Why use permanent magnet motors?

Rob Keating

March 11, 2022

Why use permanent magnet motors?

About PMSM motors

Permanent Magnet Synchronous Motors (PMSM) are advanced AC electric motors consisting of a stator, and a rotor which is connected to the output shaft. Permanent magnet motors typically use Neodymium magnets which are embedded in the rotor to create torque garnered by electromagnetism. This type of motor differs from most other electric motors, whereby the rotor generates its own magnetic field through induction, or where the field currents are transferred to the rotor by brushes and slip rings. PMSM motors are an excellent choice for a range of motion control applications.


Benefits of PMSM motors

- High efficiency
This is particularly true at lower speeds. The permanent magnet motor does not require current to be supplied to its rotor to generate the rotor field, therefore eliminating the rotor losses almost completely. When compared to induction or reluctance motors it also requires lower currents on the stator and has a bigger power factor, leading to smaller current ratings on the controller, and increasing the overall drive system efficiency.

Driving lower speeds at higher efficiency than an induction motor might delete the requirement of a speed reduction transmission, taking complexity out of the mechanical arrangement.  

Some battery electric vehicles (BEVs) utilise both motor technologies – one on the rear axle and the other on the front axle. Depending on the requirements, the more efficient system can provide propulsion power, or activate both at the same time for maximum power.

- Constant torque
This type of motor can generate constant torque and maintain full torque at low speeds.

- Size
The smaller size, lighter weight and less coil provide a higher power density.  

- Cost-effective
With the absence of brushes, there are reduced maintenance costs.  

- Minimal heat
In PMSM the heat is generated on the stator coils and there are no brushes and only minimum heat generated on the rotor, facilitating the cooling of the motor. As they run cooler than induction motors, the motor’s reliability and lifespan is increased.

- Speed range
This type of motor can have a wide speed range with the use of Field Weakening and can adopt the maximum torque/current (MTPA) control strategy during constant torque operation.

What applications use PMSM motors?

Industries that use PMSM motors include Metallurgical, Ceramic, Rubber, Petroleum, Textiles, and many others. PMSM motors can be designed to operate at synchronous speed from a supply of constant voltage and frequency as well as Variable Speed Drive (VSD) applications. Widely used in electric vehicles (EVs) due to high efficiency and power and torque densities, they are generally a superior choice in high torque applications such as mixers, grinders, pumps, fans, blowers, conveyors, and industrial applications where traditionally induction motors are found.  

Conclusion

With advanced electric motor technology, permanent magnet motors are suitable for a wide range of uses and becoming more popular in many new applications. High torque density and efficiency, combined with a host of other benefits, including constant torque, make PMSM motors a preferred choice for electric vehicles and many other applications. As synchronous motors, they can work in applications where induction motors cannot. At the rate of advancement, the PMSM motor may become the industry standard for motion control.  

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