SYNCHRONOUS ELECTRIC MOTORS

Abstract: A synchronous electric motor adapted for connection to a variable frequency solid-state supply and having rotor and stator windings so arranged that the conductor per slot density distribution of the rotor coils is preferentially designed to generate e.m.f. wave forms in the stator windings which combine with the supply voltage waveforms to create a desired waveform shape in the stator current.

Description: SYNCHRONOUS ELECTRIC MOTORS THIS INVENTION relates to an improved winding arrangement for synchronous electric motors and is beneficial to the performance of synchronous machines when supplied from solid-state frequency converters. An advantage of synchronous motors is that they are capable of operating at a leading power factor and can therefore co mutate thyristor converters. This type of commutation is often referred to as "natural" or "machine" commutation. Since no extra commutation components are required except during starting and at low speeds, the cost of naturally commutated converters is much less than that of forced commutated inverters which rely on extra devices such as capacitors or inductors to provide commutation of the output thyristors in the converter. Conventionally, a converter/motor arrangement consists of an input thyristor or diode bridge which supplies a unidirectional voltage to a d.c. link. A large inductance is often included in the d.c. link to ensure a smooth d.c. current in this part of the converter. The d.c. link current flows into an output thyristor bridge which is then controlled by suitable firing of the thyristor gates to switch the current into the stator windings of the synchronous motor in such a way as to provide a rotating magentic flux distribution in its airgap, which reacts with the rotor currents to provide a torque on the rotor shaft. The diversion of bridge current from one stator phase winding to another is often referred to as commutation. The commutation of currents in the output bridge can be achieved by voltages induced in the motor stator windings by the airgap flux. This "natural" commutation enables the output bridge to be constructed using a minimum of components, thereby reducing the cost of manufacture. A disadvantage of such a system is that the commutation process must be completed during a particular time period of each cycle when the stator voltages induced by the airgap flux are in the correct direction. The commutation process takes some time before it is completed, which further restricts the time at which commutation can be successfully accomplished during each cycle. The result of these restrictions is that the current and voltage waveforms in the motor do not have an optimum phase relationship with one another and the motor is said to be working at a low power factor. At the same time, the voltage and current waveforms in the motor contain harmonics and these can be responsible for both extra losses and pulsation of the output torque of the motor. It is an object of the present invention to provide in a synchronous motor, a rotor winding which will induce a voltage in the stator windings to determine the shape of the current waveform drawn from a supply converter so as to minimise the problems normally associated with a conventional converter/synchronous motor circuit. According to the present invention there is provided a synchronous electric motor adapted for connection to a variable frequency solid-state supply and having rotor and stator excitation systems, characterised in that the excitation system of the rotor is so arranged that e.m.f. waveforms are generated in the excitation system of the stator which combine with the supply voltage waveforms to generate a desired resultant current waveform shape in the stator, and in that the excitation system of the rotor is constructed as two component parts, the first component inducing an e.m.f. which substantially opposes the supply voltage and the second component inducing a further e.m.f. to circulate the desired current waveform in the excitation system of the stator. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:- Fig. 1 is a circuit diagram showing a conventional converter/motor arrangement; Figs. 2a and 2b illustrates a typical rotor winding arrangement for a conventional synchronous motor; Fig. 3 illustrates a typical stator winding ι voltage and current waveform of a conventional synchronous motor such as illustrated in Fig. 2; Figs. 4 and 5 illustrate typical waveforms of stator winding current and voltage for a synchronous motor made in accordance with the present invention; Figs. 6 and 7 illustrate theoretical rotor winding distribution of ampere-conductor density which will produce an approximation to the stator waveforms of Figs. 4 and 5 respectively; Fig. 8 is a diagram similar to Figs. 6 and 7 but when applied to the arrangement of Figs. 2a and 2b; Figs. 9, 10 and 11 illustrate the physical arrangements of rotor windings which produce approximations to the ampere-conductor density distributions of Figs. 6, 7 and 8 respectively; Figs. 12a and 12b show the conductor/ slot distributions of the rotor winding as two groups of coils, to produce the waveforms of Fig. 5. Figs. 13 and 14 are examples of rotor windings producing the conductor/slot arrangements illustrated in Figs. 9 and 10 respectively; and Figs. 15a and 15b are examples of rotor windings to produce the two groups of coil whose distribution is illustrated in Figs. 12a and 12b respectively. In Fig. 1 there is illustrated the conventional converter/motor arrangement including an input thyristor bridge 10, an output thyristor bridge 11 and a d.c. link 12 which includes an inductance coil 13. The motor 14 comprises stator windings 15 and a rotor winding 16. The rotor windings of a conventional synchronous motor such as those illustrated at 18 on rotor 17 in Fig. 2a are designed so that rotation of the rotor induces approximately sinusoidal voltages in the stator