Permanent Magnet AC Motors (PMAC) and Their Benefits

Energy efficiency is more important now than ever. With that in mind, the popularity of permanent magnet motor technology has increased. Thanks to their efficiency over standard induction motors, there are many benefits to permanent magnet AC (PMAC) motors.

What is a PMAC motor?

PMAC stands for permanent magnet, alternating current motor. PMAC motors have a simplified design. Powerful permanent magnets create a constant flux in the air gap, stopping the need for rotor windings or brushes. These are normally used for excitation in synchronous motors. When you replace the rotor windings and brushes with permanent magnets, you get the accurate and controllable performance of a synchronous motor with the robust design of a standard AC induction motor.

How does a PMAC motor work?

The electric motor is energized directly on the stator with the help of a variable speed drive. They are not designed for across-the-line starting. PMAC motors rotate at the same speed as the rotating magnetic field produced by the stator windings. Therefore, if the field is rotating at 1,200 rpm, the rotor also turns at 1,200 rpm.

Benefits of Using PMAC motors

PMAC motors have magnet materials within the rotor. AC motors need a current to be induced into the rotor by the stator through airgaps and conducted in the rotor laminations. PMAC motors can operate at low speeds. This eliminates the use of gearboxes, which also saves space and installation cost.

Permanent magnet ac motors have a high power density. That means there is a higher ratio of power output compared to the physical motor size. PMAC motors are also more efficient due to the motor’s ability to produce magnetizing current.

Applications Where You'll Find PMAC Motors

PMAC motors are suitable for variable speed and constant torque applications. In these applications, the drive and application parameters dictate how much torque is required at a given speed. In specialty cases, PMAC motors are employed in closed-loop configurations using speed feedback. This is done through feedback to an encoder signal, allowing the drive to track the exact rotor position. Users receive a true infinite speed range that includes full torque at zero speed.

Three Key Terms to Know About PMAC motors

Back EMF (Electromagnetic Fields)

Voltage that opposes the change in current that causes it. Occurs when there is relative motion between the current-carrying armature and the external magnetic field.

Cogging

Unwanted jerking that occurs while the motor spins. This is caused by the motor constantly overcoming the attraction of permanent magnets and the stator structure. This causes causing noise and vibration.

Permanent Magnets

Magnetized materials that create a magnetic field by their own internal structure.

Reminders and Limitations to PMAC Motors

Back EMF limits permanent magnet speed. Back EMF is directly proportional to the motor speed. If the motor speed exceeds the design speed range, it will cause the motor voltage to exceed the maximum voltage of the drive, causing motor failure.

Drives are capable of limiting motor back EMF when operating properly. However, if the drive faults and loses control during overspeed, it cannot protect itself.

Over-current or exceedingly high operating temperature can cause PMAC motor magnets to lose their magnetic properties. Once permanent magnets are demagnetized, they become permanently damaged.

For more information on PMAC motors, contact a motor expert or review our catalog of PMAC motors for your next application. You can also view our PMAC motors available, key features, and motor specs.