DC Motors in Forklift Applications

Forklifts are manufactured with varied designs, power sources, load capacities, and lifting configurations for both industrial moving and material handling applications. 1 Forklift trucks are classified into two broad categories: indoor and outdoor use. Outdoor-use forklifts are powered by an internal combustion engine fueled by either gasoline, diesel, LP gas or CNG gas. Indoor-use forklifts 2 are powered by an electric motor (12, 24, 36, or 48 VDC) and typically use large lead-acid batteries (10-75KWh capacity) for traction, steering and lifting power; the batteries also function as a counterweight to provide a counterbalancing force to oppose the lifted load in order to maintain stability of the forklift. 3. The forklift’s lifting mechanism is electro-hydraulically powered and consists of a fork, mast and lift/tilt jacks or cylinders. A hydraulic pump driven by an electric motor “forces hydraulic fluid through control valves. Hydraulic pressure raises the lift jack, which raises the fork by means of a system of chains and rollers. The forks raise on a mast. Heavy loads can be tilted backward by the tilt jacks to aid in stability. The lifting capacities of electric forklift tracks range from 1,000 to 10,000 pounds.” 4

Types of Electric Forklifts

Electric forklifts typically include walkie/rider pallet trucks and sit down, counterbalanced forklift trucks. There are two main types of electric, motorized pallet trucks: walkies and riders. Walkies can handle heavy loads for short distances. They are less expensive than riders. Since they move at walking speed, collisions when they do occur are less severe. Riders are used for long distances that would make a walkie impractical. Due to their fast speeds, they can pose a safety hazard to both the operator and pedestrians within the vicinity of operation. High lift pallet trucks are a variation of walkies and riders. They are used to move material on warehouse shelving that’s beyond the reach of most workers. High lift pallet trucks can be unstable since the center of gravity changes as the load height is increased, so it is critical to only lift loads within the manufacturer’s specifications. 5

Sit-down, counterbalanced forklift trucks are the most popular type of industrial & material handler. Similar to the fork-mast design of most pallet trucks, sit down forklifts have a place for the operator to sit that’s located between the lifting mechanism and the battery located in the rear of the machine. Fast speed and instability are the two main safety hazards of sit-downs. Order picker forklifts are used to obtain material on shelving while straddle forklifts are used to carry long material typically used in construction (pipers, boards, etc.) 6

Uses of DC Motors in Forklifts

Permanent magnet DC (PMDC) Motors 7 use a strong, permanent magnets to produce the motor’s magnetic field. Due to this constant magnetic field, PMDC motors are characterized by a consistent speed-torque response over the motor’s operating range (up to base speed). They are more compact and efficient with better thermal characteristics than wound-field DC motors so they can be used for continuous duty applications. But they have lower horsepower ratings than wound-fields; therefore, they are used in functions such as the steering (motor-pump) system and some lifting pump drive applications. In addition, PMDC motors have a lower stall torque than wound-field DC motors. Wound-field DC motors 8 use electromagnetic excitation to produce the motor’s magnetic field. When used in a series-field configuration, they have very high starting torque. They have high horsepower ratings so they can be used for high load applications at much higher speeds than PMDC motors. Typically, they will be used to power the traction drive, steering systems or the lifting system’s hydraulic pump drive. Their thermal characteristics make them most suitable for intermittent duty applications. DC motor control in electric forklifts is usually a voltage control, chopper system (DC-to-DC converter) using a buck/boost chopper circuit. 9

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