by Gus Baldini, engineering manager, Control Systems R&D, The Raymond Corporation
For many years, DC motor technology was the choice for electric lift trucks. Today, the advantages of AC motor technology have made it a cost-effective, highly efficient solution for making electric lift trucks more energy efficient, more productive, more reliable, and less costly to operate than their DC-powered counterparts. To understand why, it helps to know the history of AC in industrial applications, particularly with respect to lift truck control systems.
The Growth of AC
AC motors were invented over 100 years ago. AC motors and control systems have been the mainstay in industrial automation applications for many decades. A notable exception has been electric lift trucks.
Until recently, it was difficult to achieve variable speeds with an AC motor. DC motors and control systems were the choice for variable-speed applications, including lift trucks. AC variable-speed systems were practical only in line voltage stationary systems. AC power electronics were too large and too expensive for low-voltage battery applications like lift trucks.
Today, although the AC motor’s operation remains essentially the same as when it was invented, engineering advances in power electronics and microprocessors have made the AC controller more energy efficient and comparable in cost, with reliability that rivals or exceeds that of the best DC systems. These control system advances have made variable-speed AC motors advantageous for use in battery-powered lift trucks.
Why AC is Better
The ultimate goal for any electric lift truck user is to maximize uptime and productivity while reducing costs. AC-powered lift trucks meet that goal, making them better able to satisfy the needs of maintenance managers, lift truck operators and distribution center managers. The AC-powered lift truck’s less frequent maintenance, fewer required parts and more energy-efficient operation contribute to lower operating costs. Lift truck operators appreciate the precision control, improved acceleration, cooler running, and more efficient battery utilization of an AC truck. Overall results with AC-powered lift trucks include double digit improvements in productivity and energy efficiency, with some users reporting up to 20% gains in both areas, as well as lower maintenance and parts costs. Compare Annual Maintenance of DC vs. AC Motors
Removing Carbon Dust
Monthly Brush Inspections
Replacing Brushes & Springs
No Brush Sets
No Spring Sets
No Monthly Cleaning
No Carbon Dust
No Brushes & Springs
Fewer Parts / Less Maintenance. The use of AC controllers and motors has simplified maintenance for lift truck service technicians. AC controllers have no wearable parts and a low incidence of maintenance needs. AC motors have no brushes, springs, commutators, or independent regeneration circuits, all of which are in DC motors. Therefore, with AC motors, there are fewer parts to stock and replace, and routine maintenance is quicker and less frequent. Plus, since AC has no brushes, there is no brush replacement, and no residual carbon dust to clean from the motor compartment. AC motors also operate at a lower temperature than DC motors, so heat-related issues with wiring, bearings and brakes are dramatically reduced or eliminated.
Because maintenance is simplified, technicians find AC lift trucks faster and easier to service. Less scheduled downtime for maintenance translates to higher equipment utilization.
A large retail distribution facility finds the lower maintenance of AC trucks to be an important benefit. “There are fewer parts to change, fewer parts to wear out,” says Todd McVeigh, the company’s maintenance manager. “Everything’s pretty much internal on the new AC motors, so there’s really less of everything—we have less troubleshooting time, less downtime.”
Greater Energy Efficiency. An AC motor has at least a 10 percent greater operating efficiency than a DC motor. This extends battery life, so workers spend more productive time on the lift truck and less downtime changing batteries. Because the battery lasts longer, users replace batteries less frequently during a shift, or use smaller, less expensive batteries. Also, there is less reduction in truck performance as the battery discharges. This means that the lift truck maintains optimum performance for longer periods, improving productivity and reducing the need for premature battery changes.
This energy efficiency is due to the inherent AC design, and superior regeneration technology. The brushes and commutator on a DC motor impose electrical losses and create waste heat. Thus they consume more amp-hours without producing as much useful work. AC motors don’t have these components, so they can be much more efficient. Regeneration, another efficiency feature on the AC system, captures the momentum of the lift truck and turns it into electrical current to recharge the battery every time the operator slows down the truck. This system returns energy to the battery, improving overall operating efficiency well beyond the gain from the AC motor alone.
A major wholesale grocery warehouse operation found a significant reduction in the number of batteries they used per shift. “On the older trucks, we were going through three to four batteries a day. On the AC truck, we are barely using two batteries per day,” said Mark Paradie, a lift truck operator at the facility.
Improved precision control and acceleration. Because operators spend less time ramping up to top speed and less time making fine control adjustments to get into proper position, AC lift trucks are more productive.
DC motors exhibit a time delay when changing speed or direction. This is most noticeable when the operator plugs to a stop and reverses direction. AC motors do not have this delay, so they respond instantly to input commands. This makes it easier for operators to accelerate quickly and to more precisely position the lift truck for storing or retrieving pallets. Operators can have better control when right angle stacking a load at 15, 25 or 30 feet in the air.
The increased productivity can be easily measured, as operators at a large third party logistics facility found out. “On any given shift we can probably [move] a good 20 percent more skids with the AC [lift truck] than we can with the older reach trucks that we have here,” said lift truck operator Brian Giurich.
Cooler running. AC motors run cooler and don’t have “hot spots” like DC motors because there are no brushes or commutators. This makes the work compartment cooler and more comfortable for the operator. Comfortable operators are more productive and take fewer “cool-off” breaks.
Heat can also decrease the life of some truck parts, such as rubber hoses and wires. With the cooler-running AC lift truck, there are fewer heat-worn parts to replace. That keeps the lift truck productive and reduces operating costs.
Easier to troubleshoot. Solid-state improvements in AC controllers have improved lift truck reliability by providing on-board self-checking, and constant monitoring and regulation of motor performance. In AC lift trucks, the self-diagnostic software goes through a number of checks every time the truck is turned on, every time the operator steps on the deadman pedal, and every time the control handle is moved. This ongoing monitoring makes the truck more reliable and makes it easier to troubleshoot in the event that there is a problem.
All these benefits translate into more work done for every hour the operator uses the truck. These lower costs and increased uptime can mean improved warehouse throughput, and more business profits.
AC Can Benefit Any Lift Truck User
AC lift trucks can benefit any user, not only those who have 24 hour-a-day, 7-day-a-week applications. While the capabilities of AC-powered trucks are ideally suited for heavy-duty applications, cold storage, and high throughput warehouses, any company using an AC lift truck even for short, intermittent periods will enjoy cost savings, energy efficiency, improved uptime, and very high operator acceptance.
An ever-increasing proportion of lift trucks sold today are AC powered. AC- powered lift trucks are used in a variety of applications, including third party logistics, general and refrigerated warehousing, retail and grocery distribution, and manufacturing, among others. This growth in popularity clearly indicates that today’s lift truck users understand the benefits of AC.