15-12-2012, 01:58 PM
DESIGN, PROTOTYPING AND ANALYSIS OF A LOW-COST DISK PERMANENT MAGNET GENERATOR WITH RECTANGULAR FLAT-SHAPED MAGNETS
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Abstract
This paper presents a new structure to overcome some of the difficulties in axial-flux
permanent magnet surface-mounted generators. In the new structure with a non-ferromagnetic
holder, the centrifugal force acting on permanent magnets is counteracted. Hence, an increase in
the rotation speed and the output power of the generator is feasible. The new design is inexpensive
and easy to construct. Thus, by development of the proposed structure, it is possible to construct
high-speed, low-cost axial-flux permanent magnet generators.
INTRODUCTION
Axial-Flux Permanent-Magnet (AFPM) machines with coreless stators are regarded as high efficiency
machines for distributed power generation [1-3]. In these machines, the ironless stator avoids the direct
magnetic attraction between rotors and stators [2]. Because of the absence of core losses, a generator with
this type of design can potentially operate at a higher efficiency than conventional machines [2]. Besides,
the compactness and disk-shaped profile make these types of machines particularly suitable for
mechanical integration with wind turbines [4].
Electric generators using an axial flux configuration were developed almost 150 years ago. However,
their applications have been limited to fractional power due to some difficulties [5], [6]. One major
problem is the centrifugal force acting on magnets that tends to move them from their place. Therefore,
the rotation speed of AFPM generators is limited, and that leads to using such generators in low-speed
applications such as direct coupling with wind turbines [4]. In the past, some researchers have done work
on AFPM generators with rated speeds equal to 200 rpm [4] and close to 1950 rpm [7]. In previous AFPM
machines, to increase the power output, the pole-numbers were increased, for example, to 28 poles in [4]
and 40 in [7]. Another way to reach the desired high outputs is to increase the rotation speed of the AFPM
generator with lower pole-numbers. A higher speed leads to higher induced voltage on stator windings.
The AFPM generators with no cores always have low inductances [1, 2] and thus current increase does not
change the machine performance [8]. One advantage of fewer numbers of magnets is the decrease in the
generator cost.
EQUIVALENT CIRCUIT
The per-phase equivalent circuit of a coreless AFPM machine is shown in Fig. 4a [2]. In this circuit, RS,
LS, em, va and ia are the stator resistance, stator inductance, the induced electromotive force due to the
fundamental flux linkages of PMs in the air-gap, the fundamental instantaneous phase voltage and current,
respectively. The equivalent resistance of the eddy-current losses of the stator is depicted by Re.
THE EQUIVALENT RESISTANCE OF EDDY-CURRENT LOSS
Magnets produce both axial and tangential components of magnetic fields in the air-gap region. The
motion of the magnets over the winding produces alternating fields in the stator conductors in both the
axial and the tangential directions. These two fields individually induce eddy currents in the stator
windings. If the machine runs at high speeds, the induced eddy currents produce high losses, thus
increasing the temperature of the windings and decreasing the efficiency of the machine [10].
PROTOTYPING AND TESTING OF THE DESIGNED GENERATOR
The centrifugal forces acting on magnets increases as the rotor speed increases, and hence in previous
structures of AFPM generators, it was not safe to operate at speeds in excess of 1500 rpm [4, 7].
In this paper a low-cost and simple structure for counteracting the centrifugal forces acting on glued
magnets on the rotor disks is designed and named “non-ferromagnetic holder”. The 3-D view of the nonferromagnetic
holder is shown in Fig. 9. This holder is attached to a rotor disc with screws. It is clear that
with this provision, magnets cannot move from their places during rotation. Hence, one can increase the
speed of the generator to twice the limit used in the previous works. Fig. 10 shows one of the rotor disks
with its magnet holders.
CONCLUSION
In this paper, after the introduction of AFPM machines with no iron cores, one typical generator was
theoretically designed. Next, with the finite element analysis, the parameters of the generator were
calculated. After the construction of the generator, the performance of the generator was experimentally
evaluated. AFPM generators are usually driven at low speeds, and hence, to increase the output power, a
higher number of pole-pairs is needed. One special characteristic of the designed generator is the use of
non-ferromagnetic holders to counteract the centrifugal forces acting on the magnets during the rotation of
rotor disks. Hence, it is now possible to drive such machines at high speeds. At these speeds, the output
power increases, so there is no need to use more magnets to increase the number of pole-pairs. The
prototyped generator is relatively small and cheap. The lower cost might result in the use of these
machines in domestic and household appliances too.