16-01-2013, 10:10 AM
Brushless Interior Permanent Magnet (IPM) Motors – A New Solution for High Performance Appliances
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INTRODUCTION
Permanent magnet (PM) brushless (BL) motors have emerged in recent years as a very strong
contender to replace induction motors used in electronically controlled variable speed applications. In
most cases, BLPM motors can provide superior performance in terms of increased efficiency and reduced
noise, while the total cost differential for motor plus electronics is subject to relatively fast payback,
especially considering the increasing cost of energy.
At the core of a BLPM motor are the PMs, which are placed in the rotor and provide the magnetizing
flux. One immediate advantage is that in a typical design, there are virtually no rotor losses.
Different magnet grades can be employed for rotor manufacturing, with ceramic-ferrites and rare-earth,
especially neodymium-iron-boron (NdFeB), being the common choices. In particular, the sintered NdFeB is
a strong material with energy density higher by one order of magnitude than that of the lower cost ferrites.
Although NdFeB is a more expensive material, it enables overall motor size reduction and/or energy
increase and, through careful design, can lead to an attractive motor solution.
Why is an IPM design so attractive?
BLPM motors can be classified in two major groups:
motors with the PMs mounted on the surface of the rotor and
motors with PMs placed in the interior of the rotor core (Fig.1).
The first group is commonly referred to as surface PM (SPM),
and the typical manufacturing technology involves gluing arc
magnets and/or securing them with special tape on the outer
surface of a rotor core. While this technology may be cost
effective in conjunction with large ferrite magnets or with
bonded magnet rings, it presents challenges for the sintered
NdFeB designs. In this case, the solution is more complicated
because, in order to cope with thin magnets and to minimize
eddy-current losses in the magnets, multiple smaller magnets
are often used to make one pole.
High efficiency without electronics
Despite recent advancements, for some applications the
added cost of a variable speed drive still remains prohibitive.
In this case, the induction motor, the industry “work
horse” for the last century, appears to be irreplaceable.
However, BLPM machines can also be made to operate
directly from the mains and benefit of increased efficiency
without electronics!
The solution, called a line-fed IPM motor involves the use of a
rotor that includes, apart from the PMs, also a squirrel cage,
similar to that of an induction machine. In the cut-away rotor
example of Fig. 3, three magnets per pole are fitted in the
rectangular slots. During the motor transient start-up, the rotor
cage contributes to the production of an asynchronous torque
that overcomes a PM transient braking torque and accelerates
the rotor. In typical steady-state operation, the rotor moves in
synchronism with the air-gap revolving magnetic field.
Electronic control boosts motor and drive performance
From the point of view of electronic commutation, brushless
PM motors can be driven with “trapezoidal” or “sinusoidal”
current waveforms. The control of the first type, also referred
to as BLDC, is typically simpler, especially when done sensorless,
i.e. without a rotor position sensor. Nevertheless, the
BLAC control with sinewave currents typically has superior
performance both in terms of increased efficiency and
reduced noise.
IPM motors particularly shine when used in conjunction with
vector control. In this case, the electronic controller “tracks” the
rotor position with respect to the stator (armature) field and
“injects” the current such as to optimize torque production and
efficiency, as explained with reference to Fig. 2. The salient rotor
structure of the IPMs lends itself to robust sensorless applications.
In principle, both SPM and IPM rotor types can be mated with
the same stator design. However, the design should be carefully
completed in order to ensure that, among other characteristics,
a sinusoidal back emf is achieved so that the electromagnetic
torque ripple is reduced. In this respect, a low
harmonic content of the air-gap magnetic field is preferable
and is also beneficial for reducing core losses.