GB2350239A - Magnetic circuit laminations forming a capacitor - Google Patents

Abstract

At least one of the laminated stator or rotor of an electric machine has at least one capacitor formed by connecting adjacent laminations 1, the dielectric comprising the inter lamination insulation 2. The capacitor(s) 12 thus achieved are used in the drive circuit. Inductor arrangements 14 employing the laminations as a core, as set out in WO98/40957, may also be used with the present arrangement.

Description

2350239 ELECTRICAL MACHINE AND DRIVE SYSTEM INCORPORATES PASSIVE

COMPONENTS IN THE MACHINE.

Field of the Invention.

This invention relates to systems including electrical machines (machines for conversion between electrical and mechanical energy) and "powerelectronic drives" in their most general sense where the function of these "drives" may be to achieve a fully variable-speed drive or some other machine-related function such as active power-factor correction for induction machines, softstarting to prevent excessive in-rush currents at machine startup, or some other function.

Background to the Invention.

A class of machine-drive systems is described. 'Me term "machine" here refers to a energyconversion device which transforms electrical energy into a mechanical form or vice-versa or both. The term "drive" here refers to a (sub)system of circuits comprising both passive components and power electronic switching which is operative to exert some form of control over the machine. The drive may include some intelligence which determines the switching sequences and the machine may include some sensors for informing this intelligence. The normal understanding of the term "drive" in similar contexts to this is a variable-speed drive which may contain internal control loops for velocity and possibly also for position. The present definition of drive also includes (sub)systems whose primary function is soft-starting, active power-factor correction or any other machine-control function.

The electrical machine can be considered to comprise at least 2 major components capable of relative motion. At least one of these components includes a substantial stack of ferro-magnetic laminations. This stack (these stacks) serve as a major portion of the magnetic circuit of the machine. Most conventional machines include two such stacks. These stacks of laminations are typically present for the following reasons:

(a) For the reduction of eddy-current losses. An alternating magnetic field can be present in a stack of iron laminations (having layers of insulation between adjacent laminations) without substantial power losses occurring as a result of circulating currents. If thin laminations are used, relatively high frequencies of alternating magnetic field are possible with relatively small eddycurrent losses.

(b) For ease of production of prismatic components having a complex section. The stator poles of conventional DC machines and the sets of rotor poles from conventional salient-pole (wound rotor) synchronous machines are often constructed as sets of laminations at least in part for this reason.

The invention contained herein draws the laminations in one or more of these stacks into use as either as single discrete capacitor or as a set of capacitors.

A previous invention (Application No. PCT/GB/98/00540, Patent No. WO 98/40957) has described how a stack of laminations forming the stator core of any rotating AC machine can Page I serve to form one or more inductor or transformer cores at the same time as serving its normal function of conveying magnetic flux from one part of the airgap to another. This secondary utilisation of the stator core is achieved through the use of additional windings which drive flux in complete loops encircling the machine axis. The primary role for the inductors/transformers created in that previous invention is in connection with controlling the currents arising in the machine as a result of connection to a power-electronic drive.

Passive components (inductors, shunt resistors and capacitors) constitute a major proportion of the cost of a modem power-electronic drive. Given that inductance can be integrated into the electrical machine (WO 98/40957), it is natural to wonder whether the other two classes of component can also be achieved. Incorporating resistance is straighforward enough but the fact that resistors dissipate energy makes this much less attractive from the start since electrical machine design is virtually always limited by thermal considerations and incorporating another major heat source would inevitably require that the cooling provisions for the machine be enhanced along with its thermal mass. All motors possess at least some very small inherent capacitance since they have conductors which are designed to run reasonably parallel to each other and parallel to the ferromagnetic components. This capacitance is a nuisance in some respects since it can interact with various components of motor inductance to cause internal resonances within the motor windings at unfortunate frequencies and it often causes an undesirably easy path for currents to pass into the machine frame. It is not readily usable because (a) is it many orders of magnitude too small to be useful and (b) it would be extremely difficult at the very least - to exercise independent control over the voltage on the main current-carrying conductors since this is determined largely by the requirements of the motor duty anyway.

Virtually all significant capacitors are constructed using the concept of a dielectric material interposed between parallel conducting surfaces. Virtually all electrical machines are now AC machines and of these, more than 99% include - as a very substantial proportion of their mass - a core of ferromagnetic laminations stacked up with thin layers of insulating material between adjacent laminations. The primary function of the laminations is to act as a part of the magnetic circuit of the electrical machine. However, the laminations are always electrical conductors and any insulation has some dielectric properties. If the laminations are connected electrically in such a way as to create significant electrostatic fields across the interlaminar insulation, then useful levels of capacitance may be achieved.

Power factor correction for the lagging current drawn by induction motors and other machines is one potential motivation for using capacitors. The capacitance which is achievable through connecting the laminations in the context of a typical industrial induction machine can provide only a very small proportion of the power-factor correction desired with present-day dielectric materials and without artificially extending the motor dimensions by large proportions simply to push up the capacitor area and dielectric volume.

