Page 173 - Electric Drives and Electromechanical Systems

Chapter 6 Brushless motors 167

construction of a permanent magnet rotor is the possibility of the failure of the magnet-
rotor bond at high rotational speeds and accelerations. The preferred solutions to this
problem include encasing the rotor in a thin stainless-steel jacket or binding the outer
surface with a glass-fibre or similar non-metallic yarn. In addition, a suitable adhesive
should be used. The compound which is used should be thermally stable and it should
have a linear-expansion coefficient which is close to that of the magnets and the rotor
material.
The magnetic material selected for a motor is largely determined by the required
output specifications; in high-performance motors neodymium iron-boron, NdFeB,
has the highest energy product of commercially available magnets, typically 20 kJ m 3 ,
whereas ferrite has an energy product of 200 J m 3 . To obtain a high flux density in the
air gap, the magnet’s flux density and the pole face area need to be considered in
considerable detail during the electromagnetic design process for the motor. In prac-
tice, the limiting factors are the volume of the magnetic material required for ferrite
magnets, or the high cost of the material for NdFeB; hence careful optimisation of the
design is necessary. The net result is a small permanent-magnet motor, when
compared with brushed d.c. or a.c. induction motors with a similar power output.
When compared with the brushed d.c. motors, the advantages of the brushless design
are clear:
Due to the construction of the motor, the heat-generating stator windings are on
the outside of the motor frame, allows direct heat dissipation to the environment,
without heat flowing through the motor’s bearings or across the air gap.
Any possibility of sparking is eliminated by the removal brushes; this allows the
motors to be used in hazardous environments, and there is a considerable reduc-
tion in the radio-frequency interference (RFI) which is generated.
Maintenance costs are reduced, both for brush replacement and for problems
resulting from the dust which is generated by brush wear.
The speed-torque restrictions caused by the commutation limit, as found in d.c.
brushed motors, are eliminated.
However, these advantages do not come without a corresponding set of disadvan-
tages. In a d.c. brushed motor, the commutation of rotor currents is undertaken by the
mechanical arrangement of the commutator and brush gear. In brushless motors this
mechanical system is replaced by an electronic commutator comprising a three-phase
power bridge, a rotor-position encoder with a suitable resolution, and commutation
logic to switch the bridge’s devices in the correct pattern to produce a motoring torque
(see Fig. 6.2).
As part of any selection procedure, it is necessary to compare brushless motors
against their main competitors. Compared with brushed d.c. motors, they are more
expensive, but they are also smaller, easier to maintain, and more reliable, and there is
the additional complexity of the three-phase drive. Compared with induction motors
they are again smaller and more expensive, but the power electronic design is identical.