THREE-PHASE INDUCTION MOTOR

Abstract: A three-phase induction motor is subjected to a countermeasure for removing a harmonic component (7) of the order causing a torque ripple, among a plurality of harmonic components included in a magnetic flux generated in an air gap between a stator core and a rotor core (3). In the three-phase induction motor, conductor grooves (4) of the rotor core (3) are skewed by an electrical angle of 2π/n (where n is the order of the harmonic component (7) to be removed). And also the number of slots of the stator core, N are selected by a manner of specifying relations of winding pitch, number of poles, number of phase and non integer number of grooves per pole and per phase.

Description: The present invention relates to a three-phase induction motor and, more particularly, to a three-phase induction motor having a relatively low capacity, e.g., a power output of 1 kW or less, which can smooth torque, and which is used as a servo motor. An induction motor is structurally simple, inexpensive, strong, and hence has been widely used as a rough constant speed motor in many industrial fields. However, recently, when it is used together with a high speed microprocessor or a high speed semiconductor switching element, a strong potentiality as a wide range variable speed motor, e.g., servo motor, has been brought about by a new control technology, such as "vector control induction motor drive". In the three-phase induction motor, magnetic flux (to be referred to as "air gap magnetic flux" hereinafter) generated in an air gap between a stator or primary core and a rotor or secondary core includes various harmonic components, such as the 5th, 7th, 11th, 13th, 17th,... harmonic components, thereby generating magnetic flux variations. For this reason, the harmonic components, especially the 5th and 7th harmonic components, adversely affect to generate a torque ripple or a variation of torque. In order to remove the torque ripple, a conductor groove of the rotor core is generally skewed. Conventionally, based on a principle that skew smoothes the air gap magnetic flux, skew is performed by one slot pitch of the stator as shown in Fig. 1. In Fig. 1, reference numeral 1 denotes a stator core; 2, stator slots; 3, a rotor core; 4, skewed conductor grooves; and 5, rotor bars buried in the conductor groove 4 for constituting a secondary circuit. Fig. 1 is a developed view in which the circular stator and the rotor are developed. With this arrangement, the slots 2 and the conductor grooves 4 do not completely oppose each other, and a magnetic flux passage between the stator and the rotor is always provided. Therefore, magnetic flux variations in the air gap become small, and the torque ripple caused by the stator slots 2 is reduced. On the other hand, in order to reduce variations in the air gap magnetic flux itself, the number of slots 2 provided to the stator is increased, or an opening of the slot 2 is closed. As described above, various countermeasures have been taken to prevent the torque ripple. However, since no countermeasure directly affects the harmonic components which cause the torque ripple, the torque ripple cannot be completely removed. For example, when the conductor grooves 4 of the rotor are conventionally skewed, the harmonic components of relatively higher orders (e.g., the 11th or more) can be removed. However, for example, the 5th and 7th harmonic components which largely adversely affect cannot be removed and often generate the torque ripple. In addition, a method of increasing the number of slots 2 of the stator to smooth torque is possible and effective for a large-sized motor. However, in the case of a small-sized motor, the number of slots 2 of the stator cannot be largely increased because of manufactural limitations. Furthermore, a method of closing an opening of the slot 2 leads to complex manufacturing steps and hence can be adopted only to a motor in a limited field. US-A-4 371 802 discloses a single phase, permanent split capacitor induction motor. In order to eliminate or substantially reduce the 5th harmonic torque dip, the squirrel cage bars are spiralled through two poles of the 5th harmonic flux, i.e., one-tenth of a full turn, and the rotor bars are insulated from the laminations. The spiralling of the rotor bars results in the substantial elimination of the 5th harmonic of the flux wave because each incremental length of rotor bar has a vector 5th harmonic voltage generated therein, those voltages vectorially adding up to zero in 360° electrical. It is an object of the present invention to provide a three-phase induction motor, which can be manufactured by simple manufacturing steps, which generates less torque ripple, and which has the characteristics of a servo motor. This object is achieved by the three-phase induction motor according to the claims. According to the three-phase induction motor of the present invention, two countermeasures are adopted. In the first countermeasure in order to remove the nth (e.g., the 7th) harmonic component, which largely adversely affects, of the harmonic components included in air gap magnetic flux and causes torque ripple, a conductor groove of a rotor core is skewed by an electrical angle of 2π/n, where n is an order of harmonic component to be removed. With the motor of the present invention the harmonic components of the orders, which largely adversely affect, of the harmonic components included in the air gap magnetic flux