Permanent Magnet Synchronous Motor (PMSM) – Construction and Working Principle

Permanent Magnet Synchronous Motor, PMSM is a kind of synchronous motor in which permanent magnet is used as rotor to create field poles. No field winding is wound on the rotor.

Construction of Permanent Magnet Synchronous Motor (PMSM):

The basic construction of PMSM is same as that of synchronous motor. The only difference lies with the rotor. Unlike synchronous motor, there is no filed winding on the rotor of PMSM. Field poles are created by using permanent magnet. These Permanent magnets are made up of high permeability and high coercivity materials like Samarium-Cobalt and Neodium-Iron-Boron. Neodium-Iron-Boron is mostly used due to its ease of availability and cost effectiveness. Theses permanent magnets are mounted on the rotor core.

Based on the mounting arrangement of magnet on rotor core, Permanent Magnet Synchronous Motor (PMSM) can be categorized into two types: Surface Mounted PMSMs and Buried or interior PMSMs

In Surface Mounted PMSM, permanent magnet is mounted on the rotor surface as shown in figure below.

This type of PMSM is not robust and therefore not suited for high speed application. Since the permeability of magnet and air gap is almost same, therefore this type of construction provides a uniform air gap. Therefore, there is no reluctance torque present. Thus the dynamic performance of this motor is superior and hence used in high performance machine tool drives and robotics.

In Interior or Buried PMSM, the permanent magnets are embedded into the rotor instead of mounting on the surface. This provides robustness and hence can be used in high speed applications. Due to presence of saliency, reluctance torque is present in this type of PMSM.

Working Principle of Permanent Magnet Synchronous Motor (PMSM):

The working principle of permanent magnet synchronous motor is same as that of synchronous motor. When three phase winding of stator is energized from 3 phase supply, rotating magnetic field is set up in the air gap. At synchronous speed, the rotor field poles locks with the rotating magnetic field to produce torque and hence rotor continues to rotate.

As we know that synchronous motors are not self starting, PMSM needs to be started somehow. Since there is no winding on the rotor, induction windings for starting is not applicable for such motors and therefore variable frequency power supply for this purpose.

Applications:

Permanent Magnet Synchronous Motor can be used as an alternative for servo drives. It is widely used in various industrial application viz. robotics,  traction, aerospace etc.