Industry “workhorse” ac induction motors have been at the forefront of energy-efficient (EE) developments due to their huge installed numbers that account for a significant part of total electricity usage. Efficiency standards and mandatory minimum energy performance specs have spurred EE designs for these general-purpose motors in the 1-500 hp (0.
Industry “workhorse” ac induction motors have been at the forefront of energy-efficient (EE) developments due to their huge installed numbers that account for a significant part of total electricity usage. Efficiency standards and mandatory minimum energy performance specs have spurred EE designs for these general-purpose motors in the 1-500 hp (0.75-375 kW) output range. (See online reference 1 for more.)
However, another ac motor design—salient-pole, interior permanent magnet (IPM) rotor—offers measurably higher energy efficiency and other benefits. IPM design buries flux-producing permanent magnets within the rotor structure, thus it differs from conventional PM motor design that simply mounts magnets on the rotor surface. IPM rotors have been used for some time in servo motors for CNC, machine tool, and other high-performance incremental motion systems (see online reference 2). Yet, in such high-dynamic applications, productivity gains and throughput are considered more important than efficiency.
Baldor Electric’s interior permanent magnet (IPM) motor looks like a cast-iron frame machine at first glance.
Mainstream role for IPM
New for IPM rotors is their use in general-purpose motors for wider, industrial and OEM applications. This is an initiative enthusiastically promoted by Baldor Electric Co., whose motor design combines a laminated frame stator with the IPM rotor.
“Bar magnets embedded in the rotor form a salient-pole machine, helping to focus flux and provide more power density, efficiency, and lower temp rise,” says John Malinowski, motor products manager at Baldor. More torque comes from an extra reluctance torque component developed in the IPM motor.
As for the stator, it’s part of the laminated frame that eliminates a stator-to-frame interface, providing a more direct path for heat transfer from the motor. Fins formed in the thin steel laminations optimize heat transfer. “When stacked up and riveted through under pressure, it looks like a cast-iron frame,” Malinowski explains (see photo).
Comparing motors at 400 hp, for example, premium efficiency induction motors offer 96.2% efficiency, while IPM rotor motors have tested at up to 98.3%, he notes.
IPM motors are controlled by a standard adjustable-speed vector drive, requiring only some special software. “Open-loop control is common, but an encoder can be easily added for applications in need of tighter regulation or positioning,” says Malinowski.
Baldor sees a growing mainstream market for these high-performance motors with four to five times smaller frame size, five to six times more power density, and nearly 50% weight savings compared to ac induction machines. “They’re now tooled up through 150 hp, with larger IPM motors at prototype stage. Sizes up to 400 hp are expected in 2009; even 1,000-hp units are in planning,” notes Malinowski.
Another possible role for IPM technology could be in the emerging “above NEMA Premium” motors area. The International Electrotechnical Commission has recently defined the highest efficiency category for industrial motors (called IE4-super premium) without specifying motor type. “IE4 category should not be limited to only ac induction motors,” adds Malinowski.
Higher initial cost is a fact for IPM motors. Growing sales volumes would drive down manufacturing cost, but the real measure of energy savings resides in electricity and lifecycle costs. Internal permanent magnet ac motors have a place in wider industrial applications for energy savings and power density.
ONLINE extra
Related links follow.
1. Aug. 2007, Advancing technology www.controleng.com/article/CA6465425.html
2. Oct. 2005, IPM article www.controleng.com/article/CA6263668.html
Author Information
Frank J. Bartos, P.E., is a Control Engineering consulting editor. Reach him at [email protected] .
IPM motors add to energy-savings mix
Embedding permanent magnets inside an ac motor’s rotor makes for more complex and costly design compared to a surface mount rotor. However, extraordinary performance and benefits are obtained. As mentioned in the main article, interior permanent magnet (IPM) motors produce a “reluctance torque” component in addition to a magnet torque component. The result is greater output torque per motor size. Other IPM motor advantages include the holding of efficiency and power factor values higher across a wider range of motor loads than ac induction motors.
Baldor Electric Co. currently offers IPM motors up to 150 hp (112 kW) for general-purpose applications, with larger sizes coming soon. The laminated frame design uses one stator and rotor size per each frame size of motor. Importantly, shaft height is maintained identical to that of NEMA standard induction motors. John Malinowski, motor products manger at Baldor, notes that IPM technology also “leapfrogs” costly copper rotor design used in the latest high-efficiency ac induction motors to meet efficiency mandates.
Adjustable-speed control of pumps, fans, and compressors represents a prime mainstream application for IPM motors. In a related consideration, Baldor cites these smaller, lighter weight IPM motors as ideal for reducing detrimental “reed-frequency” vibrations common in vertical pump-mount installations.
While interior permanent magnet motors allow dramatic reduction of physical size versus induction motors, minimum size is not always the goal. “Depending on the application, some intermediate motor frame size, weight, and performance level may be the best, or best fit on a machine,” Malinowski says.
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