Flux-vector drives deliver near-servo performance

February 2, 1998

POWER TRANSMISSION

Flux-vector drives deliver near-servo performance

Packaging equipment designers at HK Systemsget superior operating performance

Karen Auguston Field Managing Editor

Most designers are well aware of the benefits of servo motors: higher acceleration rates, faster speeds with excellent speed regulation, and the ability to change profiles on the fly, adding up to greater throughput, precise positioning, and superior performance.

Despite the advantages, however, many designers currently looking to take the next step beyond wound-rotor or single-speed dc motors may not be prepared to make the immediate leap to--nor able to justify the added cost of--servo technology. Fortunately, many engineers are finding a happy medium with ac flux-vector drives. They can deliver near-servo performance in a variety of applications at about 80% of the cost per kW.

In a closed-loop configuration, these drives achieve high-performance torque and speed control by providing shaft-position feedback to compensate for any error in the output speed. And unlike servos, which require a motor designed for very low inertia of the rotors, a flux-vector drive only requires a standard, squirrel-cage motor.

Keep it simple. That particular aspect of the technology was appealing to designers at HK Systems, who had worked exclusively with single-speed, two-speed, and variable frequency ac motors up until they set out to design a next-generation palletizer that can handle small to large cases and bags.

The palletizer works by forming product in layers and then stacking the layers on a pallet. As the diagram shows, product enters the system through an elevated (9-ft high) belt infeed. A slat sorter directs individual products into a rake accumulation area, where case turners form the desired layer pattern. A mechanical rake then pushes the entire layer onto a bi-parting slide plate area, while the hoist moves the pallet to the appropriate position.

"Speed control was an inherent concern because of the potential for product damage," says Mechanical Engineering Manager Phillip Edwards. Because of the fast cycle times, designers immediately considered using servo motors to power the rake and hoist.

Although servos had the performance characteristics designers were looking for, the cost and learning that would be required to overcome a lack of familiarity with the technology appeared difficult to justify. According to HK's preliminary cost analysis, the use of servo technology would bump up significantly the electrical content of the machine.

An alternative solution designers considered was a flux-vector drive, which in its own right appeared to have several advantages. In addition to the superior speed range (both below 30 hz and above 60 hz) and regulation, its ability to provide variable-speed control would address the problems experienced with very high torque spikes when switching from low speed to high speed. No mechanical switching of the power circuit, fewer hard-wire power connectors and relays, and fewer moving parts were other  pluses.

Because of the fast cycle times required for some products, designers wanted variable-speed drives on this new palletizing machine to ramp up to speed very quickly, yet not jostle products.

Open or closed? Initially, designers considered running the vector drive in an open-loop configuration. "Our first reaction was that open loop would be much simpler," says Paul Davis, electrical engineering manager. "There is no closed feedback cards, no encoder, and it doesn't need a lot of wiring."

But designers changed their minds after building prototypes. "We found that with a closed-loop configuration, you don't have to program anywhere near as much to get the necessary feedback," says Davis. "And the drive can take its own readings instead of relying on the designer to feed it the theoretical calculations."

In fact, the designers were pleasantly surprised by the overall performance of the flux-vector drive. "To be honest, I'd have to say that we would have loved to use servo technology, because you can't beat the performance," says Kurt Lloyd, director of engineering. "But we are looking at the technology very, very closely, and at some point it is going to win out."