This invention relates to a winding arrangement for an electrical machine. In particular, the invention relates to a stator winding for a synchronous machine which is driven in use by a variable speed prime mover. The invention finds particular utility where electrical power is provided to a network or load via a 12 pulse rectifier arrangement
Permanent Magnet, PM, electrical machines provide robust and compact motors and generators in many industries, such as aerospace for powers up to several 10's of kW's and marine propulsion systems and wind turbine plants up to and potentially above 5 MW.
The fixed magnets located on the rotor of a PM machine result in a rotating magnetic field having a constant magnitude. This means that the magnitude and frequency of the generated output voltage varies with the rotor speed. In aerospace applications, the rotor speed will be generally controlled by and proportional to the speed of the gas turbine jet engine and the drive shaft taken from one of the spools. In current applications, this speed may vary during a flight cycle by as much as 10:1 which relates to significant changes in voltage and frequency beyond acceptable limits for many applications. Consequently, the variable voltage and frequency is conditioned by a fully rated AC to DC converter which is arranged to produce a constant DC voltage by controlling the reactive power flow in the machine stator windings.
In marine propulsion systems shaft generators driven by the main propulsion engine(s) to provide electrical power are limited by the fact that the ship's electrical system normally requires a fixed frequency. This means that engine speed has to be kept constant which is generally inefficient. The use of a power electronics converter in a marine electrical network has the benefit that the main engine can operate at an optimum speed to maximise its efficiency, effectively decoupling it from the requirement to operate at fixed speed to support the fixed frequency marine electrical network.
In many applications, the PM machine is only required to act as an electrical power generator in which case the fully active, bidirectional converter as shown in
A drawback with the simple arrangement shown in
It is known that providing a 12-pulse system can help reduce the 5th and 7th harmonic currents in the PM machine. A two channel approach is known to construct 12-pulse transformer-rectifier system in high power electrical power transmission and distribution systems, and a similar approach may be applicable to a PM machine. Another known configuration is a 6 pulse synchronous system achieved by connecting the stator windings in a star and delta configuration which provides two channels electrically displaced 30 degrees.
The arrangement 310 shown in
Although this provides an improvement over the PM machine with conventional stator windings, the YD stator windings 314 are overly complex and do not address the additional current flow in the windings which still suffer from parasitic power dissipation as a result of the 5th and 7th harmonics.
The present invention seeks to provide an improved PM machine for DC systems.
The present invention provides an electrical system comprising an electrical machine, comprising: a rotor having a magnetic flux source mounted thereto; a stator winding arrangement having a first set of electrical connections to provide a first output channel, and a second set of electrical connections to provide a second output channel, the first and second channels being electrically out of phase, wherein the stator winding arrangement is constructed from a plurality of winding portions connected in electrical series, wherein each output channel is connected to a load or network.
Having a stator winding which is tapped at various points to provide a plurality of winding portions allows a two channel machine to be provided in a compact way with fewer losses than known systems.
The winding portions may comprise a plurality of alternately connected long and short windings.
The output channels may be poly phase and the short windings may provide the phase difference between the two output channels in that each phase of the first channel is taken from one end of a short winding. Each phase of the second channel may be taken from the respective other end of the short winding.
The first and second output channels may be approximately 30 degrees out of phase.
A third channel may be taken from the midpoint of the short winding for each phase.
The stator may include slots and turns of each long winding may be located in the same slot as turns of at least one short winding. All of the stator slots of a stator pole may be occupied by both the long and short windings. The long and short windings may be arranged around the stator such that the circumferential pole length of the long and short windings are the same.
The magnetic flux source may be a permanent magnet.
The stator winding arrangement may be three phase.
The electrical machine may have three or more output channels.
Each channel may be connected to the load or network via a DC convertor.
In a yet further aspect, the invention provides an aircraft, marine vessel or wind turbine having the electrical machine or the electrical system of the preceding aspects.
Embodiments of the invention will now be described with the aid of the following drawings in which:
a shows a PM machine with a star-delta winding arrangement which provides two channels which are 30 degrees out of phase.
b shows the voltage waveforms for the star and delta windings with the respective 6 pulse current harmonics.
a and 5b show an axial section of a 2 pole electrical machine and the associated winding arrangement.
a and 6b show the phasor arrangement of the windings.
In order to provide a constant electrical output for a network or load, the electrical machine 410 has two electrical outputs or channels 412a,b which are electrically offset from one another. That is, the electrical outputs are electrically out of phase by a predetermined amount.
The determination of the degree of phase shift between the channels 412a,b is described in detail below with reference to
Thus, as shown in
More generally, there is provided a polyphase stator winding arrangement having a plurality of serially connected windings arranged to provide n phases, where in each phase includes at least two electrical terminals which are electrically offset from one another by a predetermined and equal amount so as to provide the electrical machine with 2n phases sets which are electrically out of phase from each other.
