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 .




No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Leaders Under 40 program features outstanding young people who are making a difference in manufacturing. View the 2013 Leaders here.
The new control room: It's got all the bells and whistles - and alarms, too; Remote maintenance; Specifying VFDs
2014 forecast issue: To serve and to manufacture - Veterans will bring skill and discipline to the plant floor if we can find a way to get them there.
2013 Top Plant: Lincoln Electric Company, Cleveland, Ohio
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Bring focus to PLC programming: 5 things to avoid in putting your system together; Managing the DCS upgrade; PLM upgrade: a step-by-step approach
Balancing the bagging triangle; PID tuning improves process efficiency; Standardizing control room HMIs
Commissioning electrical systems in mission critical facilities; Anticipating the Smart Grid; Mitigating arc flash hazards in medium-voltage switchgear; Comparing generator sizing software

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

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.