GB2321560A - Alternating current electric motors and generators suitable for variable speed operation. - Google Patents

Abstract

An alternating current electric motor or generator has a single stator, with two or more sets of single-phase, or multi-phase main field windings, together with appropriate control windings, providing interactive magnetic field systems whose effects are summated by a single rotor to produce, in one variant, a brushless equivalent of the d.c. motor and, in another variant, a brushless,functional equivalent of the three-machine, Ward-Leonard, drive system, which can be controlled by voltage adjustment alone and is suited to system enhancement by operation with associated variable-frequency inverters. Various alternative arrangements are described, having specific benefit to particular applications including those involving directly cooled or super-cooled machines; wavepower generators; industrial, direct-on-line, soft-started machines for single or multiphase supplies and high performance requirements.

Description

ALTERNATING CURRENT ELECTRIC MOTORS AND GENERATORS SUITABLE FOR VARIABLE SPEED OPERATION This invention relates to an alternating current, rotating electrical machine, suited to variable speed operation 5y means of control of voltage. This can be regarded as the alternating-current analogy cf the shunt-field, d.c. rotating machine.

Conventional, direct current rotating machines are well known devices, typically comprising a magnetic yoke, providing a steady magnetic field, within which an armature rotates. The armature comprises a laminated steel rotor carrying 2 set of electrically conductive coils, each of which is connected at the appropriate times to a direct current supply, via a commutator and associated brushes. so as to produce electric current in suitable directions in conductors surrounded by the main field, thus summating a stream of torque impulses over a wide range of rotational speeds. The essence of the action is that the stationary field, the current carried by the rotor conductors and the direction or their permitted motion are all mutually at right angles, producing s torque reaction upon the rotor, and that the stationary magnetic field flux is locally distorted, but largely unaltered by the armature current. This form of electric machine has advantages of speed and tcrque control, but in zommon use, the need to convert the public a.c. supply to a d.c. supply at variable voltage introduces disadvantages of harmonic currents and poor power factor in the a.c. supply. Moreover, the brushes gradually wear away and require periodic replacement, neglect of which can engender sericus damage.

Conventional alternating current machines are well known devices typically comprising a magnetic yoke, carrying a set of coils connected to a multiphase mains supply and physically arranged to produce a rotating magnetic field within which is a rotor. The rotor is of laminated steel and may have one of several typical forms, as follows:- It may have magnetic saliency by virtue of shape, or by virtue of the injection of a.c. or d.c. electrical power via slip-rings and brushgear. It then rotates in magnetic synchronism with the stator field.

It may heve a set of peripheral conductors, which are short circuited, typified as the squirrel cage. It then rotates in equilibrium, asynchronously with the stator field. Sometimes the conductors are connected to slip-rings, so that external electrical resistance can be introduced for starting purposes.

The essence of the action is that the magnetic field rotates ard the rotor is constrained to follow. Low-power, a.c. machines are usually connected to a single-phase, stator supply and equipped with additional devices to ensure that they rotate only in the desired direction. These forms of the electrical rotating machine have advantages of simple and robust construction. but have disadvantages for speed adjustment and torquf control in that the source of the main power generally must be a variable frequency supply. which typically is generated by elaborate electronic control equipment switching the stator currents at relatively high frequencies.

According to the present invention, there is provided an alternating current electrical rotating machine of induction type, but having the action of the direct current machine without use of the comutator brushgear and without requiring variable frequency contro' of the main power supply. In effect, the machine combines on one stator and rotor. the essential functions of the 2-machine, Ward-Leonard motor generator set and associated d.c. motor.

Specific embodiments of the invention will now be described by way of example nly. with reference tc accompanying schematic drawings, in which the electrical sense of winding direction is narked with triangle, conductors directed into the diagram are marked with cross. . emergent conductors are marked with a dot and where appropriate to clarity, instantaneous magnetic polarities are shown in parentheses, for the moment of red phase R1 at maximum flux.

