PM Electric Motors

Which advantages you can get using these types of motors?

A permanent magnet (PM) motor is an ac motor that uses magnets imbedded into or attached to the surface of the motor’s rotor. This article provides an elementary understanding behind the terminology, concepts, theory, and physics behind PM motors.

It's All in Super Magnets

The magnets used are neodymium and iron-boron magnets, also called Super-Magnets. This name due extreme concentration of magnetic field per cm2 which makes them very attractive despite its small size, their high magnetic field density favours the strength and efficiency.

Given the characteristics of the PM motors and the high magnetic field density which allow to design motor with a third the size of a motor with the same performance. In addition, the high efficiency allows a minimum consumption of electricity to run Evs. Last but not least, super magnets have an estimated magnetic lifespan of around 400 years. Guaranteeing their efficiency and reliability over time.

EVs make use of permanent magnet synchronous motors for their high torque density and efficiency. The main result of the proposed analysis is to put in evidence the different behavior of IPM and SPM motors.

A permanent magnet motor (also called PM) can be separated into two main categories: Interior Permanent Magnet (IPM) and Surface Permanent Magnet (SPM). Both types generate magnetic flux by the permanent magnets affixed to or inside of the rotor.


surface permanent magnet

A type of motorin  which permanent magnets are attached to rotor circumference.


interior permanent magnet

A type of motor that has a rotor embedded with permanent magnets is called IPM.

Picture Courtesy of KEB AG

Perhaps the biggest advantage of IPM designs, one that gives them an edge in vehicle applications like traction motors, is the high-speed performance. The power versus speed curve for SPM motors is roughly hyperbolic, rising to a region of quasi-constant power over a narrow speed range, then falling off.

From SPM to IPM

For decades, surface permanent-magnet (SPM) motors dominated the market for permanent magnet motors. In recent years, however, the emerging hybrid and electric vehicle market, have boosted demand for interior permanent magnet (IPM) motors. With advantages like near-constant power over a broad speed range and a magnet-retaining design, IPM motors provide a good solution for applications like traction and auxiliary motors.

For vehicle applications IPM technique provides big benefits compared to SPM motors. The IPM configuration allows more control over the magnetization of the magnetic circuit.

IPM motors, in contrast, provide a much broader region of more or less consistent torque. Using a technique called field weakening, designers can apply current to modify performance. Field weakening essentially involves tuning the magnetic field of the stator to partially oppose the effect of the permanent magnets.

Comparative Charts



SPM motors have the magnets affixed to the exterior of the rotor surface, their mechanical strength is so weaker than IPM one. The weakened mechanical strength limits the motor’s maximum safe mechanical speed. In addition, these motors exhibit very limited magnetic saliency (Ld ≈ Lq). Inductance values measured at the rotor terminals are consistent regardless of the rotor position. Because of the near unity saliency ratio, SPM motor designs rely significantly, if not completely, on the magnetic torque component to produce torque.



IPM motors have the permanent magnet imbedded into the rotor itself. Unlike their SPM counterparts, the location of the permanent magnets make IPM motors very mechanically sound, and suitable for operating at very high speeds. These motors also are defined by their relative high magnetic saliency ratio (Lq > Ld). Due to their magnetic saliency, an IPM motor has the ability to generate torque by taking advantage of both the magnetic and reluctance torque components of the motor making them adapted to various EVs.

Why you should choose an IPM motor instead of a SPM?

For high-speed applications like traction motors IPM motor is the best choice. You use less magnet material because you get some of the torque from the saliency of the rotor plus you have the superior control of the magnetization of the magnetic circuit by control of the current, which allows you to operate very efficiently over very wide speed range. The fact that the rotor is robust is a bonus.

High Torque

High torque is achieved by using reluctance torque in addition to magnetic torque.


Mechanical safety is improved as, unlike in a SPM, the magnet will not detach due to centrifugal force.

Energy Saving

IPM motors consumes up to 30% less power compared to conventional electric motors.

High Speed

It can respond to high-speed motor rotation by controlling the two types of torque using vector control.

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