ELECTRIC MOTOR

Abstract: Electric motor comprising a rotor (1), a stator (2) and at least two pole pairs. One particularly preferred application of the electric motor according to the invention is small-size accurate servomotors of high capacity. At present the feed back information required by servo motors is generally obtained from at least partially separate transduced means attached to the servomotor, such as an optical or magnetic coder, a tachogenerator or resolver. A drawback of these known solutions is mainly that the volume and weight of the motors is high. This problem is eliminated in the motor according to the invention by providing it with an integral resolver so that primary windings (4a, 4b) of the resolver are arranged in the rotor (1) of the motor, and that secondary windings (11a, 11b) of the resolver are wound round the stator (2) of the motor in such a manner that the winding wire goes alternately inside and outside the stator.

Description: Electric motor The invention relates to an electric motor com¬ prising a rotor, a stator and at least two pole pairs. One particularly preferred application of the elec¬ tric motor according to the invention is small-size accurate servomotors of high capacity. At present, feed-back information required by servomotors is generally obtained from at least par- tially separate transducing means attached to the mo¬ tors, such as an optical or magnetic coders, a tacho- generator, or resolver. A drawback of these known sol¬ utions is that the volume and weight of the motors is high. In addition, the adjusting system is liable to disturbances, which is due to the clearances and flex¬ ibility of the coupling. The object of the present invention is thus to eliminate the drawbacks described above. According to the invention it has been found that the resolver can be integrated in the basic structure of the motor if the number of pole pairs in the motor is at least double as compared with that of the resolver. With this kind of motor, the above-described drawbacks of the prior art are eliminated by providing it with an integral resolver so that the primary winding of the resolver is arranged in the rotor of the motor and that the secondary winding of the resolver is wound round the stator of the motor in such a manner that the winding wire goes alternately inside and outside the stator. As is known, the absolute angle position, velo¬ city, and direction of the rotor can thus be deter¬ mined on the basis of signals generated in the se¬ condary winding of the resolver. These can be used for further controlling the commutation, velocity, and parking of the motor. According to the invention it is possible, if desired, to integrate a synchro or some other trans¬ ducing means of the type with a rotating transformer in the basic structure of the motor. Amongst these, however, the resolver is the most useful alternative in view of the control of the servomotor, for in¬ stance, wherefore it is used in this particular case. As used herein, the term "resolver", however, has to be considered to have a wider meaning in such a way as described above. A motor effected according to the invention is clearly smaller and lighter than known motors compris¬ ing a separate resolver for obtaining feed-back infor- mation. Consequently, it is particularly suited for uses in which a small size and lightness are among the basic requirements set for the motor. Such uses in¬ clude e.g. aeroplanes. In addition, the structure ac¬ cording to the invention eliminates the liability to disturbances caused by the flexibility and clearances. One more major advantage is that the motor is cheaper than previously. In the following the invention and its pre¬ ferred embodiments will be described in more detail with reference to the examples of the attached draw¬ ings, wherein Figure 1 is a partial longitudinal cross-sec¬ tion of a brushless permanent magnet motor when the number of pole pairs in the resolver is one, Figure 2 is a cross-sectional view of the motor of Figure 1 in the direction of the line A-A, Figures 3a and 3b illustrate one way of winding the secondary windings of the resolver in the motor shown in Figures 1 and 2, Figures 3c and 3d illustrate an alternative way of winding for the way of winding of Figures 3a and 3b, Figure 4 is a simplified view of the secondary winding of the resolver as a planar view for the de- monstration of noise voltage. Figure 5 illustrates a noise voltage occurring in the secondary winding of the resolver in the case of a quadripole motor, and Figure 6 illustrates a noise voltage occurring in the secondary winding of the resolver in the case of a bipolar motor. Figures 1 and 2 illustrate a hexapolar brush- less permanent magnet motor the rotor and stator of which are indicated with the reference numerals 1 and 2, respectively. The rotor periphery is formed by six magnetic poles Nl to N6, of which the three upper poles Nl to N3 are visible in the sectional view of Figure 2. The primary or rotor winding of the resolver is formed by two windings 4a and 4b which, in the spe- cific motor shown in the figures, are wound round pole magnets disposed at an angle of 180 degrees with respect to each other. Thus only the upper winding 4a is visible in Figures 1 and 2; however, for the sake of clarity, the lower winding 4b positioned at an angle of 180 degrees