Alternative One Fig 1 shows s a schematic arrangement of a 6 pole machine with cardinal references M,N,P,Q, in which two sets of 3-phase, starconnected, stator windings R1,4;Y2,3;B1,4 : R2,3:Y1,4;B2,3. are magnetically opposed and balanced against each other to produce, by transformer action, nett equal and opposed e.m.f's. and consequently zero induced currents in a single, continuous rotor winding of the form depicted. The currents which flow in the stator windings comprise only magnetising and loss components and, ignoring the effects of stray flues, an electrical balance exists across an: diametric plane through the machine and a multiplicity of diametric links across the rotor windings may be incorporated, without passage of currents under the balance conditions. Consider now the effect of energising windings R5,6;Y5,6;B5.6 : R8,7:Y8.7:B8.7, from an adjustable voltage, 3-phase control supply which is in phase with the main supply. These windings are wound equally and in the sense of the dots and crosses. on corresponding limbs of the stator, to form primaries of three, phase displaced, single-phase transformers, by combining R5,6 & R8,7; Y5.6 > Y8.7: B5.6 & B8.7 respectively.

Other configurations are possible, to achieve similar effect. which is to form 3, phase disp1aced, crossed-field configurations. Because the magnetic circuits coincide with those cf the main field poles the resulting currents buck or boost the magnetising our rents of the individual phases of each 3-phase transformer, in consequence of which the under-excited transform@ will try to absorb current from the supply, to feed a perceived "load", while the over-excited transformer will try to return current from a perceived "source".

Assuming the main supply is adequately robust and the secondar windings are appropriately connected the result will be a potentially very large power circulation between the two, 3-phase transformers.

In the configuration depicted in Fig 1, the continuous, cross-linked, zig-zag winding can be, alternatively, a short-chorded, single, or multiple lap winding. occupying 12 slots and having a chord of slots. plus connecting links, similar to prior known arrangements for "equalising" connections in d.c. machines. The relative potential differences are depicted in Fig 2. other. symmetrical rotor winding arrangements. preferably incorporating regular, lap, zig-zag. or wave windings, a multiple of 12 slots and a multiple of 6 diametric links. are possible in order to achieve the same objective cf avoiding undesired circulating currents amongst the transformer secondaries.

Consider now. the effects of various operating states upon these transformer secondaries. In the absence of currents R5.6.8.7:Y5.6.8.7:B5.6.8.7 the vector diagram of secondary voltages is as in Fig 2 and no currents flow, because, for a balanced load. the vector sum of voltages within any group of 3 adjacent phases: i.e. any half of the circuit, is zero and the total round the circuit is also zero. When. for example, currents flow in Y5,S,0.7 then by single-phase, transfomer action, currents will try to flow in opposing directions in the rotor conductors encompassing the associated flux along axic PQ, but these intersect pairs of magnetic fields having similar polarity and negligible torque will result. This will only happen because the magnetic balance of the 3-phase transformers has been disturbed, resulting in 3-phase transformer secondary currents which flow in the direction P-Q, around both halves of the winding and return via the diametric link. Neglecting, for the moment. the phase lag due to leakage reactances in the transformer, this neatly arranges the phase of the current waveform in the rotor windings relative to the flux waveform in the Y field poles, maximising their potential interaction. Similarly, balancing currents flow under poles of the R and l B groups, returning via median links displaced, appropriately about PQ. resulting in threephase currents in the stator main windings, together with a potentially large, total torque. The number of independent, rotor windings and diametric links employed influences the variation of the current paths from the instantaneous, ideal positisns in synchronous space and. therefore, the extent of torque ripple resulting from rotation of the rotor. The leakage veactances. to which reference was made above, introduce a phase lag in the 3-phase transformer secondary currents, which may be important in some instances, but which can bo offset by various means, including applying a suitable phase-shift to the voltage supplied to the buck-boost, control windings to -e-establish the required relationship.

If the rotor is stalled, then the circulating currents are limited by the rotor impedance and the extent of magnetic imbalance. Hence for any given machine it is possible to prescribe a starting torque limit by controlling the applied initial imbalance: that is to say the voltage applied to the control windings.