In this invention, the logic of integrating what are essentially passive components of the powerelectronic drive circuit into the electrical machine is extended to include capacitance. The capacitors may be deployed at the "machine-end" of the drive for filtering the voltage harmonics appearing at the motor windings (thereby reducing the high transient electrical stresses occurring internally in the main machine windings). Alternatively they may be used internally in the drive particularly as the DC fink capacitors which are invariably present in those drives which have a DC link between a rectifier stage and an inverter stage. Finally, they may be used as part of the filter (sub)system present at the interface between the drive and the supply where they can serve Page 2.

to help prevent unacceptable levels of conducted electromagnetic interference from passing back to the supply from the drive and/or to help reduce the susceptibility of the drive to damage from high levels of conducted interference from other sources.

EMBODIMENT.

The central concept of the invention is that electrical connections are made to individual laminations thereby "commoning" two or more subsets of the complete set of laminations in a stack such that the stack becomes active as one or more capacitors and that these capacitors are then connected into the power electronic drive.

Fig. I illustrates, schematically, the concept of connecting the laminations electrically. A stack of laminations comprises permeable and conducting laminations (1), each of which is separated from its immediate neighbours by a layer of insulation (2). In normal practice, this layer of insulation is achieved by coating each individual lamination on both sides with a thin layer of varnish or similar insulating coating so that the total thickness of insulation between two adjacent laminations is twice the thickness of insulation in each side of each individual lamination. However, it is possible that the insulation layers could be discretely formed layers interposed between bare metallic laminations. Each of the laminations in the laminated stack has a tag, (3) protruding from it with which an electrical connection can be made to the lamination with the effect that electrically commoned subsets of laminations (4) are achieved. No attempt has been made in Fig. 1. to show the detailed shape of the laminations. In the case of a laminated stator core of a rotating machine, they would ordinarily have a circular outer edge (apart from the connecting lug (3)) with an inner edge which had teeth formed in it to accomodate stator windings. The exact shape of the laminations is of little consequence. The fact is that such stacks are present in electrical machines for primary purposes which are not connected with achieving capacitances using them. By connecting to the edges of these laminations, some useful capacitance can be achieved for very little cost. Normally the entire stack of laminations used for capacitive purposes would be electrically isolated from the framework of the machine.

Fig. 2 illustrates, schematically again, the concept of integrating the capacitors formed within the electrical machine into the drive circuits. The system comprises a drive (5) connected to a mainly electro-magnetic "machine" (8). The drive comprises a power-electronic section (6) and may also include a control section (7) but this may not be present. The powerelectronic section of the drive receives supply power through a set of conductors (17) and it provides electrical power directly to the main motor windings (10) through the set of conductors (9). The drive's power electronic section (6) exchanges bundles of energy with the capacitances realised within the motor (12) through a set of conductors (11).

The power-electronic section may also exchange bundles of energy with some inductances (14) also realised as an integral part of the "machine" through the set of conductors (13). We expressly note here that there may not be any discrete inductances realised integrally with the machine stator and that even if there are, these may not have independent connection to the drive but may be part of a resonant or filter circuit including the realised capacitances and the inductances.

The machine may incorporate some sensors (16) (typically Hall-effect probes to detect flux and resolvers/encoders for position and velocity measurement) and the signals from these sensors might be returned to the control portion (7) of the drive through sensor conductors (15). The command signal for the entire drive enters through the port (18).

Figure 3 shows how a single capacitance may be achieved by simply connecting alternate laminations to one of the two common points.

Figure 4 illustrates how a star configuration of capacitances can be achieved and Figure 5 shows how a delta configuration can be achieved.

it is straightforward to see that thin foil conducting layers may be inserted between the relatively thick laminations to double the capacitor area and half the thickness of the capacitor dielectric. By treating the connection of the foils in an identical way to the connection of the lamination plates, the same effects decribed and illustrated in the case of "normal" laminations alone can be achieved.

it is also straightforward to see that it is possible to establish two or more quite-independent capacitors in the motor stator core if some insulating spacer pieces can be used to break the stator core into two or more axial sub-lengths. The spacer pieces must be several times thicker than the dielectric thickness for obvious reasons but this may still be a very small distance. Fig. 6 illustrates how a stator core may be divided axially using spacer pieces (19) for the achievement of several uncoupled capacitors.

SPECIFIC EMBODIMENTS.

The following specific embodiments are described. No further figures are provided and the descriptions are very brief because in all cases, the embodiment comprises substituting what we shall term lamination capacitors of a conventional capacitance in some established circuit types. In all of these embodiments, consideration would be given to choosing an appropriate thickness of dielectric to withstand the voltages which might be applied and to provide reasonable levels of capacitance.

(a) The lamination capacitor is utilised as one or more major capacitors employed as the "DClink capacitor" of any power-electronic converter which transforms into DC before the final inversion stage.