windings. Any departure from a pure sinusoidal waveform is mainly due to the slotted nature of the rotor core 17. Fig. 3 shows the relative phase of the voltage and current and the waveform shapes in a conventional motor as illustrated in Fig. 2a. Referring now to Figs 4 and 5, typical waveforms are shown of stator winding current and voltage for a synchronous motor using a rotor winding system in accordance with the invention. For the particular waveforms shown the large inductance in the d.c. link of Fig. 1 has been removed and the bridge is said to be voltage-forced. It can be seen that the stator voltage and current waves are in-phase, and the shape of the current waveforms can be selected by choosing different rotor winding arrangements. It will be understood that many more waveshapes than those shown in Figs. 4 and 5 can be achieved by use of the proposed rotor windings. The design of such windings may be described as follows. It is well known that the flux density distribu'tion in the airgap of an electric machine can be calculated if the distribution of both rotor and stator ampere-conductors is known. If the inner stator surface and outer rotor surface are considered cylindrical, then if the difference between these diameters is small compared with the magnitude of the diameters, the relationship between the magnetic flux density in the airgap and the ampere-conductor distributions is given by equation (1) as follows . b(x,t) ( (jj-,(x,t) + j r (x,t)) dx where b(x,t) is the expression for flux density j s (x, t) i s the expression f or stator ampere/conductor density j r (x,t) i s the expression f or rotor ampere/ conductor density g is the airgap dimension which is approximately equal to half the difference between the stator and rotor diameters. U-o is the permeabililty of free space. x is the circumferential distance around the airgap from a reference point on the inner stator bore. t represents time For a given required voltage and current waveform in the stator windings, expressions for b(x,t) and j_(x,t) can be obtained and equation (1) can be solved to produce an expression for j r (x,t). Once this is known the ideal distribution of rotor ampere conductors can be calculated. The resultant winding and current is then such that they will produce a value of ampere-conductors equal to j r (x,t). A simplified winding arrangement results if only those components of j r (x,t) which travel at the same velocity as the rotor surface are considered when selecting a rotor winding distribution. The current required in the rotor windings is then a d.c. or constant current. Examples of the proposed rotor windings, are illustrated in Figs. 6 and 7 which relate to the stator waveforms of Figs. 4 and 5 respectively. - 3 - The theoret ical amper e conductor dens ity distribution produced by a conventional synchronous motor w inding typi f ied by the example shown in Fig. 2a is illustrated in Fig. 8. Comp r ison of Figs. 6, 7 and 8 shows the difference between the sinusoidal distribution of rotor ampere-conductor density produced by a conventional winding in Fig. 8 and the di stributions of Fig. 6 and 7 produced by the proposed windings containing harmonic components of appropriate magnitude and phase. Windings which produce approxima t ions to the ampere-conductor density di stributions of Figs. 6 , 7 and 8 are shown in Figs. 9, 10 and 11 respectively. These figures illustrate the number of conductors in each slot on the rotbr, and again emphas i se the di f ference between the sinusoidal di stribution of Fig. 11 and the harmonic di stributi on of Figs. 9 and 10. The rotor windings di scussed so far are suitable for machines required to operate at a constant magnitude of stator current and with constant flux densi ty magnitude. This condition is usually achieved by making the stator terminal voltage linearly proportional to motor speed. A variation of the proposed rotor windings allows a synchronous motor to be operated with control of stator current waveform at any stator terminal voltage and any stator current magnitude. The rotor winding to achieve this consists of two groups of coils. The current in one group is varied to control the stator current magnitude and the current in the other group is varied as the airgap flux varies due to changes in the ratio of stator terminal voltage to speed. Figs. 12a and 12b show the two conductor/slot distributions to produce the waveforms of Fig. 5 where the magnitude of either the stator current or stator voltage waveforms can be varied to achieve a required output. The design of these winding groups can be determined using equation (1) in a similar manner to that- described above for the first single group windings. From equation (1) it can be seen that the rotor ampere conductor density, j r (x,t) can be obtained if both the airgap magnitude flux density b(x,t) and the stator ampere conductor density j s (x,t) are known. b(x,t) can be split into two components: b Q (x,t) which is the flux density wave resulting from equation (1) with no stator current, and b'(x,t). The remainder of the flux density occurs due to armature reaction. It can easily be shown that the component b Q (x,t) depends entirely on the applied voltage to the machine and that b'(x,t) depends entirely on the stator current waveform. Equation (1) can be rewritten as equation (2) in terms of the two components of b(x,t) as follows fj r (x,t) dx = g b n (x,t) + g b'(x,t) -f j s (x,t)dx Since the right hand side of equation (2) can be conveniently divided into two parts, g__ b n (x, t) and g b'(x,t) - j s (x,t) dx, the solution of j r will contain two parts, one part being proportional to g b^x,t) and the other part being proportional to cr -~-^o t <~*o b'(x,t) - I j s (x,t) dx. The two parts of the solution for the particular required waveform shapes given in Fig. 5 are given in Figs. 12a and 12b. As for the single group windings described above, only the components of the solution travelling at rotor