are removed. That is, when the conductor grooves of the rotor core are skewed by the electrical angle of 2π/n, i.e., 2/n of an interval between magnetic poles, voltage vectors induced by the nth harmonic component in one rotor bar have opposite directions and are cancelled with each other, so that a current of the nth harmonic component no longer is supplied. By the skew of the conductor grooves of the rotor described above, the torque ripple is largely reduced, but the torque ripple caused by the harmonic component of the order (e.g., the 5th harmonic component) which is not removed sometimes remains to pose a problem. The second countermeasure for this problem is taken by a manner of selecting the number of slots (grooves) of the stator core and selecting the pitch of the stator winding. Such a manner is disclosed by EP-A-229.018 which falls under the terms of Article 54(3)EPC. An embodiment of the present invention will now be described in detail with reference to the accompanying drawings, in which Fig. 1 is a schematic view showing a relationship between a stator and a rotor having skewed grooves in a conventional induction motor; Fig. 2 is a schematic view of an embodiment according to the present invention; and Figs. 3(a) to 3(c) are schematic views showing a method of removing harmonic component torque which can be used together with the present invention. Fig. 2 is a schematic view for explaining an embodiment of the present invention, and showing a relationship between the skew of the conductor grooves of a rotor and a waveform of an air gap magnetic flux. Fig. 2 shows a case wherein an influence of the 7th harmonic component included in the air gap magnetic flux is removed, and a description thereof will be made with reference to a 2-pole motor. In Fig. 2, reference numeral 6 denotes a fundamental waveform of the air gap magnetic flux; and 7, the 7th harmonic component waveform thereof. Reference symbol L denotes a magnetic pole interval; and W, a rotor core width. The magnetic pole interval L is a half cycle width of the fundamental waveform 6, i.e., π in the case of a 2-pole motor since an electrical angle equals to a mechanical angle. As shown in Fig. 2, assuming that the harmonic component order is n (n = 7 in this case since the 7th harmonic component is in question in this embodiment), conductor grooves 4 of rotor core 3 are skewed by one cycle of the harmonic component, i.e., 2π/n of the electrical angle. As a result, the nth (7th) harmonic component voltages are induced at, e.g., points P1 and P2 of rotor bars (secondary circuit) 5 buried in the conductor grooves 4, and the induced voltage vectors have a direction and a magnitude represented by arrows. Therefore, the nth (7th) harmonic component current at the rotor bar 5 is cancelled and hence is no longer supplied. That is, in an induction motor having a rotor of the above arrangement, no torque ripple by the nth harmonic component is generated. Note that in the above embodiment, the description has been made with reference to the case wherein the harmonic component order n = 7. However, the present invention is not limited to this embodiment, and values of n may be determined by selecting the harmonic components, to be removed, having a large influence. For example, a desired one may be selected from the 5th, 7th, 11th, 13th, 17th,... harmonic components. In addition, the present invention can be applied to, e.g., a 2-, 4-, or 6-pole motor irrespective of the number of poles. Another countermeasure for reducing the torque ripple wherein the number of slots of the stator core is selected so that the number of slots per pole per phase of the stator core becomes a non-integer, a three phase balanced winding condition is satisfied, and the pitch of the stator winding is determined by the rth (r ≠ n) harmonic component, thereby to remove the rth harmonic component, will be described below. The number of slots of the stator core is determined by the following equation: N = l x m x q where N denotes the number of slots of the stator core, l the number of poles, m the number of phases, and q the number of slots per pole per phase, N being selected so that q becomes a non-integer. In addition, assuming that the order of the harmonic component which has a great influence and hence is to be removed is r, a winding pitch s of the stator is selected in the case of a fractional pitch winding in accordance with the following equation: s = (r - t)/r where t is an odd number. The number N is determined to satisfy the above relationships and to obtain a three-phase balanced winding. Figs. 3(a) to 3(c) are views for explaining the countermeasure of selecting the number of slots of the stator core. Figs. 3(a) to 3(c) show the case wherein an influence of the 5th harmonic component is removed and the number of slots of the stator core becomes a minimal value so as to realize harmonic removal in a small-sized induction motor. More specifically, in the case shown in Figs. 3(a) to 3(c), the number of poles ℓ = 2, the number of phases (m) = 3, the number of slots per pole per phase (q)= 2.5, that is, non-integer, the number of slots of the stator core (N)= 2 x 3 x 2.5 = 15, and the stator winding pitch (t)= 1 (a fractional pitch winding). Fig. 3(a) is a developed view of the stator slots 11; Fig. 3(b) shows a magnetic flux distribution showing a fundamental wave 8 and the 5th harmonic component 9 to be removed; and Fig. 3(c) shows a winding 12 of the stator, left and