Why the Permanent Magnet AC Motor Is Special March 23, 2022

At Duke, we often deal with the weird and wonderful when it comes to motor repair and replacement. Our sales guys and motor techs are specialists when it comes to custom and complex repairs. One of the unicorns we see every so often is the permanent magnet AC motor (PMAC), which is not the same as your regular AC synchronous motor and is very application-specific.

Permanent magnet AC motors (PMAC) use rare-earth magnets embedded into the surface of the rotor, which provides a strong magnetic field around the rotor. PMACs use the natural magnetic field to create motion as opposed to induction motors which induce a magnetic field in the rotor. PMACs also require a variable speed drive and sometimes an encoder to control them.

Advantages of Permanent Magnet AC Motors (PMACs)

Because PMAC motors do not require any electrical current, they can be more efficient because they are not incurring electrical losses. This also means that less heat is generated, which leads to improved operational life and, therefore, reliability.

PMAC’s also have a higher torque capacity and better torque utilization with faster acceleration. Further, its compact design with high torque density and less weight mean it can fit into smaller spaces. A higher continuous torque over a wide range of speeds is an added benefit, with lower rotor inertia and higher dynamic performance under load.

Disadvantages of Permanent Magnet AC Motors (PMACs)

PMACs are expensive, often three times the price of induction motors. They can also be hard to maintain and operate as magnets on the rotor make the rotor hard to remove and replace. The magnets can even attract nearby equipment and machinery, which can be a real hazard. And believe it or not, permanent magnets aren’t always permanent and can have limited capabilities. Certain forces can demagnetize the magnets if the material is strained, permitted to reach high temperatures, or is impacted by a significant electrical disturbance.

Another disadvantage of PMACs is that they generate electricity in a freewheeling load situation (freewheeling or flyback is defined as a sudden increase in voltage across the inductive load when the current through the circuit shows a reduction), which makes them dangerous to work on because they can cause electric shock or death.

Applications of Permanent Magnet AC Motors (PMACs)

Although the initial cost of permanent magnet motors can be high, this can be offset by the efficiency gains and longer operational life. If speed control is required and the motor is meant to operate at reduced speeds, permanent magnet motors have greater appeal. Typical industries include water and wastewater, pulp and paper, and air handling. Common applications include pumps, fans, mixers, power generation, and paper machines.

If you’ve got a broken unicorn on your hands or just a regular old motor, we’d love to see it and get it back up and running again. From big iron to high-tech servo, marine generators to water treatment plant pumps, hospital HVAC systems to aggregate kilns, we’ll get your AC and DC equipment up and running – or tell you what your options are.