AC permanent magnet motors

Conventional ac induction motors are universal in application to the point that you would be hard pressed to find an industrial facility or even a residence without one. However, in this energy conscious age, manufacturers are hitting the limits of efficiency in this design. On the other hand, permanent magnet (PM) motors can offer higher efficiency levels than induction motors since they have ...

09/01/2009


Conventional ac induction motors are universal in application to the point that you would be hard pressed to find an industrial facility or even a residence without one. However, in this energy conscious age, manufacturers are hitting the limits of efficiency in this design. On the other hand, permanent magnet (PM) motors can offer higher efficiency levels than induction motors since they have no I2R losses in the rotor.

In spite of this and other advantages, wider adoption of PM motors has been hobbled by the expense and trouble of having to include a speed encoder for control. This problem has been mitigated thanks to developments of more sophisticated strategies that use an open-loop vector control method, eliminating the encoder. With that problem out of the way, users have discovered that ac PM motors offer some

Rotor construction of ac PM motors is much different from traditional induction designs, but eliminating the rotor copper losses gives PM motors a big efficiency boost.
Rotor construction of ac PM motors is much different from traditional induction designs, but eliminating the rotor copper losses gives PM motors a big efficiency boost.

attractive packages with smaller frame sizes and weight for a given horsepower rating weighed against conventional induction designs. On the other hand, induction motors are still generally less expensive and more able to withstand rough applications.

Two rotor types

Like their induction motor counterparts, ac PM motors use a wound stator, but that's where the similarity ends. PM motors have the magnets attached to the rotor, either on the surface or embedded inside. Consequently, there is no rotor current, which reduces copper losses. This is the main source of the efficiency gain.

The differences between surface mounted permanent magnet (SPM) and interior permanent magnet (IPM) go well beyond manufacturing considerations. This apparently subtle variation causes major changes to the operating characteristics of the motor.

IPM designs embed the magnets inside the rotor. This allows for greater strength which permits higher running speeds. It also creates magnetic saliency with variations of inductance that are measurable at the terminals according to the rotor position. While the specifics of this concept deserve more extensive discussion, the practical effect is that the motor also develops reluctance torque in addition to permanent magnet torque.

SPM designs usually fix the magnets on the surface of the rotor with some sort of adhesive, so the strength of that bond is a practical determinant for maximum speed and overall robustness. Moreover, surface mounting does not create saliency, so there is no reluctance torque.

Speed control

There are two main open-loop (no speed encoder) speed control strategies for ac PM motors. The first open-loop vector approach uses voltage control. The voltage control block calculates a voltage reference according to the speed command and motor current. It calculates the required output voltage to generate the needed amount of torque. This approach is typically used with SPM designs in pump and fan applications.

The second open-loop vector approach uses current and speed control algorithms. This approach effectively creates a virtual speed encoder by tracking magnetic pole positions while the motor is running. This uses a speed estimator, a speed controller, and a current controller block capable of powerful and fast computing. This works particularly well with the characteristics of IPM designs, and allows for very sophisticated speed and torque control.

While ac PM motors aren't for every situation, their growing list of capabilities offers advantages that might solve your next application problem.


Author Information
Peter Welander is process industries editor. Reach him at PWelander@cfemedia.com .




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