right conductors 12 a and 12 b of the winding 12 shown in Fig. 3(c) being fitted into the slots 11 a and 11 b shown in Fig. 3(a) respectively, showing a relationship between a pitch and positions of the slots. This example shows the case wherein the order of the harmonic component to be removed r = 5 and the stator winding pitch s = (r - 1)/r = 4/5 = 0.8. That is, since the number of slots per pole is 15/2 = 7.5, the pitch of the winding in terms of the number of slots is 7.5 slots x 0.8 = 6 slots, as shown in Fig. 3(c). With this arrangement, the left and right conductors 12 a and 12 b of the winding 12 are positioned at a starting node 10 a and the fourth node of the 5th harmonic component 9 as shown in Figs. 3(b) and 3(c), so that the effect of the 5th harmonic component 9 is no longer induced. In order to remove the r=5 harmonics, the widths in which the stator winding exists are, as shown in 10a-10b of Fig. 3(b), set in such a way as to cancel the '+' part of harmonics 9 (the part above 10a-10b) with the '-' part of harmonics 9 (the part below 10a-10b). In the N pole region of Fig. 3, the number of crests in harmonics 9 is in the total (both '+' and '-' ) or "r" (an odd number). The number of widths wherein the winding exists is the same for both '+' and '-', namely, an even number. "t" is the difference obtained by subtracting from "r" the number of crests in harmonics 9 (the even number) contained in the widths wherein the winding exists. The resulting "t" is thus the odd number; where the pitch s is given by the formula s = (r-t)/r. From the efficiency viewpoint, it would be better for the value of s to be closer to 1. As for '+', the values to be assumed are 1, 3, 5, and so forth, but from the efficiency point of view, the value t=1 is used often. In the embodiment, t=1 has been used. It will be examined below whether the three-phase balanced winding can be realized in the case wherein the number of entire slots 11, that is N = 15 and windings are provided at every 6 slots. Whether the three-phase balanced winding is possible or not is determined by the following equation. Assume that a value obtained by dividing N by the greatest common divisor of N and P is N₀, where P denotes the number of pole pairs, that is, l/2, and the number of phases is m. When N₀/m = integer, a two-layer winding is possible, and when N₀/2m = integer, a single-and two- layer windings are possible. But when N₀/m is not equal to integer, a three-phase balanced winding cannot be realized, so that such a number of slots cannot be adopted. In this example, N₀ = 15 because N = 15 and p = 1, and N₀/m = 15/3 = 5 because the number of phases m = 3, so that integer is obtained to realize the three-phase balanced winding. That is, this example is a design example which can be realized. When the stator is designed as described above, an influence of the rth harmonic component (the 5th harmonic component in this example) is removed. Therefore, when the skew of the conductor grooves of the rotor core, the number of slots of the stator, and the winding pitch are determined according to the present invention, the influences of the nth and rth harmonic components can be eliminated at the same time. When the higher (e.g., the 7th) harmonic component is removed at the rotor and the lower (e.g., the 5th) harmonic component is removed at the stator, skew can be performed at a small angle to facilitate manufacture of the rotor, resulting in a great advantage. As has been described above, according to the present invention, the harmonic components of the orders which largely adversely affect to generate the torque ripple can be selectively and directly removed, thereby realizing a three-phase induction motor with less torque ripple. Therefore, a small-sized driving source of a serve mechanism which requires a torque smoothness can be provided.

Claims: A three-phase induction motor having a stator core (1) and a rotor core (3), a plurality of harmonic components to be removed being included in a magnetic flux generated in an air gap between said stator core (1) and said rotor core (3), comprising: a plurality of conductor grooves (4) formed on a surface of said rotor core (3), said conductor grooves (4) being skewed by an electrical angle of 2 π/n, where n is the order of one of said plurality of harmonic components (7) to be removed; a plurality of slots (2) formed on a surface of said stator core (1) so as to satisfy the condition N = ℓ x m x q where N denotes the number of said slots (2); ℓ, the number of poles; m, the number of phases; and q, the number of said slots (2) per pole per phase; said N being selected so that said q becomes a non-integer; and a stator winding provided in said slots (2) so as to satisfy both a three-phase balanced winding condition and a condition represented by the equation, s = (r - t)/r where s denotes a pitch of said stator winding; r, the order of another one of said plurality of harmonic components (7) to be removed; and t, an odd number; thereby two harmonic components of the orders n and r are directly removed to minimize a torque ripple. A motor according to claim 1, wherein t = 1. A motor according to claim 1, wherein n = 7 and r = 5.

Inventors: ENDO, TETSUO NIPPON ELEC. IND. CO, LTD., IIJIMA, HIROSHI NIPPON ELEC. IND. CO, LTD., MIYAMOTO, YUKIO NIPPON ELEC. IND. CO, LTD., OKAMOTO, KIYOKAZU C/O NEC CORPORATION
Applicants: NEC CORPORATION
Application year: 1987
Main classification: H02K17
IPC codes: H02K17, H02K1