If the rotor is not stalled. it accelerates from rest. to follow the rotating flux and immediately to produce proportionate. induced, back-e.m.fs, in all conductors intersecting fluxes of the field poles. in accordance with the known rules for asynchronous, induction machines. These are almost completely in mutual opposition and therefore have stile, total effect. However , there is an essential imbalance in magnetic fluxes, due to the single phase transformer action and the over-excited transformer therefore associates with larger back-e.m.fs. so that provided that the imbalance is not greater than that which can just sustain the required speed of rotation; i.e. the demand is not more than synchronous speed, against the applied actual load, then the motor accelerates tc that speed. Accordingly, at a speed which just extinguishes the recirculating primary currents, a stable situation exists and the input power matches the load plus losses. The contrcl currents required in windings R5,6,8,7Y5,6,8,73B5,6,8.7 are the magnetising currents and loss components appropriate to the respective, single phase transformers and are substantial, but tn general can be, proportionately, much less than the current components for delivering the main power of the machine.

The control voltage may be increased or decreased by means such as tap-changing, or variable auto-transformers or electronic controllers, to raise or leer the speed of rotation. It follows that if the running condition can he preset. then the motor can be direct-on-line started, without incurring unduly large current surges, but acceleration will decrease almost exponentially as the target speed is approached. It also follows that if higher dynamic performance is required, then additional servo-control must be exercised, to demand are than full load current and therefore to begin to utilise the inherently high short-term overload capability of the machine. A number of theoretical and practical advantages can be ascribed to a machine of such design.

It offers adjustable, preset speed operation , with direct-on-line, soft starting and with constant maximimum torque capability. from g fixed frequen , 3 phase. alternating current mains supply. without need of electromagnetically noisy or complex. high power control systems.

Its electrical power factor, even at less than maximum speed operation, can be advantageous because it is, in any case, desirable that the leakage reactances are made as low as is practical. In order to reduce variation of the parameters with speed or supply frequency. The proportionately short, closed magnetic circuits and relatively short, rotor end connections are benefIcial in this quest. Morever. by connecting R5.6,8,7:Y5.6,8,7:B5,6.8,7 with a phase-advanced supply. it becomes practical to adjust the power factor. even to a leading value. if required.

If offers high reliability because it incorporates neither commutators, sliprings. nor electrical brushgear. except as may be required for earthing the rotor assembly to satisfy statutory safety. cr protection criteria. tt is a magnetically closed, 3-phase, balanced system, offering scope to minimise any generated electromagnetic @ interference.

Short-term overload capacity 5 potentially that robust value which typically relates to a three-phase distribution transformer.

Runaway under internal fault conditions cannot result in higher speed than synchronous speed.

It does not require compensating windings to make it suitable for heavy loads and therefore it can have relatively sirooth polefaces, conducive to minimising eddy current losses and thus enhancing efficiency.

I' can take advantage of the improved control and / or higher frequency operation offered b: incorporation of electronic other inverter type power supplies.

If used with an electronic typs of inverter for control enhancement. but with the stator connected to the mains supply for fixed frequency operation, then even at high power ratings. by using the inherent current amplification and overload capacity of the machine to achieve servo-type, system performance, it can provide high response. motoring regenerative capability, without requiring bi-directional currents, or main current capability in the inverter.

It can be powered from battery-fed electronically generated 3 phase supplies which. together with its high overload capacity, should enable it to satisfy many traction applications, without need of change-speed gearboxes.

It offers potential to use a simple arrangement of heavy-current, low-voltage, continuous, rotor conductors, which could be made very robust to suit # igh speeds, for test rigs or turbo- generator applications. or could be tubes permitting relatively simple. direct cooling, possibly Including supercooling, for high efficiency operation, or compactness.

It is inherently suitable for unidirectional operation as e ither a motor or a generator, at any speed within the operating range and may be simply reversed, by swapping any two of the three line connections to the mains supply, thus reversing the phase sequence.

Multiples of the six-pole configuration may be be used within machine. to achieve enhanced performance where required; for example in large, slow speed, hydro-electric generators.

Other configurations of this design afternatlve are possible-. including both linear thrusters formed by "unrolling" the design onto a flat plane. and rotary versions with axially displaced stators acting on either the periphery, or the radial faces of a single rotor.