with respect to the winding 4a is provided with its own reference numeral. The alternat¬ ing reference voltage of the resolver is applied to the windings 4a and 4b by means of an inductive coupling. In this particular motor, this takes place simultaneously by means of inductive switches (wind¬ ings) 5 positioned at both ends of the rotor. From these inductive switches, the reference voltage is induced in windings 7 positioned at the ends of the rotor; the windings 7, in turn, are connected in series with the two rotor windings 4a and 4b of the resolver. In this particular motor the inductive coup¬ ling is designed so that it utilizes a shaft 8 of the motor as a part of the magnetic circuit. Thereby the shaft has to be of special construction. Within the area of the rotor, it has to be ferromagnetic and mag¬ netically soft, within the area of the shaft ends pre¬ ferably non-ferromagnetic. Of course, other solutions for obtaining an inductive coupling are also possible as well as other ways of applying a reference voltage to the rotor windings 4a and 4b of the resolver, e.g. slip-rings. The inductive coupling operates without brushes, as is desireable in all parts of a brushless motor. A stator winding 10 is arranged in winding slots 9 formed in the stator 2 in a manner known per se. The secondary or stator winding of the resolver is wound round the stator 2, which secondary winding is formed by two windings 11a and lib positioned at an angle of 90 degrees with respect to each other. In the cross-section of Figure 2, the winding wires are vis¬ ible in pairs so that one wire belongs to the winding 11a and the other to the winding lib. The winding wire in both windings goes alternately on the stator 2 and in the winding slot 9. Figure 1 shows two wire pairs of the secondary winding of the resolver in the upper half of the motor in a cross-section so as to show that they extend in an oblique manner. Figures 3a and 3b illustrate in more detail the way of winding of the secondary windings 11a and lib of the resolver in this particular tooth-type motor in which the winding slots 9 are bevelled in the longitudinal direction over one spacing. Figure 3a il¬ lustrates the winding 11a, and Figure 3b the winding lib. Both windings are shown in a planar view; the winding has been cut in the middle, and the halves are coupled in such a manner that one half in the coupling is the reverse of the other. The winding wire portion positioned in the winding slot 9 is indicated with a broken line and the winding wire portion positioned on the stator 2 with a continuous line. As appears from Figure 3a, the winding wire of the winding 11a first goes round through 180 degrees in every other winding slot 9, returns thereafter to the starting point and goes in every other winding slot 9, whereafter it goes over to go round the other half of 180 degrees in a similar way. The winding wire of the winding 11a shown in Figure 3b goes in the corresponding way, how¬ ever, with a displacement of 90 degrees with respect to the winding 11a. When the winding is carried out as described above, the winding is distributed evenly in this specific dc motor in which the winding slots are bevelled over one spacing. In principle, a single winding turn of one winding of 180 degrees shown in Figures 3a and 3b is enough for obtaining a resolver signal, the rest only amplifies the signal. Figure 3c shows an alternative for the way of winding shown in Figure 3a. In this case, the winding wire first goes round through 180 in every other winding slot 9, then goes over to the other half of 180 degrees, round which it goes twice in every other winding slot 9, whereafter it goes back to the first half of 180 degrees and goes round it in every other winding slot 9. A corresponding alternative for the way of winding of Figure 3b is shown in Figure 3d. The winding wire of a winding 12b shown in Figure 3d goes similarly as the winding wire of the winding 12a in Figure 3c but with a displacement of 90 degrees with respect to it. As to the windings shown in Figures 3a to 3d, it should be noted that their starting point may be positioned arbitrarily along the periphery of the stator whereby the way of presentation may change correspondingly. It is further to be noted that irre¬ spective of the way of winding the winding wires may be wound several times, that is, a single winding wire may be replaced with a skein comprising several wires. In the following, possible noise voltages and the requirements they set to the motor will be dealt with. A noise voltage may occur in the secondary wind¬ ing 11a, lib or 12a, 12b of the resolver due to (i) the rotation of the permanent magnetic field with¬ in the stator 2, and (ii) alternating magnetic fields caused by stator currents. Their effect on the second¬ ary winding of the resolver can be illustrated by an imaginary planar view of the secondary winding of the resolver shown in Figure 4 and by simplifying the winding to one typical winding in each winding half (a and b) which are connected in series as shown in Fig¬ ure 4. As known, the electromotoric force E induced in the winding can be calculated by the formula E = - N dt wherein N is the number of winding turns and $ is mag¬ netic flux. When the winding turns are positioned evenly round the stator, the area of the graph of change of the magnetic field illustrates the induced total voltage within ranges a and b. When the winding halves are coupled as described above, a condition for a value zero of the pole voltage is that the total change of the field within the range a reduced with the total change of the field within the range b is e- qual to zero. A special case of this condition is that the total change of the field within both the range a and the range b is equal to zero. Case (i) will be discussed first, that is, the noise voltage induced in the secondary winding of the resolver by the permanent magnet field rotating within the stator. It is assumed that the field caused by the poles is sinusoidal, whereby the change of the field, too, is sinusoidal, though 90 degrees out of phase therewith. Accordingly, the value of change (-≤H?