Permanent Magnet Synchronous Motor

3Φ AC synchronous motor

Revolving armature type
Revolving field type

Conventional 3Φ Synchronous Motor : (Revolving Field Type)

Stator : Stator is made up of silicon steel stampings. Stator slots carry a balanced 3Φ armature winding, wound for a specified even number of poles. The ends of the armature windings are connected to the terminals of the motor.

Rotor : Rotor is made up of forged steel with outward projected poles. The number of rotor poles must be same as that of stator. These rotor poles carry field coils. They are suitably connected to form a field winding. The ends of the field windings are connected to the two slip rings which are also mounted on to the same shaft.

Carbon (or) graphite brushes make sliding contact over the slip rings. These brushes are electrically connected to the terminal board.

Merits

It runs at a constant speed provided the supply frequency is constant. Even under loaded condition the speed is maintained constant.
The power factor of operation of the motor can be controlled by suitably controlling the excitation.

Demerits

It is not a self starting motor special arrangements are needed to start the motor.
It requires a regular maintenance because of the sliding contacts.

3Φ BLPM Synchronous Motor

Second stage of evolution of synchronous motor is 3Φ BLPM Synchronous motor. The stator is similar to that of conventional synchronous motor. The rotor is made up of forged steel and it carries permanent magnets. Number of magnets is equal to the number of poles. They set up a magnetic field in the airgap.

Merits

It runs at a constant speed.
No field winding, no field loss, better ‘η’
No sliding contacts. So it requires less maintenance.

Demerits

Power factor of operation cannot be controlled as field current can’t be controlled.

Permanent Magnet Synchronous Machines

Permanent magnet synchronous machines generally have the same operating and performance characteristics as synchronous machines in general operation at synchronous speed, a single (or) polyphase source of ac supplying the armature windings, a power limit above which operation at synchronous speed is unstable, reversible power flow, A PM machine can have a configuration almost identical to that of the conventional synchronous machine with absence of sliprings and a field winding.

Construction

Figure shows an cross section of a very simple PM synchronous machine.

Stator : This is the stationary member of the machine. Stator laminations for axial airgap machines are often formed by winding continuous strips of softsteel. Various parts of the laminations are the teeth slots which certain the armature, windings, yoke completes the magnetic path. Lamination thickness depends upon the frequency of the armature source voltage and cost.

Armature windings are generally double layer (two coil sides per slot) and lap wound. Individual coils are connected together to form phasor groups. Phasor groups are connected together in series/parallel combinations to form star, delta, two phase (or) single phase windings. AC windings are generally short pitched to reduce harmonic voltage generated in the windings.

Coils, phase groups and phases must be insulated from each other in the end-turn regions and the required dielectric strength of the insulation will depend upon the voltage rating of the machine.

In a PM machine the airgap serves an role in that its length largely determines the operating point of the PM in the no-load operating condition of the machine. Also longer airgaps reduce machine windage losses.

Rotor : The PMs form the poles equivalent to the wound-field poles of conventional synchronous machines. Permanent magnet poles are inherently “salient”, of course and there is no equivalent to the cylindrical rotor pole configurations used in many conventional synchronous machines.

Many permanent magnet synchronous machines may be cylindrical or “smooth rotor” physically but electrically the PM is still equivalent to a salient, pole structure. Some of the PMSM rotors have the permanent magnets directly facing the airgap. It is shown in figure. Rotor yoke is the magnetic portion of the rotor to provide a return path for the PMs and also to provide structural support. The yoke is often a part of the pole structure.

Damper winding is the typical cage arrangement of conducting bars, similar to induction motor rotor bars and to damper bars used on many. other types of synchronous machines. It is not essential for all PM synchronous machine applications, but is found in most machines used in power applications. The main purpose is to dampen oscillations about synchronous speed, but the bars are also used to start synchronous motors in many applications. The design and assembly of damper bars in PM machines are similar to the other types of synchronous machines.

Rotor Configurations

Synchronous machines are classified according to their rotor configuration. As a starting point in a rotor for PM synchronous motor, we can start with the conventional salient-pole synchronous machine rotor and merely replace the soft-iron field poles or a radial section of them with permanent magnets. Figure snows salient pole rotor using alnico magnets.

There are four general types of rotors in PM synchronous machines.

Peripheral – The permanent magnet are located on the rotor periphery and PM flux is radial. Figure shows a peripheral type.

Interior – The PMs are located in the interior of the rotor and flux is generally radial. it is shown in figure.

Claw Pole or Lundell – The PMs are generally disc-shaped and magnetized axially. Long, soft-iron extensions emanate axially from the periphery of the discs like ‘Claws’ or lundell poles.

There is a set of equally-spaced claws on each disc which alternate with each other forming alternate north and south poles.

Transverse – In this type, the PMs are generally between soft-iron poles and the PM flux is circumferential. Figure shows transverse type rotor. Here, the rectangles in the soft-iron poles indicate damper bars.

Magnetically, this configuration is similar to a reluctance machine rotor, since the permeability of the PMs is very low, almost the same as that of a non-magnetic material. Therefore, reluctance torque-as well as torque resulting from the PM flux is developed.