Alternative Two Xs depicted in Fig 3, which is not exhaustive, it 1.5 possible to achieve a secondary voltage balance of two, or more, transformers either with all windings connected in the same sense or with the primary and secondary windings of any transformer, together with its magnetize fluxes, all reversed. For its purpose, rig 3 assumes all coils commence at the triangular dot, are wound In the same direction and have suitable numbers of turns. Alternative one, above, used indings of the same sense. configured to achieve the necessary balance and physically opposed field poles were wherefore of the sam. electrical phase and magnetic polarity. Consider now Fig , in which the physically opposed field poles of the same electrical phase, are of opposite magnetic polarity: i.e. the set R1,Y1,B1 is opposite the set R2.B2.Y2. which also has the reversed phase sequence. For simplicity only one of several identical rotor windings is shown.

Each rotor winding is a continuous series of regular, zig-zags each spanning one sixth of the rotor circumference; i.e. one pole-pitch.

The assembly is. in effect, a pair of equally excited, three-phase transformers with interleaved yokes and a set of opposed secondaries. which are in overall balance. because for example. during rotation the sense of the rotor conductor at position R2 is automatically adjusted by the winding, relative to the adjacent pair of conductors. at B1 and Y1. However, potential differences will exist across certain rotor winding diameters and electrical linking across the rotor diameter cannot be employed in this alternative arrangement. Moreover, to apply an imbalance, as in alternative one. would be more difficult because there simply is not the equivalent magnetic symmetry. or balance. If now we apply an imbalance, say by reducing the supply to R1,Y1,B1 current will be induced conductors under each pole and the rotor will accelerate. but because one side of the rotor is operating super-synchronously, while the other is sub-synchronous, a complex waveform containing two, speed related frequencies of rotor, back-e.m.fs. Ill be produced, which can be considered as sum and difference frequencies. The difference component will al always be at the main power supply frequency and will be potentially useful, hut the sum component will be at supply frequency plus twice the slip frequency and may cause currents likely to interfere with satisfactory operation at some condition.

If the balance of the stator field excitation is maintained and currents are induced in the rotor conductors by means of conductors set into each poleface, depicted as delta connected windings BY. RY BR in Fig 5 and energised with an adjustable three-phase supply, which is in phase with the field supply voltages, then single-phase rotor currents will flow. . causing a corresponding tercue in the rotor. which will accelerate if free to rotate, generating speedrelated. back-e.m.f. which maintains an operational balance at a suitable running speed. This variation is therefore capable cf operating without circulating power between the state windings, but still providing adjustable speed by voltage control from a fixed frequency supply.

If the balance of the primaries in Fig s is maintained and the rotor is rotated in either direction. then 3-phase, s.m.fs. will be induced in conductors under field poles of each triad and current Will flow in opposite directions in pairs of conductors under opposed field poles of the same phase, attempting to oppose the motion. By transformer action, power will be transferred to the mains supply. It is noted that the system is internally symmetrical for one phase only ( R1,/R2) and relies upon external symmetry being provided by the mains, 3phase supply. to ensure that the vector sum of currents in the and B phases s indeed equal and opposite to the R phase current.

Nevertheless. it will he so and this alternative arrangement offers the following. eminent advantages as a generator of mains electricity from marine waves.

It is capable of power generation from a low-speed, or an oscillatory source of mechanical power: e.g. waves in water, driving a float, paddle, wheel, or turbine.

It is inherently asynchronous, stable and low-loss when at rest, naturally Inert when not being driven and s suited to direct 3-phase power generation at the applied system voltage and frequency, with only relatively simple protective devices such as built-in fuses or circuit-breakers. and without additional control systems.

It requires and augments an existing mains supply, but is a slave. not a stand-alone generator, so system protection and the control of a population of such machines is anticipated to be straight-forward.

It and be preset to a selection of power or voltage ratings, merely by changing winding taps, within a previously designed range.

The detailed design can he firmly based in known art relating to d.c. and a.c. rotating electrical machines.

By building-in leakage reactance, load or fault limitation can be automatic. within each small member of what might be a large population of devices in one locality.

It is largely benign, , being capable only of braking action in normal use and merely behaves as ar. unloaded transformer, continuously connected to the mains supply, when not actually generating power.

Any singly occurring, significant electrical fault is likely tc result in electromagnetic imbalance. leading to internal overload and tripping of the local protection device and therefore to prompt, self-disconnection from the system.

