Lift Truck Tips: For those about to lift

Around the turn of the millennium, lift truck manufacturers began releasing equipment with alternating current (AC) drive motors in place of the direct current (DC) motors, which had been in use for decades. Shortly afterward, AC motors also began powering the lifting functions of many models. Ease of maintenance is a key advantage, but the performance characteristics, efficiency and user-friendliness of AC has proved critical to modern fleets.

The AC/DC rivalry goes back to Thomas Edison and Nikola Tesla, and according to electric product planning specialist for Toyota Material Handling USA Martin Brenneman, factions still debate whether lift trucks’ new motors are can be classified as AC. After all, they are powered by direct current from the battery.

“There are some semantic arguments that it’s not truly an AC motor, but a brushless DC motor,” Brenneman says. “Both are true.”
When graphed, alternating current looks like a sine wave, with curving peaks and valleys. The AC in a lift truck depicts a square wave, with lines that go straight up, across and down. Brenneman says this is because the motor controller is taking the straight-line DC current and using transistors to flip it back and forth.

Let’s dig a little deeper: In a DC motor, energized magnets surround an armature inside the motor to make it spin. Electricity is delivered to the magnets by carbon brushes, which apply pressure to a portion of the armature to complete the circuit.

“That contact creates dust and heat, and if you look closely you can actually see some sparking, all of which is normal,” Brenneman says. “But because you have something stationary rubbing against something in motion, there is inherent inefficiency and something has to wear. One of the biggest benefits to AC motors is getting rid of the brushes and the related maintenance.”

In an AC motor, instead of transferring energy to magnets to make them work, there are multiple magnets around the motor housing that pulse in sequence to make the stator spin. The motor can tell where the stator is in the rotation, and selectively fires the A magnet, then B, then C, to control acceleration, speed and torque.

“This is where AC motors offer greater control and efficiency,” Brenneman says. “All a DC motor magnet can do is pull or push. With AC, both can happen at the same time.”

The A magnet can pull the stator until it passes A, at which point A will begin to repel the stator toward B as B begins to pull. This precision creates greater efficiency as well as greater customization of performance.

“The lift truck can be set to slow down as soon as the operator lifts his foot from the accelerator, and an experienced operator might rarely take his foot off the pedal,” Brenneman says. “It’s also tunable, so if an operator prefers to coast, that’s fine, or you can set it to aggressively decelerate.”

One operator might use equipment in a tight area where slow, fine movements are ideal. Another operator needs to travel long distances on the same equipment. When each operator enters his code on the dash display, the truck parameters are set for him, and he’s back in black.