=- ) of the field has alternatively the minimum and the maximum value at shift points between the poles, at which points the value ( y ) of the field has zero crossings. Figure 5 shows a situation when a quadri- polar permanent magnet field rotates within the stator 2 at a given moment. The curve thus illustrates the d S value of change eft- of the field; for the sake of sim¬ plicity, the curve is not shown in the sinusoidal form; instead, it is formed by rectilinear parts be¬ cause the shape of the field does not affect the final result in this case. However, the fields have to be equal in shape at each pole (Nl, N2...). The horizon¬ tal axis in Figure 5 represents the length of the periphery of the stator, and the section of each pole is indicated with the respective reference Nl to N4. As appears from the figure, the areas of the graphs of change compensate each other separately both within the range a and the range b, whereby the induced pole voltage is zero. This will happen when the number of poles is even and at least four. In case (ii), the magnetic fields caused by stator currents are analogous with the above-described permanent magnetic fields, and so are the changes thereof. As compared to case (i), the only difference is the phase shift of 90 degrees when the motor oper¬ ates ideally. Accordingly, the magnetic fields caused by the stator currents do not cause an additional vol¬ tage in the secondary winding of the resolver when the number of the poles is even and at least four. Figure 6 further illustrates a situation cor¬ responding to that of Figure 5 when a bipolar magnetic field alternates within the stator 2. As appears from the figure, the area of the graph of change has a de- termined total value (unshadowed area) both within the range a and the range b, depending on the position of the poles with respect to said ranges a and b. Consequently, a voltage is induced in the secondary winding of the resolver. Correspondingly, it appears from the figure that a voltage is induced in the se¬ condary windings of the resolver if a magnetic field having an even number of poles alternates within the stator. An absolute prerequisite for the motor accord¬ ing to the invention is that it comprises an even num- ber of poles which are at least four. Similarly as described above, it is also poss¬ ible to prove that when the magnetic fields and the graphs of change thereof are asymmetrical in shape, no additional voltage occurs in the secondary windings of the resolver if the number of poles in the field is at least two. A prerequisite is that all the fields are similarly asymmetrical. The combined effect of the reference current and the rotatory movement of the rotor 1 also causes a noise signal proportional to the speed of rotation of the rotor in the secondary windings 11a and lib or 12a and 12b of the resolver. This is undesirable because the amplitude of the resolver signal should not be de¬ pendent on the speed of rotation. However, it can be proved that this noise signal is fairly insignificant, and its effects can be compensated for in known manners, if this is considered necessary. Disturbances may occur for the following rea¬ sons: A. The permanent magnetic poles (Nl, N2...) have unequal flux values with respect to each other, which causes an imbalance therebetween. Unequal flux values may be due to e.g. a slightly different magnet¬ ization of the magnets. B. The mounting of the rotor is eccentric and the size of the air gap may vary due to manufacturing tolerances. C. The secondary winding of the resolver is not distributed quite evenly round the stator. However, the above sources of disturbance can be minimized in a suitable way if they distort the output signals of the secondary windings of the resol¬ ver too much. This minimizing can be carried out e.g. by magnetic balancing of the rotor and electronic com- pensation of the disturbances. A noise voltage may possibly also occur when the winding currents and magnetic fields act on the inductive coupling of the reference voltage of the re¬ solver, which coupling is indicated with the reference numerals 5 and 7 in Figure 1. The effect of the cur¬ rents of the stator winding 10, however, can be com¬ pensated for by applying a reference voltage simulta¬ neously to both ends of the rotor, as shown in Figure 1. The directions of the currents should thereby be chosen so that the stator current amplifies the refer¬ ence voltage at one end and weakens it at the other end. The shape and dimensions of the stator winding should be equal at both ends. In principle, it is