Permanent Magnet Technology
The use of permanent magnets (PMs) in construction of electrical machines
brings the following benefits:
No electrical energy is absorbed by the field excitation system and thus there are no excitation losses which means substantial increase in the efficiency,
Higher torque and/or output power per volume than when using electromagnetic excitation,
Better dynamic performance than motors with electromagnetic excitation (higher magnetic flux density in the air gap),
Simplification of construction and maintenance,
Reduction of prices for some types of machines.
Permanent Magnet Classification
Permanent Magnet
Permanent Magnet Synchronous
Machine (PMSG)
Permanent Magnet Brushless Machine
(BLDC)
Introduction
PM Synchronous Machine are widely used in
Wind mile generation
Industrial servo-applications due to its high-performance characteristics.
General characteristics
Compact
High efficiency (no excitation current)
Smooth torque
Low acoustic noise
Fast dynamic response (both torque and speed)
Expensive
Construction
PMSM
Stator Rotor
Inner Rotor & Outer Rotor
Longitudinal & Transversal
Radial Flux & Axial Flux
Radial & Axial Rotor
If the normal vector is perpendicular to
axis, machine is called Radial. If the
normal vector is parallel with the axis, the
machine is called Axial.
Radial Rotor
Higher power rating achieved by
increasing the length of machine.
Used in
Ship propulsion
Robotics
Traction
Wind systems
Radial & Axial Rotor
Axial Rotor
Smaller than Radial machine
High torque density
Used in
Gearless elevator systems
Rarely used in Traction
Generation
Longitudinal & Transversal Rotor
In transversal flux machines, the
plane of flux path is perpendicular to
the direction of rotor motion.
Transversal flux machines can be
adjusted independently current
loading and the magnetic loading.
Used in
Applications with high torque
density requirement.
Free piston generators for hybrid
vehicles.
Ship propulsion and wind system.
Inner Rotor
The interior-magnet rotor has radially
magnetized and alternately poled magnets.
Because the magnet pole area is smaller
than the pole area at the rotor surface, the
air gap flux density on open circuit is less
than the flux density in the magnet.
The magnet is very well protected against
centrifugal forces. Such a design is
recommended for high frequency high
speed motors.
Outer Rotor
The surface magnet motor can have
magnets magnetized radially or sometimes
circumferentially. An external high
conductivity non-ferromagnetic cylinder is
sometimes used. It protects the PMs against
the demagnetizing action of armature
reaction and centrifugal forces, provides an
asynchronous starting torque, and acts as a
damper.
The magnet is very well protected against
centrifugal forces. Such a design is
recommended for high frequency high
speed motors.
Surface and Buried Magnet
Surface Magnets Simple construction
Small armature reaction flux
Permanent magnets not
protected against armature fields
Eddy-current losses in
permanent magnets
Expensive damper
Buried Magnets Relatively complicated
construction
High armature reaction flux
Permanent magnets protected
against armature fields
No eddy-current losses in
permanent magnets
Less expensive damper
Operating Principle
In the permanent magnet synchronous
generator, the magnetic field is obtained by
using a permanent magnet, but not an
electromagnet. The field flux remains
constant in this case and the supply required
to excite the field winding is not necessary
and slip rings are not required.
All the other things remain the same as
normal synchronous generator.
The EMF generated by a synchronous
generator is given as follows
Equivalent Circuit – rotor side
Flux Linkage equations
The Flux Linkage can be generated field current