IPM motors for highest energy efficiency

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
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 braunbart@sbcglobal.net .

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.

For more info, visit

The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
Pipe fabrication and IIoT; 2017 Product of the Year finalists
The future of electrical safety; Four keys to RPM success; Picking the right weld fume option
A new approach to the Skills Gap; Community colleges may hold the key for manufacturing; 2017 Engineering Leaders Under 40
Control room technology innovation; Practical approaches to corrosion protection; Pipeline regulator revises quality programs
The cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Power system design for high-performance buildings; mitigating arc flash hazards
VFDs improving motion control applications; Powering automation and IIoT wirelessly; Connecting the dots
Natural gas engines; New applications for fuel cells; Large engines become more efficient; Extending boiler life

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
This digital report explains how plant engineers and subject matter experts (SME) need support for time series data and its many challenges.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me