An older electric sit-down counterbalance forklift (running a DC motor and SCR control) was brought in with a violent acceleration issue. The moment the operator touched the accelerator pedal-even just a fraction of an inch-the forklift would violently lunge forward, almost throwing the operator into the steering wheel. If they pressed the pedal slowly, it still just jerked forward in ugly increments.
The maintenance team assumed the SCR (Silicon Controlled Rectifier) motor controller was failing and dropping the full battery voltage to the motor all at once. They spent $2,500 on a rebuilt controller, installed it, and the problem was exactly the same.
On DC drive systems, the controller needs to know exactly how fast the motor is spinning to properly regulate the voltage going to the armature. This job belongs to a small tachometer generator bolted to the end of the motor. As the motor spins, the tach generates a small voltage signal (usually 0 to 10 volts) and sends it back to the controller.
We disconnected the two tiny wires coming from the tach generator and put a multimeter on them. When we spun the motor by hand, the multimeter read zero. The tach generator was completely dead. Here is why that caused the violent lurching: When the operator pressed the pedal, the controller tried to apply a small amount of voltage to accelerate smoothly. Because the tach signal was dead, the controller thought the motor was not spinning at all. The controller's software panicked and immediately dumped the full battery voltage to the motor to "wake it up." The motor jerked forward, the sudden spin generated a tiny burst of voltage in the dead tach, the controller saw the speed spike, instantly cut the voltage, and the cycle repeated. It was a violent electrical feedback loop. We replaced the $50 tach generator, and the forklift accelerated as smooth as glass. On older DC electric trucks, if the acceleration is violent, always check the tach feedback signal before you buy an expensive motor controller.
