A 3-phase AC electric forklift was brought in for a "Main Contactor Welded" fault. When the operator turned the key on, the contactor slammed shut with a bang, but the truck would not move. The diagnostic error codes were generic "Motor Overcurrent."
We isolated the motor cables from the traction inverter and tested the motor windings with a megger; the motor was perfect. We disconnected the battery and opened the inverter box. Inside, the cooling fins were covered in a fine layer of aluminum dust from the brakes.
We removed the heat sink and looked at the Insulated Gate Bipolar Transistors (IGBTs)-the high-speed switches that convert DC battery power into 3-phase AC. Using a multimeter in diode mode, we tested the collector-emitter junctions. Three of the six IGBTs were completely shorted.
The aluminum dust on the heat sink had bridged the high-voltage terminals, causing a phase-to-phase short. The failure was so fast and violent that the main battery fuse didn't even have time to blow, but the IGBTs vaporized instantly. This is a common issue on forklifts where the inverter cooling fan sucks air from the chassis, which is dirty with brake dust. Replacing the IGBT module requires specialized soldering (often reflow ovens), so the entire inverter housing usually has to be replaced-a $3,000 component killed by five cents of brake dust.
