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Advanced AC Lift-Truck Motors Offer Powerful Alternative to Internal-Combustion Models

Wed, 01/21/2004 - 8:36am
Dirk von Holt, President Jungheinrich Lift Truck Corp.

For years the forklift, powered by an internal combustion (IC) engine has been the workhorse for industrial and commercial material handling of heavy loads. The noise, emissions and performance limitations of conventional forklifts was accepted as a necessary evil. However, one of the most significant trends in the forklift industry is the increased use of electric-powered forklifts. And, now that an 80-volt, 5000-6000 pound capacity truck is available the applications arena for electric forklifts is limitless.

Twenty years ago, the ratio of forklift sales in the U.S. was 55% IC engine-powered and 45% electric-powered; and the electric vehicles were typically used for lighter duty applications. Today, electric-forklift sales approach 60% of the overall market. The growth can be attributed to several factors, including performance and operational improvements, extended range of vehicle capacities, environmental and OSHA concerns and operating cost benefits.

Performance enhancements

Material handling equipment functions by moving an item from point A to point B. The advantage held by IC-powered forklifts is power: the ability to move heavy loads quickly and efficiently over any terrain. More specifically, they can climb ramps and grades, accelerate faster, reach higher top-end speeds and lift loads faster. Overall they are regarded as the true workhorse of industry.

Interest has been generated due to recent advancements in electric forklifts with 48V and now 80V AC motors. AC motors offer several advantages: high performance, reduced maintenance and improved energy consumption, to mention a few. However, the load capacity limits for electric trucks have been a drawback. Operators want the freedom and flexibility to convey all loads with a single truck.

Operators are also getting more comfortable with electric forklifts, realizing they operate cleaner and quieter. Also, less vibration is transferred from the engine through the seat, floorboard or steering wheel. Tests have shown reduced vibration keeps operators alert and less fatigued, especially in operations that require long periods of forklift use.

Advances in detail

Several improvements and enhancements have increased the popularity of electric forklifts. The switch to AC-drive technology versus DC has provided several operational benefits, as well as environmental advantages. Also, ergonomic controls and digital displays provide a more user-friendly vehicle.

An AC-powered pallet truck still uses a DC battery. An inverter in the truck's controller converts the DC current to three-phase AC current. AC power is then delivered to the truck's motor, controlling the speed and acceleration of the vehicle. The end-user realizes the following benefits:

Higher performance. AC motors allow for quicker speeds up to 12.5 mph, better acceleration and gradeability, and lift speeds up to 108 fpm. Operators notice a faster and smoother response when changing from forward to reverse and vice versa. This improvement in directional changes increases productivity.

Reduced maintenance. Unlike DC motors, AC motors do not have brushes, controller contactors for switching the direction of the motor rotation, or motor commutators. Fewer moving parts means reduced maintenance costs. In addition, the elimination of brushes allows AC motors to be much smaller than comparable DC motors.

Reduced energy consumption. One of the weak points of DC-powered trucks has been the decrease in performance as the battery loses its charge. An AC power system offers the control needed to maintain power, even as battery charge runs low. Up to 30% less energy is consumed with an AC system, compared with a DC system. Typical AC systems recover battery energy using three forms of "regenerative" braking: when the accelerator lever is released (coasting), when the brake is applied, and when the directional lever is operated (switch back or plugging). The inertia energy created by these actions is converted to electrical energy and returned to the battery, extending overall operating times and operating cycles.

Apart from the elimination of emissions issues, not having to buy fuel also adds to the acceptance of electric trucks. For example, to accomplish the same amount of work, an electric forklift may cost $4 in electricity, whereas an IC forklift may need $10 in fuel. The initial costs of an electric truck with battery and charger may be higher, but the overall true operating costs over time will provide significant savings.

Also, electric vehicles have digital displays providing operators with instant information regarding the status of their vehicle's condition. Speedometer readings, battery-discharge gauges, warning messages and multiple-hour meter readings are common. A final key advantage, and one that can significantly impact maintenance costs, is that electric vehicles typically include built-in analyzers. With these self-diagnostic capabilities, electrical forklifts take less time to troubleshoot.

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