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 ...

Peter Welander, Control Engineering

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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

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.