The shift to 80V AC asynchronous (induction) traction motors has eliminated brush wear, but introduced a severe thermodynamic limitation: continuous torque derating at high duty cycles. An AC motor's power density is limited by the thermal class of its stator winding insulation. Most forklift traction motors use Class H insulation, rated for a maximum continuous operating temperature of 180°C.
In a cold store at -25°C, the motor performs flawlessly. But in a hot warehouse (35°C ambient) doing continuous heavy shuttling, the stator's Internal Temperature Sensor (PTC thermistor) monitors the winding temperature. When the motor draws 350 Amps at a low 12 Hz frequency (creeping with a heavy load), the skin effect and proximity effect in the copper windings increase AC resistance by 15%, generating massive I2R heat.
Once the stator temperature hits 155°C, the motor controller (SEVCON or Curtis) initiates a thermal derate. It reduces the PWM (Pulse Width Modulation) duty cycle, cutting the available current by 50% within 2 seconds. The forklift suddenly slows to a crawl, leaving the operator stranded with a 5-ton load 20 feet in the air. The motor requires a full 8-minute soak at 0 amps to drop below the 130°C reset threshold. The engineering fix is not a larger motor, but increasing the airflow from 150 CFM to 400 CFM by upgrading the external IP56-rated blower and programming a 30-second post-run cooling purge.
