Permanent Magnet and Hybrid

25.6.3.4 Permanent Magnet and Hybrid

Synchronous Machines

The permanent magnet synchronous machine designed with high-energy rare-earth magnets operates with high efficiency, high power density, low rotor inertia, and low acoustic noise. The excitation from the permanent magnets is fixed and, there- fore, the regulation of the output voltage of the machine is not as straightforward as in a synchronous machine with a wound rotor. For generator operation, machines of this type can use switched-mode rectifiers to regulate the output volt- age [42, 43]. The boost rectifier of Fig. 25.14 is one possible implementation of this approach. Alternatively, a diode rec- tifier followed by a dc/dc converter can be used to regulate the generator system output [44]. Another method proposed

25 Automotive Applications of Power Electronics 659

FIGURE 25.17 Circuit for a switched reluctance machine.

for this type of system involves the use of tapped windings and two three-phase SCR bridges [45]. The taps on the phase

windings are connected to one bridge, while full phase wind- S ings are connected to the other bridge. The bridge connected

to the full phase windings is used to supply power to the dc

bus at low engine speeds, while the converter connected to the N taps is used at high speed. The use of a tapped winding and

dual bridges helps the system cope with the wide speed range

of the alternator and limit the losses associated with the pul- S sating output currents. In the case when both motoring and

generating modes are desired, a full-bridge converter can be

Stator

Rotor

used. Again, as this is a synchronous machine, some form of

Magnets

Shaft

position sensing or estimation is necessary. Also, in all of these FIGURE 25.18 Structure of an axial-airgap permanent magnet systems the effects of failure of the power electronics must be machine. carefully considered as there is no possibility of regulating the back voltages by field control.

Attempts to develop a simpler voltage regulation scheme for Fig. 25.18 [47]. The stator of the machine can be slotless or permanent magnet synchronous machines have led to a per- slotted. Two different magnetic circuit configurations are pos- manent magnet/wound-rotor hybrid synchronous machine in sible. In the NN configuration, the magnetic polarities in one which the rotor consists of two parts: a part with permanent pole pitch on both sides of stator are the same so that there magnets and a part with a field winding [46]. The two parts are two main fluxes with symmetrical distribution through are placed next to each other on a common shaft. The rotor the stator. In this case, the conductors can be wound into two with the field winding can employ claw-pole, salient-pole, or back-to-back stator slots to make one coil. The machine has cylindrical structure. The field current generates a flux that is

a large stator yoke dimension because the flux passes through used to either aid or oppose the permanent magnet flux and the yoke, but less copper loss because of short end windings.

regulate the output voltage of the machine. One possible fail- In the NS configuration, the magnetic polarities in one pole ure mode of this approach that can lead to catastrophic failure pitch on the opposite sides of stator are the opposite of each is if the field winding breaks while the machine is operating other so that there is only one main axial flux through the sta- at high speed. In this case, the generated output voltage will tor. In this case, the stator yoke dimension is small, but the end become large and uncontrolled. Some means of mechanically windings are long because the direction of current in the back- disconnecting the alternator at the input or electrically discon- to-back stator slots is the same. The iron losses are small due to necting it at the output may be necessary to limit the impact small yoke dimension and the copper losses are high because of this failure mode.

of long end windings. Heat removal is more challenging due to small stator dimensions. The structure shown in Fig. 25.18 is that of an axial-airgap permanent magnet machine with

surface magnets. In an axial-airgap machine with interior per- The principle of operation of an axial-airgap, or axial-flux, manent magnets, the magnets are embedded in the steel of machine is the same as that of a radial-airgap machine. An the rotor. axial-airgap machine is characterized by a short axial length

25.6.3.5 Axial-airgap Machines

The axial airgap versions of other types of machines, such and large diameter. The structure of an axial-airgap per- as the induction and switched reluctance machines, are also manent magnet machine with surface magnets is shown in possible. The structure of an axial-airgap induction machine

660 D. J. Perreault et al. is similar to that of an axial-airgap permanent magnet machine

except that windings are used instead of permanent magnets.