speed are considered to simplify the winding. For various stator terminal voltages, the distribution shown in Fig. 12a will vary in magnitude, the abscissa at each point on the curve simply being scaled by a constant amount. The distribution can therefore be produced by a fixed winding distribution (conductors per slot) and by varying the d.c. current supplied to the group of coils. Similarly the distribution shown in Fig. 12b will scale proportionally with the required stator current magnitude and can be produced by a fixed winding distribution. By varying the current supplied to this second group of coils the current waveform shown in Fig. 5 will scale proportional to this current. The two components of rotor current distribution described above can be considered to serve two functions, as follows a) the first component g b Q (x,t) induces an e.m.f. into ~*o the stator coils which substantially opposes the applied voltage; and b) the second component, _g b'(x,t) - J j g (x,t) dx induces an e.m.f. into the stator coils which circulates the desired stator current waveform. Examples of the new rotor windings are given in Figs.13 to 15. Figs. 13 and 14 are examples of achieving the conductor per slot distribution of Figs. 9 and 10. Figs. 15a and 15b show an example of a rotor winding comprising two groups of coils producing an approximation to the ampere conductor density distributions of Figs. 12a and 12b. The winding examples shown in Figs.13 to 15 are of double layer construction, but it will be understood that the required conductor/slot distribution could be achieved by any other well known winding technique such as single-layer and irregular concentric grouping of coils. It will be understood that although this description of a synchronous motor/converter circuit includes illustration of a three-phase stator winding arrangement it is possible to use the proposed rotor windings in conjunction with any number of stator phase groups. In many cases it is advantageous to increase the number of phases from three. It is also understood that although the invention has been described specifically for naturally commutated converters it can equally well be beneficial when the motor is used in conjunction with forced commutated converters using any solid-state switching device in the bridge. It is also understood that at very low speeds a naturally commutated converter cannot be commutated by a conventional synchronous motor since the induced emf in the stator windings is insufficient to commutate the current. The proposed rotor winding technique can be used with any of the well known methods of starting such converter/synchronous motor systems. Whilst a motor has been described having rotor coil windings as its excitation system, this may instead be provided by a number of permanent magnets fixed to the rotor and having various field strengths so distributed as to generate in the stator windings the desired current waveform shape.

Claims: CLAIMS 1. A synchronous electric motor adapted for connection to a variable frequency solid-state supply and having rotor and stator excitation systems, characterised in that the excitation system of the rotor is so arranged that e.m.f . waveforms are generated in the excitat ion system of the stator which combine with the supply voltage waveforms to generate a desired resultant current waveform shape in the stator, and in that the exc itation system of the rotor is constructed as two component parts, the first component inducing an e.m. f. which substantially opposes the supply voltage and the second component induc ing a further e.m.f. to circulate the desired current waveform in the excitation system of the stator. 2. A synchronous electric motor according to Claim 1, wherein the exc itat ion system of the rotor is provided by a number o f rotor co i l w indings p re f e rent i a l ly di stributed around the rotor to create the des ired e.m. f . waveforms in corresponding coil windings of the stator. 3. A synchronous electric motor according to Claim 2, wherein at least some of the rotor coils are arranged so as to be non-concentric about, the direct polar axi s. 4. A synchronous electric motor according to Claim 2 or Claim 3, wherein the rotor coils are wound such that in operation, the electromotive forces generated in the stator coils contain at least one harmonic of the fundamental frequency, the phase and magnitude of said harmonic being determined by the conductor per slot distribution of the coil windings, and pre-selected to produce the desired current and voltage waveforms in the stator coil windings. 5. A synchronous electric motor according to Claim 1, wherein said two component parts are combined in a single excitation system. 6. . A synchronous electric motor according to Claim 1, wherein said two component parts are provided' by two separate excitation systems. 7. A synchronous electric motor according to Claim 2, wherein the rotor coil windings are arranged as two groups of coils such that the current in one group may be varied to control the stator current magnitude, whilst the current in the other group can be varied as the air gap flux varies due to changes in the ratio of stator terminal voltage to speed. 8. A synchronous electric motor according to Claim 2, wherein the rotor coil windings are of double layer construction . 9. A synchronous electric motor according to Claim 1, wherein the excitation system of the rotor is provided by a plurality of permanent magnets attached thereto and of various field strengths distributed around the rotor so as to generate the desired resultant wave form shape in the stator . 10. A synchronous electric m otor adapted f or connection to a variable frequency solid-state supply, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

Inventors: MCLEAN GRAHAM WILSON, MCLEAN, GRAHAM, WILSON
Applicants: UNIV MANCHESTER, THE VICTORIA UNIVERSITY OF MANCHESTER
Application year: 1986
Main classification: H02K19
IPC codes: H02K19