(b) The lamination capacitor(s) is(are) utilised as one or more major capacitors in a filter circuit. Wherever a separate discrete capacitor could be found in a conventional machine-drive system, this might be replaced by the integrated laminations capacitor. The filter circuit might be placed between the mains supply and the drive electronics to prevent high frequency harmonic distortion on the supply caused by the drive switching events or it might be connected between the drive and the motor in order to prevent the sharp voltage transients caused by the switching events in the drive inversion stage from damaging the motor windings.

(c) The lamination capacitor(s) is(are) utilised to achieve a "resonantbank" inverter with virtually 100% exchanges of energy between the capacitor(s) and one or more inductors. These inductors may or may not be realised as integral elements to the machine core as described in (WO 98/40957). By selecting the way in which the capacitors and inductors are connected Page 4.

together, either a high or low resonace frequency may be obtained. The switch transitions are timed so as to incur minimal power losses and may also be influenced by the desire not to propagate particular frequencies back into the supply.

This particular application is especially serendipitous when one examines the storable energy in the magnetic iron and in the dielectric of the insulation. A continuous constant-cross-section magnetic circuit of an inductor which is operated up to flux densitities of LOT in iron whose relative permeability is between 1000 and 2000 may store energy in the magnetic iron at a density of around 300 - 600 J.m. Because we can effectively reduce the apparent permeability of the iron core material by partially saturating it with the main motor field, the energy density can be pushed up considerably from these numbers to numbers in the order of 2. 5 - 5U/m. The energy density achievable through electrostatic storage within a modem capacitor dielectric can reach up to 250 kJ/d.

In a typical motor, the total volume of interlaminar insulation is generally in the order of 1% of the total volume of magnetic iron. Hence there is a reasonably fortuitous natural balance between the energy storable in capacitance and that storable in inductance using the back of core.

(d) The lamination capacitor(s) are utilised in static VAR-compensation circuits. These are particularly relevant for induction machine systems but are relevant to a major fraction of all AC machine-drive pairs. These circuits may or may not also use significant amounts of discrete inductance and if they do, that inductance may be provided either separately or as an integral part of the machine following WO 40957.

Static VAR compensation can be attempted - at least in theory - with any values of capacitance and inductance. If very small energy-storage capabilities are available, then the power-electronic switches will have to operate extremely frequently in order to "charge-up" these small energystores and then discharge them into the supply in such a way that leading current is drawn. Along with fast-switching comes high harmonic distortion and heavy switching losses. It is attractive - if not mandatory - to employ some degree of resonance behaviour in static VAR compensation (by incorporating both inductance and capacitance). This is achievable through connecting capacitance and inductance into a single resonant circuit which is deliberately excited into resonance.

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Claims (7)

1. I An electrical machine and drive system wherein the laminations of one or more of the stacks of laminations within the electrical machine are individually connected electrically in such a way that they collectively form one or more discrete capacitors drawing the interlaminar insulation into use as a dielectric and which capacitors are then connected into the circuits of the drive for use therewith. in this context "electrical machine" may mean any form of electromagnetic machine usable for converting between mechanical and electrical energy forms and "drive" is any (sub)systern of circuits comprising passive and active (switched) components which may be used to exert any control over the behaviour of the electrical machine.
2. 2 An electrical machine and drive system as described in (1) in which the stack of laminations from which capacitors are being formed is subdivided into two or more discrete lengths separated by insulating spacers which are several times thicker than the dielectric thickness such that one or more capacitors can be realised in each discrete length which are uncoupled from the capacitor or capacitors realised in any other discrete length.
3. 3 An electrical machine and drive system as described in (1) or (2) in which the number of conductive surfaces is increased by a factor of up to 2 by interposing very thin foil conductors between laminations which have dielectric coating on each side.
4. 4 An electrical machine and drive system as described in (1),(2) or (3) in which one or more multi-phase capacitors are achieved which may be of "ring" or "star" configuration wherein the "ring" configuration is achieved by connecting successive individual lamination to successive phase terminals or the "star" configuration is achieved by connecting, say, all of the odd-numbered laminations to earth and connecting successive even-numbered laminations to successive phase terminals.
5. An electrical machine and drive system as described in (1)-(4) above in which the electrical machine is a "rotating machine" and in which machine discrete inductors are simultaneously realised by driving magnetic flux in complete loops in the back of stator core encircling the machine axis and in a direction generally parallel to the airgap of the machine (as described in WO 98/40957) and which discrete inductors (transformers) are also incorporated into one or more circuits of the power-electronic drive.
6. 6 An electrical machine and drive system as described in (5) in which at least some of the discrete capacitors formed from the electrical connection of machine laminations and at least some of the inductances formed by driving oscillatory magnetic flux in complete loops around the back of stator core are connected together into some dynamic circuit which is incorporated as a part of the power electronic drive.
7. 7 An electrical machine and drive system as described in (6) in which the dynamic circuit is a resonant circuit which is utilised to advantage within the drive to provide further control over voltages across switches either at the switching instants or during the "on" phases of a given switching cycle.

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