By Jim Montague, Executive Editor
Industrial motors are comparatively mature technologies. Most of their major innovations were introduced and refined decades ago, so motors seem to stand still while younger, quicker advances in drives, controls, networking, onboard intelligence and software seem to swirl around them.
However, it would be a mistake to think that motors aren't experiencing some significant evolutions as well. For instance, Tower Engineering (www.tei-usa.com) in Fort Worth, Texas, uses a specially designed RPM ac direct-drive cooling tower motor and VSI cooling tower drive from Baldor Electric (www.baldor.com) to replace right-angle gearboxes and other moving parts serving high-inertia fans in institutional HVAC systems (Figure 1). This motor combines the power-dense, laminated-frame RPM ac motor with a permanent-magnet, salient-pole rotor design.
"I always wanted to get rid of these gearboxes and all of the other moving parts," says Rod Applegate, Tower's owner and president. "Misalignment, excessive vibration and noise are all inherent problems with this system. With the high speeds, the gearboxes generate too much heat, and the seals and bearings can have very short lives. There are just too many things that can go wrong. Also, when gearboxes run at these high speeds, they generate a lot of heat, and that's lost energy. Based on the test data, we anticipate that the RPM ac direct-drive solution will be 13% more efficient than a conventional drive train. This is an important discussion to have in a time when everyone is concerned about reducing the amount of energy they consume."
Just as the recent advent of $4-per-gallon gasoline spurs hybrid and electric autos, other skyrocketing energy costs push users to finally invest in premium and ultra-efficient motors that many suppliers have offered for years. This demand also reinvigorates motor builders to use even better-quality materials and construction to gain a few more percentage points of efficiency. And, although better steel and copper certainly can reduce power losses, better designs and assembly methods help achieve even greater gains in efficiency.
"We always seek new materials to reduce power losses and increase flux in our motors," says Nick Johantgen, North American technical product manager at Oriental Motors USA (www.orientalmotor.com). "However, we also recently developed better assembly techniques for our rotor and stator laminations, which allow us to preserve more of the electrical isolation in our motors. As a result, we've released a stepper motor that can produce the same torque as our existing stepper, but with 50% less power. And, because our motor accomplishes these gains by reducing losses, it runs much cooler."
Besides improved winding and lamination methods, the biggest shift in motors has been the recent move to greater use of permanent magnets (PMs) and rare-earth magnets (REMs). These magnets help increase torque and efficiency, and typically allow motors to deliver more horsepower in a smaller package.
"All the drill-rig, traction, test-stand and other applications that use our motors of 1,000 hp and less always seek more motor density and torque in smaller packages," says Rich Schaefer, product manager for variable-speed and specialty motors for Baldor. "This means using more costly steel and copper to build motors, but it also means using REMs made from neodymium iron boron (NdFeB).
NdFeB technology has been available for many years and it's scalable, but it was more costly and produced in lower volumes, making it hard to justify its use in larger motors, Schaefer explains. "REMs only recently have been adapted for larger motors, machines, hybrid cars and computers, and these higher volumes fuel use in more diverse applications," he adds. "Now, in applications where induction motors would be too big, too heavy and completely ineffective, machine builders can use smaller, lighter PM motors. I think it helps to think of PM motors as large-scale servo motors."
Two common types of PM motors include surface-mounted PM (SPM) motors with magnets attached on the surface of the rotor, and interior-buried PM (IPM) motors with magnets integrated inside the rotors (Figure 2). "We see PM motors replace induction motors in many applications," says Mike Massie, drives product marketing director for Yaskawa America (www.yaskawa.com). "However, while PM technology doesn't reduce the capital investment cost, it does reduce lifetime costs due to improved efficiency. In fact, we were able to lower the overall cost and improve efficiency in a compressor application by using a multiple-pole PM motor that eliminated a gear box and simplified the OEM's design."