also possible that a noise voltage is induced in the primary windings 4a and 4b of the resolver if an external magnetic field moves with respect to them. Fields caused by permanent mag¬ nets do not, however, induce a voltage in the primary windings of the resolver because their position with respect to the primary windings is fixed. On the other hand, it can be proved that the total voltage induced by the stator windings 10 in the primary windings 4a and 4b of the resolver is zero if the windings 4a and 4b are connected in series. To sum up the above discussion on noise vol¬ tages, it is obvious that the magnetic fields of the motor do not in principle affect the resolver signals. However, disturbances caused by manufacturing inaccur¬ acies may occur but these can be eliminated by careful planning. In addition to that, an insignificant addi¬ tional signal proportional to the speed of rotation of the rotor is formed similarly as in other resolvers. The above discussion refers to a resolver inte¬ grated in the basic structure of the motor and having one pole pair. A resolver having a higher number of pole pairs than this is obtained in the following way: it is imagined that a combination of a one-pole-pair resolver and a motor is split at one side thereof; the stator and the rotor are spread into a sector having an angle of 360° divided with a desired number of pole pairs in the resolver and this is attached to other sectors obtained in the same way. Couplings between these windings are carried out according to the same principles as what is disclosed above concerning the couplings of a resolver comprising one pole pair. It follows from the above that an absolute prerequisite for integrating a resolver comprising several pole pairs in the basic structure of the motor in that the number of pole pairs in the motor has to be at least double with respect to the number of pole pairs in the resolver. The motor according to the invention can be applied particularly to small-size accurate servomo¬ tors of high capacity, because the volume and weight of the feed-back transducing means of such motors form a substantial part of the entire motor. As to the dif¬ ferent types of motor, the following may be stated. In principle, this particular structure is suitable both for synchronous and asynchronous motors irrespective of whether they comprise a tooth-type or toothless stator. The rotor may be of a cylinder, ring or disc type; cylinder and ring type rotors, however, are per¬ haps the most practical in this respect. One application is a stepping motor type sali- ent pole motor if it is constructed so that at least four poles are magnetized concurrently and identically except that successive fields have to have different directions. Even though the invention has been described above with reference to the examples of the attached figures, the invention is not restricted thereto, but one skilled in the art can, of course, modify it with¬ in the inventive idea defined in the attached claims and his own professional skill.

Claims: .Claims: 1. An electric motor comprising a rotor (1), a stator (2) and at least two pole pairs, c h a r a c- t e r i z e d in that it comprises an integral re¬ solver or the like transducing means of the type with a rotating transformer so that a primary winding (4a, 4b) of the resolver is arranged- in the rotor (1) of the motor, and that a secondary winding (11a, lib; 12a, 12b) of the resolver is wound round the stator (2) of the motor in such a manner that the winding wire goes alternately inside and outside the stator. 2. A permanent magnet motor according to claim 1, c h a r a c t e r i z e d in that the primary winding (4a, 4b) of the resolver is arranged between magnet poles (Nl, N2...) positioned along the rotor periphery. 3. An induction motor according to claim 1, c h a r a c t e r i z e d in that the primary winding (4a, 4b) of the resolver is arranged on the periphery of the rotor (1). 4. A motor according to claim 2 or 3, c h a r¬ a c t e r i z e d in that the primary windings of the resolver are formed by two windings (4a, 4b) arranged at an angle of 180 degrees with respect to each other and connected in series with each other. 5. A motor according to claim 1 or 4, c h a r¬ a c t e r i z e d in that the secondary winding (11a, lib; 12a, 12b) of the resolver is arranged so that the winding wire goes alternately in a winding slot (9) of the stator (2) and on the stator (2). 6. A motor according to claim 5, c h a r a c¬ t e r i z e d in that the winding wire of the second¬ ary windings (11a, lib; 12a, 12b) of the resolver is arranged so that it goes round the stator through 180 degrees, going in every other winding slot (9). 7. A motor according to claim 6, c h a r c¬ t e r i z e d in that the winding wire of the second¬ ary windings (11a, lib; 12a, 12b) is arranged so that it goes round the same 180 degrees two times, whereby the winding slots (9) of the first and the second winding turn are different slots. 8. A motor according to claim 6, c h a r a c¬ t e r i z e d in that both secondary windings (11a; lib; 12a, 12b) in the resolver are formed by two halves going through 180 degrees round the entire stator (2) .

Inventors: PORO ANTTI, PORO, ANTTI
Applicants: PORO ANTTI, PORO, ANTTI
Application year: 1988
Main classification: H02K29
IPC codes: G01B7, G01D5, H02K24, H02K29