A beverage distribution center reported that a Linde E25 48V electric counterbalance forklift drove perfectly smoothly in Forward, but when the operator shifted to Reverse and pressed the accelerator, the truck jerked violently (juddered) and the dashboard flashed a "Motor Phase Sync" fault. If the operator feathered the pedal extremely gently, the truck would reverse slowly without the judder.
The technician recognized this as an AC traction motor commutation fault. The 48V AC motor relies on a speed encoder (a Hall-effect sensor reading a magnetic ring on the motor shaft) to tell the drive controller the exact rotor position. The controller then sequences the three-phase current accordingly. If the controller applies current to the wrong phase at the wrong time, it pushes against the rotor's momentum instead of pulling it, causing a violent judder.
He removed the motor's rear encoder cover. The magnetic pickup ring was loose on the motor shaft. A retaining grub screw had backed out due to vibration, allowing the ring to slip and lag behind the actual rotor position. In Forward, the motor's momentum naturally pushed the ring tight against the shaft shoulder, resulting in accurate timing and smooth operation. However, in Reverse, the motor's momentum pushed the ring the opposite way, creating a 15-degree rotational delay. The Hall sensor reported the rotor position 15 degrees late. The AC controller energized the stator coils based on the delayed position, applying torque in the wrong phase vector, causing the motor to fight itself and shudder.
The repair involved re-aligning the magnetic pickup ring on the shaft, applying a medium-strength thread locker to the grub screw, and torquing it to specification. The drive controller was then recalibrated using the Linde diagnostic laptop to re-zero the encoder offset. After reassembly, the forklift accelerated smoothly in both Forward and Reverse with zero phase sync faults.
