IPM and SPM Motors: Surface-mounted or internal magnets – what's the difference?

Opening the hub motor of an electric scooter: the magnets are glued to the surface of the rotor. Opening the motor of a recent electric car: often, the magnets are embedded in slots, inside the stack of laminations of the rotor. This difference in position gives rise to two families of permanent magnet synchronous motors: SPM and IPM. It changes the way the motor produces its torque and its ability to spin quickly.

There are two ways to mount the magnets

SPM: Surface-mounted magnets

SPM stands for "Surface Permanent Magnet." The magnets are glued to the surface of the rotor, facing the windings. The construction is simple, and all of the magnetic flux from the magnets works directly. Most electric scooter hub motors, with an external rotor, are of this type: the magnets line the inside of the bell that rotates.

Limitation: At very high speeds with an internal rotor, the centrifugal force tends to pull the magnets away, which then need to be secured with a retaining ring.

IPM: Internal Permanent Magnet

IPM stands for "Interior Permanent Magnet." The magnets are inserted into recesses cut into the laminations of the rotor, often arranged in a V-shape. They are mechanically protected, and the shape of the iron surrounding them creates an additional effect, described below.

Reluctance torque

Iron is attracted to a magnetic field, even without a magnet: this is called reluctance. A piece of elongated iron tends to align itself with the field, like a compass needle.

In an IPM (Interior Permanent Magnet) motor, the rotor is not magnetically "round": it allows the magnetic field to pass more easily in some directions than others, due to the magnet housings, which are essentially small voids. This is referred to as salience. In vector control terminology, its inductance is not the same on the d-axis (aligned with the magnets) and on the q-axis (at 90°).

Result: The total torque of an IPM motor comes from two sources.

• The magnetic torque, as in a SPM (Surface Permanent Magnet motor), is produced by the q-axis current;
• The reluctance torque, produced by the combination of the d and q currents, is due to the difference in inductance between the two axes.

What does this change for the control system?

In a Surface Permanent Magnet (SPM) motor, the "d" current does not produce torque; the vector control keeps it at zero. In an Interior Permanent Magnet (IPM) motor, a carefully chosen, negative "d" current adds reluctance torque. Therefore, for a given torque, there is a combination of "d" and "q" currents that requires the fewest possible amps: this is the MTPA strategy, which stands for "Maximum Torque Per Ampere," meaning the most torque per amp.

The controller must then know both motor inductances and calculate the correct distribution at each instant, which requires a more in-depth characterization of the motor.

Why IPM is dominant in the automotive industry

• It spins fast: The magnets are protected within the iron casing, allowing it to handle high rotational speeds.
• It provides good field weakening: Beyond its base speed, it maintains useful power over a wide speed range thanks to the current. See our guide on field weakening.
• It saves on magnets: A portion of the torque comes from reluctance, which allows for a reduction in the number of magnets, which are expensive.

Some manufacturers go even further with so-called "magnet-assisted reluctance synchronous" motors, where reluctance provides the primary torque and magnets provide supplemental torque.

And what about an electric scooter?

A scooter hub motor rotates slowly (the wheel is driven directly) and requires a lot of torque from a standstill. The outer rotor SPM is very well suited for this application: it has a large diameter, many magnets, provides direct torque, and has a simple construction. The IPM (Interior Permanent Magnet) design is more advantageous for motors that rotate quickly and drive the wheel through a reduction gear, as is common in electric motorcycles and cars.

Our electric scooter motors are hub motors of this type, available in High Speed and High Torque versions.