GB2428902A - Salient pole electrical machine - Google Patents

Abstract

A salient pole electrical machine (2) comprises a salient pole passive rotor having five salient poles (4). A three phase stator (6) has six slots, each slot comprising a winding (8) and a permanent magnet (10) which is polarised circumfentially. The magnetic material may extend circumferentially or radially.

Description

Salient Pole Electrical Machine The present invention relates to salient

pole electrical machines. The invention may be applied to motors and to generators.

The paper "The Flux-reversal Machine: A New Brushless Doubly-salient Permanent-Magnet Machine" by Rajesh P. Deodhar, Svante Andersson, Ion Boldea and Timothy Miller, in IEEE Transactions on Industry Application, Volume 33, no. 4, July/August 1997 describes a flux reversal machine. The machine described is a generator. The generator is shown in the accompanying Figure 1. The generator comprises a rotor I rotating on a shaft 3. The rotor has three salient poles. The stator 5 comprises an armature having a single phase winding 9. The armature also comprises two permanent magnets 7 arranged diametrically opposite one another and radially polarised.

Another such machine is described in lEE Proc. Electrical Power Applications, Volume 146, no. 2, March 1999. The machine is a three phase machine having a rotor with eight salient poles. The stator has six radially polarised permanent magnets, adjacent magnets being oppositely polarised. The stator has six windings arranged in three phases.

In the machines of Figure 1 and of lEE Proc. Electrical Power Applications Volume 146, no. 2, March 1999 the permanent magnets are arranged around the inner circumference of the armature.

In the paper "Switching flux permanent magnet polyphased synchronous machines" by Emmannuel HOANG, Abdel Hamid BEN AHMED, and Jean LUCIDARME, published in the EPE'97 conference proceedings pages 3.903 to 3.908,1997 there is described a salient pole machine having a rotor with ten salient poles and a stator having twelve windings arranged in three phases and twelve permanent magnets. Unlike the two previously described machines, the magnets extend radially through the stator being polarised circumferentially.

It is apparent from the foregoing papers that salient pole motors having both permanent magnets and windings in the stator can have a wide range of number of salient poles on the rotor and of windings and magnets on the stator. Furthermore, the foregoing papers discuss machines having one phase, two phases, three phases, and five phases.

The present invention seeks to provide a three phase, high-speed machine having beneficial properties.

According to the present invention there is provided a three phase salient pole electrical machine comprising: a salient pole passive rotor having five salient poles; and a three phase stator having six slots, each slot comprising a winding and a permanent magnet which is polarised circumferentially.

Such a machine is able to operate at higher speeds than the prior art machines described above. With 5 poles (as compared to 3) the frequency of operation is lower resulting in better controllability and lower iron losses resulting in higher efficiency.

Also, the 5 pole six slot configuration results in reduced torque ripple and better torque capability compared to a 4 pole, 6 slot configuration.

By operating the machine with three phases torque ripple is reduced. The frequency of operation of a machine is related to the number of salient poles on the rotor. The larger the number of salient poles the higher the frequency of operation.

Normally, such a machine is operated using an inverter and the lower the frequency of operation of the inverter the better. Accordingly it appears logical that the number of salient poles on the rotor should be minimised. However, unexpectedly, the inventors have found that for a three phase machine there is no benefit in reducing the number of salient poles of the rotor below five. Using five salient poles on the rotor produces a closely sinusoidal back EMF which is beneficial for better operation and control of the machine.

With three phase excitation at 5,000Hz such a machine can operate at 60, 000 rpm For a better understanding of the present invention and to show how the same may be carried into effect reference will now be made by way of example to the accompanying drawings in which: Figure 1 is a schematic cross-sectional view of a known salient pole machine; Figure 2 is a crosssectional view of a salient pole machine according to the present invention; Figure 3 illustrates the distribution of flux in the machine of Figure 2; Figure 4 is a block diagram of the machine of Figure 2 in combination with an inverter; Figure 5 comprises graphs showing variation of electromotive force in the machine of Figure 2; Figure 6 is a graph showing the variation of electromotive force in a comparative example; and Figure 7 is a schematic diagram of another motor in accordance with the present invention.