The electrical machine 410 finds particular utility for providing an electrical generating system which provides power to DC convertors which suffer from 5th and 7th harmonic current distortions as described above. Thus, in the described embodiment, the two electrical channels 412a,b are connected to two six diode bridge rectifier 424a,b and convertor 426a,b arrangements which rectify the alternating current outputted from the PM machine 410 into DC which is the passed through a basic chopper circuit for adjusting the magnitude of the output DC voltage. Other arrangements of rectifying convertor arrangement are possible and other areas of application for the two channel machine are envisaged within the scope of the invention.
The invention is particularly advantageous as it provides a machine having low harmonic current distortion in the dominant stator windings, low levels of induced eddy current flow in the rotor and reduced torque ripple on the mechanical drive train which provides torque to the rotor. The improvement in stator and rotor current waveforms will lead to less localised heating and therefore the requirement for less cooling.
The stator windings 418 are made up from a plurality of long and short windings 422R,r,Y,y,B,b, which are connected in electrical series and arranged so as to provide a 30 degree phase shift across the short windings 422r,y,b. The stator arrangement is three phase, with each phase being represented by a short 422r,y,b and a long 422R,Y,B winding, with the two being opposite one another as shown in
The windings of a pole may partially overlap or fully overlap around the stator pole pitch. Where there is a partial overlap, it is necessary to ensure that the mid-point of the long and short windings are co-located with respect to the stator. This will ensure that the voltages in the long and short windings are in phase with one another. Hence, in a stator having six slots per pole, all six slots may be occupied by turns of the long winding, with the middle two additionally occupied by the short windings. In this way, the mid-points of the long and short windings are co-located with each other and the mid-point of the pole. As shown below, a preferred embodiment includes both the long and short windings being common to all of the slots of a given pole to reduce the space harmonics.
a shows a schematic representation of a stator 510 in axial section having circumferentially distributed slots 512(1-18) for the windings 522R,r,Y,y,B,b arranged around a rotor.
In the described embodiment a two pole machine is shown having a stator 510 which includes eighteen slots 512 evenly distributed around the circumference of the machine. This provides three slots per pole per phase. The selection of a two pole machine is for illustration only and should not be taken to limit the scope of the invention.
With reference to
Both the long 522R,Y,B and the short 522r,y,b windings are distributed windings and the connections between the phases are such that it provides the polygonal arrangement shown in
Once the distributed windings are in place for each of the long 522R,Y,B and short 522r,y,b windings and for each phase there will be two conductor ends per winding which are connected to provide the polygonal winding arrangement shown in
To provide the necessary phase shift between the ends of the short windings (and long windings) whilst having the windings in the same slots requires the windings to have the necessary relative electrical length. In order to do this the number of turns for the long and short windings are selected to provide a desired voltage ratio. The equations set out below and explained with reference to
With the polygon long winding 422R,Y,B voltage being defined by:
And the polygon short winding 422r,y,b voltage being defined by:
From the polygon vector drawing it can be found that:
And so for a turns ratio given by
the angle φ can be calculated as:
Where φ is the phase shift required across the short winding 422r,y,b. Hence, setting φ to a predetermined amount, 30 degrees in the described embodiment, provides the required S turns ratio for the long 422R,Y,B and short 422r,y,b windings via n. For a phase shift of approximately 30 degrees, the short windings 422r,y,b have approximately a third of the turns of the long windings 422R,Y,B.
The difference in the winding lengths and the voltages leads to different currents flowing through the long windings 422R,Y,B relative to the short windings 422r,y,b. It has been shown in one exemplary machine that the power provided from the long windings 422R,Y,B was 47.75 kW and the power from the short windings 422r,y,b was 13.21 kW, making the total power from machine 60.96 kW. Thus, the short windings 422r,y,b provided 27.6% of the power of the long windings 422R,Y,B whilst having 36.6% of the voltage due to the different electrical length of the windings. This results in a short winding 422r,y,b current flow of around three-quarters of that in the long winding 422R,Y,B. This means the short winding 422r,y,b can be made from thinner section conductor whilst operating at the same current density as the long winding 422R,Y,B.
The principles of the invention can be extended to cover other embodiments. For example,
The electrical machine described in the above examples is a permanent magnetic machine. However, it will be appreciated that the invention may be applied to other suitable electrical machines where appropriate. For example, the invention may find application in a wound field synchronous machine. Further, although the present invention is particularly advantageous to applications having variable speed drives, it may be applicable in scenarios which utilise a constant speed prime mover. It will also be appreciated, that the phase shift of 30 degrees and 20 degrees achieved across the windings should be taken as approximate. A phase shift which achieves significant cancellation of the 5th and 7th harmonics in 12 and 24 pulse machines respectively is considered to be within the scope of the claims.
Number | Date | Country | Kind |
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1313684.1 | Jul 2013 | GB | national |