By Don Talend
As manufacturers aim to further increase the profitability of their operations and products, more demands are being placed on the motors that power their equipment. Efficiencies that enhance production speed as well as energy savings are top priorities.
Examples from plastic extruder and industrial compressor builders demonstrate how recent changes in motor design and their enclosures are meeting the demand for ever-higher efficiency.
The Move to AC
In recent years, John Clemens, manager of electrical engineering for Pawcatuck, Conn.-based Davis-Standard noticed that AC induction motors have increased in horsepower yet come down in price, making them much more competitive with DC motors. Davis-Standard is a diverse equipment manufacturer serving companies that make plastic sheeting, pipe, medical tubing and film in addition to rubber hoses and wire and cable (Figure 1).
“Motor design has come a long way in reducing frame size, which has made AC motors much more applicable for higher-horsepower applications,” says Clemens, whose company is converting from DC to AC motors.
A competitive initial cost makes the choice a simple one for Clemens. “The price break used to be at 75–150 hp,” he adds. “If you went above that point, AC became very expensive. But over the past five to 10 years, that has changed and AC drive and motor combinations actually are the same price or lower than DC, even up to 500–600 hp.”
AC now offers a higher horsepower range at a price that is more competitive with DC. “Ten years ago, a 200-hp AC drive and motor might be $15,000, while 200-hp DC was $7,000,” continues Clemens. “Now, that 200 hp is about $7,000 on the AC side as well; that’s where they’ve really made a lot of progress.”
Extrusion is an application that requires a constant high torque and the AC motors with which Davis-Standard equips its extruders are well-suited to the task. “If everything’s set up right and the temperature profile is good, the extruder probably runs about 80% load because we typically oversize it to have that safety factor,” says Clemens. “And we do a lot of variable speed where we’re above 60 Hz.”
V*S Master from Baldor-Reliance is an inverter-duty AC motor that Davis-Standard uses for extruders that require less than 125 hp. According to Rich Schaefer, variable speed motor product manager with Dodge Reliance, the motor has a 1,000:1 torque ratio and standard 200% overload torque. The totally enclosed unit is particularly suited to the company’s PVC production, which uses wood flour that can potentially clog a motor, adds Clemens. This unit has a standard NEMA T-frame enclosure and doesn’t require a blower.
Above the 125-hp threshold, Davis-Standard specifies Baldor’s RPM AC motor, an even more robust inverter-duty unit, according to Schaefer. The standard overload torque is equivalent on the RPM AC unit and can be specified at 300% or 400% if necessary.
The Enclosureless Motor
According to Schaefer, a key feature of the RPM AC motor is a finned, square, laminated steel frame—the first finned frame configuration for an AC motor—designed to allow higher horsepower ratings in a smaller enclosure. The steel laminations that make up the stator are punched in a square shape instead of a round shape before the laminations are put in a press and bolted together. The result is an “enclosureless” motor that, unlike a motor with a cast-iron enclosure, does not trap heat and this design allows more horsepower in less area, points out Schaefer. This size economy, adds Clemens, allows Davis-Standard to fit the motors under its extruders.
The RPM AC motor is available in various ventilation configurations, including drip-proof forced ventilated (DPFV), which Davis-Standard mostly specifies. The DPFV configuration allows even smaller sizing of the motor enclosure. “When you force-ventilate the motor, that also allows the frame to be smaller,” notes Clemens. “By reducing the frame, you increase the mounting area around the extruder base, which allows you to mount other enclosures on that base.”
In March 2008, Dodge Reliance unveiled a new synchronous-rotor, permanent-magnet version of the RPM AC motor, which formerly had only an induction rotor, for more precise speed control with synchronous operation as opposed to slip-speed operation, says Schaefer. The salient pole permanent magnet version of the RPM AC that was unveiled has a rotor with protruding, salient, poles around which the windings are concentrated.
Schaefer points out that the new version of the motor will use neodymium-iron boron magnets designed to be an improvement in terms of resistance to demagnetization at high operating temperatures vs. the samarium cobalt magnets traditionally used in PM motors. More extensive mining of neodymium-iron boron magnets should close the price gap between PM motors that use these types of magnets and induction motors, claims Schaefer.
Energy savings is a major concern in industrial applications that require compressed air. Gavin Monn, president Americas for industrial compressor manufacturer CompAir, Piqua, Ohio, estimates that 70% of a compressor’s operating cost is devoted to electricity and fully 10% of energy consumed in manufacturing goes toward generating compressed air. CompAir provides compressors for industries and applications such as printing, automotive, medical, pharmaceutical, electronics, bulk handling and fabrication, many of which generate variable demand for compressed air.