Electric Ship Research &
Development Consortium

Permanent Magnet Synchronous Motor (PMSM)

Publication Date

  • 5/13/2002


  • S. Lentijo


A Permanent Magnet Synchronous Motor (PMSM) is constructed by fitting the
magnet inside the rotor cage, which is necessary for induction starting. The PMSM are
usually considered a linear device over its entire operating range of torque and speed,
with practically linear speed-torque characteristic.

The main advantage of Permanent Magnet Synchronous Motors (PMSM) is the
absence of the excitation winding. An important application area for the synchronous
machine is large-scale power generation. In the majority of power stations synchronous
machines operate as generators and their design depends on the rotational speed required.
Multipole machines with salient poles are used for relatively slow rotation speeds
whereas for higher speeds (for example gas turbine driven generators) the machines have
lower pole numbers and cylindrical rotors - so-called turbo rotors. But such big machines
are also employed in electrical drives for traction, rolling mills, mining etc. A special
type is the Permanent Magnet Synchronous Machines (PMSM), which is often used for
servo systems up to 100 kW.

The stator has usually cylindrical shape with slots on the inner surface where the
stator windings are placed. In general the number of slots is large for distributed windings
(typically 6 per phase). The stator is manufactured using laminated metallic sheets to
minimize the eddy currents induced by the rotating flux. (figure 1).

The rotor of a permanently magnetized synchronous machine can have the
magnets applied either on the rotor surface or buried deep into the rotor. The most
common materials for the permanent magnets are Samarium-Cobalt and Neodynium-
Boron Iron, which are very durable (resist to vibration and to relatively high
temperatures) and allow high magnetic flux densities.

This type of motors uses a permanent magnet to generate the magnetic field in
which the armature rotates, the electrical circuit in the armature alone can model the
motor. In this model Rs and Ls indicate the equivalent armature coil resistance and
inductance respectively. The model for the electrical part is shown in figure 3.