Referring to Figure 2 the machine comprises a motor 2 having five salient poles 4. The machine comprises a stator 6 having six slots containing three pairs of windings 8. There is one pair of windings per phase. There are three phases A, B and C. The windings of each pair for one phase are diametrically opposite each other and connected in series. Thus the two windings 8 of phase A are arranged diametrically opposite and connected in series.

Around the outer circumference of the rotor 2 and the inner circumference of the stator 6 are six radially polarised permanent magnets 10. Adjacent magnets are oppositely polarised. Thus proceeding counter-clockwise from the "12-0-clock" position in Figure 2 the magnets are polarised N-S, S-N, N-S, S-N, N-S, S-N. It will be noted from Figure 2 that adjacent windings 8 are polarised with the same polarisation unlike the magnets.

The motor of Figure 2 has an outside diameter of 90mm. The rotor has an outside diameter of 25mm. RadSh equals 8mm where RadSh is shaft radius.

Lm equals 4mm where Lm is length of magnet also called magnet thickness.

TWS equals 8mm where TWS means Tooth Width-Stator. The outside diameter at the bottom of a slot containing a winding is 74mm.

The rotor pole-arc, at the rotor outside diameter, is 18 mechanical degrees. The stator pole-arc at the rotor bore is 50 mechanical degrees.

The stator comprises six single-tooth coils with diametrically opposite coils connected in series to form the three phases.

The magnets in this example are NdFeB magnets. However ferrite, SmCo or any other type of magnet can be used.

Referring to Figure 4, when the machine of Figure 2 is operating as a motor, the three phase windings of the motor are energised by a three phase inverter 12 which is in turn energised from, for example, a DC source.

Referring to Figure 5, Figure 5 shows on the vertical axis electromotive force plotted against time on the horizontal axis for the two coils Al and A2 of a single phase A. It will be seen that whilst the individual EMF's of coil A and coil B are neither symmetrical nor sinusoidal the sum (coil A and coil B) of the electromotive forces generated by the two coils of one phase is symmetrical and sinusoidal.

This is advantageous because a sinusoidal EMF results in better operation and control of the machine. By way of comparison, Figure 6 shows the variation of electromotive force for a machine having six slots and four poles on the salient pole rotor. It will be seen that the total electromotive force (coil A and coil B) is much less sinusoidal and much less symmetrical than for the machine shown in Figure 5.

Referring to Figure 7, the present invention may be applied to a machine of the type described in the article "Switching flux permanent magnet polyphased synchronous machines" EPE'97 conference proceedings, 1997 as described above. Such a motor comprises a rotor 2 having five salient poles and a stator 6 having six slots as described above with respect to Figure 2. However, the machine of Figure 7 differs from the machine of Figure 2 in that the magnets 10- which are polarised circumferentially extend radially through the stator.

Claims (10)

1. A salient pole electrical machine comprising: a salient pole passive rotor having 5 salient poles; and a three phase stator having 6 slots, each slot comprising a winding and a permanent magnet which is polarised circumferentially.

2. A machine according to claim 1, wherein the said permanent magnets are provided by sections of permanently magnetic material which extends circumferentially between the rotor and an armature of the stator.

3. A machine according to claim 1, wherein the permanent magnets extend radially through the stator.

4. A machine according to claim 1, 2 or 3, wherein adjacent magnets are oppositely polarised and diametrically opposite windings are connected in series to form the three phases.

5. A machine according to claim 1, 2, 3 or 4 wherein adjacent windings have the same polarity.

6. A machine according to any preceding claim which is a motor.

7. A machine according to any one of claims 1 to 5 which is a generator.

8. A machine according to claim 5 comprising means for energising the stator.

9. A machine according to claim 8 wherein the energising means is an inverter.

10. A salient pole electrical machine substantially as hereinbefore described with reference to Figure 2 or 7 of the accompanying drawings.