A pharmaceutical distribution center reported that a UniCarriers TX3 1.5-ton three-wheel electric counterbalance forklift had developed a thermal-performance fault. The truck operated normally for the first 20 minutes of a shift, then began to lose travel power progressively - acceleration became sluggish, top speed dropped from its normal 14 km/h to roughly 8 km/h, and on the dock ramps the truck struggled to climb with even light loads. Simultaneously, the operator reported a distinct "burning electrical insulation" smell from under the truck's seat panel. Battery charge level remained normal throughout, verified with the truck's battery discharge indicator.
The technician first connected the UniCarriers diagnostic tool and retrieved live drive-controller data. During the first 10 minutes of operation, the controller's heatsink temperature read 45°C, and motor current was in the normal 20-to-35-amp range. After 20 minutes, the controller's heatsink temperature had climbed to 78°C, and the controller had begun derating motor current to 15 amps - confirming thermal derating as the cause of the power loss. The burning smell originated from the controller's heatsink area, where thermal paste and nearby insulation had begun to decompose at the elevated temperature.
He inspected the "Traction Controller" - the power electronics unit that converts battery DC power into variable-voltage output for the traction motor, using insulated-gate bipolar transistor (IGBT) power modules mounted on a heatsink. The controller's cooling fan was verified functional, but the heatsink's fins were heavily clogged with dust and paper debris - a chronic issue in the facility's wash-down environment. More significantly, the IGBT module's thermal interface pad showed visible degradation - the pad had dried and cracked, and the IGBT case was discolored from localized overheating, indicating the module had been operating at elevated junction temperatures for an extended period.
He further inspected the traction motor's carbon brushes. The TX3 uses a DC series motor with carbon brushes; three of the four brushes showed advanced wear, with the springs taking a permanent set and applying reduced brush-to-commutator pressure. The commutator surface showed mild carbon buildup consistent with increased arcing. The elevated brush-to-commutator resistance was drawing additional motor current to deliver the same torque, compounding the controller's thermal load.
The root cause was traced to a combination of controller cooling restriction and brush wear, both accelerated by the facility's dusty, wash-down environment. The clogged heatsink had raised the controller's operating temperature, degrading the IGBT thermal interface and triggering progressive thermal derating; the worn brushes had increased motor current draw, further heating the controller. The two faults had compounded over time until the truck's travel power dropped noticeably mid-shift.
The repair involved cleaning the controller's heatsink fins with compressed air, replacing the IGBT module's thermal interface pad with a new pad rated for the controller's operating temperature, and replacing all four traction-motor carbon brushes with new units. The commutator surface was cleaned with a commutator stone to remove carbon buildup and restore brush contact. The controller's cooling fan was verified functional and its guard cleared of debris. After reassembly, a continuous loaded test cycle was run for 90 minutes - controller temperature stabilized at 55°C, no derating occurred, and the burning smell did not return. Heatsink cleaning and brush inspection were added to the truck's 250-hour maintenance checklist.
