A modern AC electric reach truck was brought into the shop after a terrifying near-miss. The operator was driving fully loaded down a long, steep ramp in the warehouse. When he pressed his foot on the brake pedal, the truck didn't slow down. He pressed harder, the pedal went solid, but the truck kept accelerating. He had to crash it into a steel guardrail to stop it.
When a forklift loses its brakes, the immediate assumption is that the master cylinder failed or the brake pads fell off. But this was an AC electric truck, and like most modern electric warehouse equipment, it doesn't actually use friction brakes to slow down while driving. It uses regenerative braking.
When you step on the brake pedal of an AC forklift, the pedal position sensor tells the motor controller to stop sending power to the motor and instead turn the motor into a generator. The kinetic energy of the moving truck is converted back into electricity, which creates a massive magnetic resistance inside the motor, slowing the truck down. The friction brakes are only a mechanical backup for the final few feet of a stop, or for emergency parking.
We hooked up the diagnostic laptop and checked the brake pedal sensor-it was reading perfectly. We checked the mechanical friction brakes-they were in great shape. The problem was inside the motor controller. Inside that heavy aluminum box are massive electronic switches called IGBTs (Insulated-Gate Bipolar Transistors). These switches control the rapid firing of electricity to the motor. One of the IGBTs responsible for the regenerative braking circuit had physically cracked and shorted out. When the operator pressed the brake pedal, the controller tried to engage the regen circuit, but because the IGBT was shorted, it couldn't create the magnetic resistance. The truck effectively thought it was in neutral. The driver pressed the pedal harder, but since the regen system was dead and the mechanical brakes weren't designed to stop a 10,000-pound truck from a high speed on a ramp, it just kept rolling. We replaced the motor controller, but the key takeaway for the warehouse was crucial: you cannot treat an electric forklift like a car. The friction brakes on these trucks are relatively small and will overheat quickly if the electronic regenerative system fails. Operators must be trained to immediately hit the emergency power disconnect if the pedal ever feels "dead" on a ramp.
