently lock the drive tires, throwing the operator against the restraint bar. It happened intermittently, usually when the mast was raised high or the truck hit a bump.
The shop replaced the traction motor controller, assuming the MOSFETs were failing and shorting power to the brake. It didn't fix the issue. We looked at the electrical architecture of the AC drive system. The controller doesn't just send power to the motor; it has to know the exact rotational position of the rotor to fire the stator coils at the precise microsecond. This is read by a resolver or an encoder disc mounted on the end of the motor shaft.
We pulled the motor cover and removed the encoder. The encoder disc is a thin, fragile piece of etched glass or metal that slides onto the motor shaft and is secured by a tiny set screw or a roll pin. The roll pin had sheared. The encoder disc was spinning loosely on the shaft. When the truck hit a bump, the disc slipped. The controller suddenly received false data showing the motor rotor was in the wrong position. The controller's software instantly panicked, interpreting the slip as a runaway condition, and fired the stator coils in reverse to dynamically brake the motor to a dead stop to prevent a crash. We secured a new encoder disc with a high-strength roll pin and a drop of Loctite, and the phantom braking vanished.