It is inherently robust, requires little maintenance and using known art. can be made weatherproof. or even submersile in seawater.

It will keep itself slightly warmer than its environment and therefore free of damaging condensation, without additional equipment.

It is well suited to quantity production and to long periods of unattended operation in remote locations.

Alternative Three For low power ratings, up to perhaps a few. kilowatts each machine. on normal industrial mains supplies, the design described in this alternative. which is a variation of alternativ ne, is capable of being opcrated with only a single-phase supply. This situation is depicted ing Fig 6. If B1.Y1,B2,Y2 are of equal s

Claims (6)

1. CLAIMS 1. An alternating current adjustable electric motor or generator made suitable for use with three phase electricity supplies at either fixed or adjustable frequency by having: a stator with 12 field poles or a multiple thereof wherein diametrically opposite field poles are energised in the same magnetic polarity by means of two or more sets of overlapping exciting windings each coil of which spans 3 poles and is electrically energised with the same phase of the mains supply so as to produce rotating 6 pole magnetic fields between the poles in accordance with prior known art and having in addition one or more sets of crossed-field buck- boost windings upon appropriate triplets of diametrically opposite poles so that the rotating magnetic fluxes can be systematically unbalanced at will to unbalance interleaved and opposed star-connected transformer primary circuits thus established by application of any suitable 3 phase electricity supply howsoever derived: a rotor or armature free to rotate on or about an axis between the poles and having conductive single or multiturn coils arranged as one or more continuous windings in which direction of current is reversed at each increment of one pole pitch in a closed circuit bridged at suitable equipotential points in the winding to create in zig zag form two or more sets of transformer secondaries balanced in opposition and passing negligible electric currents until systematic imbalance is applied by energising the buck-boost windings of the stator or by any alternative means thereby engendering turning moment or actual rotation together with the transfer of electrical power to or from the main electrical supply.
2. 2. An alternating current adjustable electric motor or generator made suitable for use with three phase electricity supplies at either fixed or adjustable frequency by having: a stator with 6 field poles or a multiple thereof wherein opposite field poles as viewed in a plane mirror located on a diameter are energised in the opposite magnetic polarity by means of exciting windings electrically energised with the same phase of the mains supply so as to produce contrarotating magnetic fields between the poles in accordance with prior known art: a rotor or armature free to rotate on or about an axis between the poles and having conductive single or multiturn coils such that direction of current is reversed in each increment of one pole pitch and arranged as one or more independant continuous windings in closed circuits to create in zig-zag form two or more sets of transformer secondaries electrically balanced in opposition overall and passing negligible electric currents until systematic imbalance is applied by adjustment of winding taps in the windings of the stator or by any alternative means or by externally driven rotation of the rotor thereby engendering turning moment or actual rotation together with the transfer of electrical power to or from the main electrical supply.
3. 3. An alternating current adjustable electric motor or generator made suitable for use with a single phase alternating current supply by having: a stator with at least 4 stator field poles wherein diametrically opposite field poles are of the same magnetic polarity and one or more such pole-pairs have in addition to the exciting windings either buck-boost windings or prearranged tappings on the exciting windings or any other means capable of engendering controlled imbalance between opposed field poles: a rotor or armature free to rotate on or about an axis between the poles and having conductive single or multiturn coils arranged as one or more continuous windings in which direction of effective current is reversed at each increment of one pole pitch in a closed circuit bridged at suitable equipotential points in the winding to create in zig zag form two or more sets of transformer secondaries balanced in opposition and passing negligible electric currents in the diametric links until systematic imbalance is applied by energising the buckboost windings of the stator or by any alternative means thereby engendering power circulation between stator field pole windings via the rotor windings by single phase transformer action so as to cause turning moment or rotation of the rotor together with transfer of electrical power to or from the main supply.
4. 4. An alternating current electric motor or generator as claimed in any previous claim wherein the electrical power supply incorporates any suitable number of phases and derives from electrical storage batteries or any other means and is converted by intermediate electronic equipment or any other means whether mobile or stationary.
5. 5. An electromechanical thruster or brake imparting or absorbing controllable linear or angular effort to or from any load by being an adaptation of any preceding claim.
6. 6. An alternating current electric motor or generator substantially as described herein.