BACKGROUND OF THE INVENTION The present invention relates to a synchronous generator and a synchronous generator system driven by a windmill. In order to use an AC-excited synchronous generator in a wind power generation system, it is essential to reduce the weight of the AC-excited synchronous generator as compared with the conventional counterparts. The requirement for weight reduction comes from the following reason. When a wind power generation system is constructed, the generator must be lifted up to the nacelle by a crane. In a wind power generation system using a large-sized windmill, the nacelle is located at more than 100 m above the ground level. Since the strength and therefore the diameter of the pillar for supporting the windmill is determined by the weight of the generator, the reduction of the weight of the generator is taken into much more consideration for the wind power generation system than for other types of power generation systems. One way of reducing the weight of an AC-excited synchronous generator is to reduce the length of the air gap between the stator and the rotor. Reducing the air-gap length leads to reducing the magnetic resistance in the air gap so that the exciting current flowing through the field winding can be decreased. Accordingly, the cross section area of the conductor of the field winding can be diminished so that the resultant generator can be decreased in size and weight. However, if the air-gap length is reduced, the spatial change in the magnetic resistance in the air gap becomes large due to the existence of slots in the stator and rotor so that the distortion of the armature current waveform is enhanced. JP-A-3-270664 discloses a technique which is designed to improve the armature current waveform, wherein the numbers of the slots in the stator and rotor and the winding pitches for the stator and rotor are optimized to improve the waveform. JP-A-7-15901 discloses a technique designed for improving the armature current waveform, wherein the winding pitches are so chosen that the winding factors relating to the slot-associated higher harmonics can be minimized. JP-A-2005-304271 discloses a technique for suppressing the distortion of the armature current waveform by equating to ±6 the number of slots per two poles in stator minus the number of slots per two poles in rotor. SUMMARY OF THE INVENTION According to the technique disclosed in JP-A- 03-270664, the stator winding is in the form of the fractional-slot winding so as to smooth the waveform of the output voltages. However, the effect of reducing the distortion of the armature current waveform is sometimes small in a rotary electric machine with fractional-slot stator winding. The AC-excited synchronous generator generally has the same structure as conventional wound-rotor type induction machines. And the techniques disclosed in JP-A-7-15901 and JP-A-2005-304271 both employ the values recommended in the design of conventional induction machines which are disclosed in the article " Transformers, Induction machines and AC commutator machines", Communicational Education Group of lEEJ, p.112, 1967. Accordingly, the value indicating the number of slots per two poles in stator minus the number of slots per two poles in rotor is set between +6 and -6. On the other hand, the inventors of the present invention has revealed the fact that the influence by the higher harmonics in the armature current waveform seems to decrease as the absolute quantity of this value becomes larger. It, therefore, seems to be considered that the influence by the higher harmonics in the armature current waveform cannot be sufficiently suppressed, that is, the armature current waveform cannot be sufficiently smoothed, if this value is maintained between +6 and -6. The object of the present invention is to rovide an AC-excited synchronous generator which can reduce the distortion of the armature current waveform and such an AC-excited synchronous generator for use in a wind power generation system. According to the present invention, there is provided a synchronous generator comprising a rotor in the plural slots of which the field winding is embedded, and a stator in the plural slots of which the armature winding is embedded, wherein the number of slots per two poles in the stator minus the number of slots per two poles in rotor is equal to or greater than +9, or equal to or smaller than -9. Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows in cross section the principal parts of an AC-excited synchronous generator according to the present invention; Fig. 2 is a table listing various values of the number of slots per two poles in stator minus the number of slots per two poles in rotor, calculated when the number Npp of slots per pole per phase in either of the stator and the rotor falls within a range of 3 ~ 7; Fig. 3 shows the relationship between the quotient (stator short-pitch winding factor fpRs) / (order ks of higher harmonic), and the order ke of higher harmonic; Figs. 4A and 4B show the relationship between the value of the number Ni of slots per two poles in stator minus the number N2 of slots per two poles in rotor, and the magnitudes of the higher harmonic components; Fig. 5 graphically shows the relationship between the ratio of the armature winding pitch to the stator pole pitch in an AC-excited synchronous generator with the value of Ni - N2 equal to ±12, and the factor of inclusion of higher harmonic currents, the relationship having been observed as the result of actual measurement; Fig. 6 shows a table listing the values for the armature winding pitch and the corresponding values for the ratio of the armature winding pitch to the stator pole pitch in the AC-excited synchronous generator wherein the number Nspp of slots per pole per phase in the stator is such that 3 < Nspp < 7, the nuber of poles is equal to or less than 12, and the number of slots in the stator is equal to or less than 144; Fig. 7 graphically shows the relationship between the pulsation factor of magnetic flux and the ratio of width s of slot opening in the rotor to gap length g; Fig. 8 shows the shape of the rotor slot in an AC-excited synchronous generator as an embodiment of this embodiment; Fig, 9 shows the shape of a circumferentially symmetric semi-closed rotor slot; Fig. 10 shows the shape of a circumferentially symmetric open rotor slot; Fig. 11 shows the shapes of the stator slot and the rotor slots with magnetic wedges provided in their openings, according to an embodiment of the present invention; Fig. 12 schematically shows the entire structure of an AC-excited synchronous generator system according to the fifth embodiment; Fig. 13 schematically shows the entire structure of a wind-driven AC-excited synchronous generator system according to the sixth embodiment; and Fig. 14 graphically shows the relationship between the size and the pole number of an AC-excited synchronous generator. DESCRIPTION OF THE EMBODIMENTS Embodiments of the present invention will be described below in reference to the attached drawings. [First Embodiment] The first embodiment of the present invention will be described below. In the first embodiment of the present invention, the armature current waveform is smoothed by controlling the value indicating the number of slots per two poles in the stator minus the number of slots per two poles in the rotor. Fig. 1 shows in cross section the principal parts of an AC-excited synchronous generator according to the present invention. In Fig. 1, an AC-excited synchronous generator 1 comprises a stator 10, a rotor 20, an armature winding 13 and a rotary shaft 25. The armature winding 13 comprises bottom coil sides 131, top coil sides 132 and coil ends 133. The coil ends 133 electrically connect the bottom coil sides 131 and the top coil sides 132. The stator 10 has the bottom coil sides 131 and the top coil sides 132 inserted in the stator slots 12 cut in a stator iron core 11 and immobilized by means of stator wedges 14. In the example shown in Fig. 1, both the bottom coil sides 131 and the top coil sides 132 of the armature winding 13 are seen labeled from #1 through #90 in the counterclockwise direction. The rotor 20 has a field winding 23 inserted in rotor slots 22 cut in an armature iron core 21 and immobilized by means of rotor wedges 24. The rotary shaft 25 is fitted in the rotor iron core 21. As shown in Fig. 1, the coil ends of the field winding 23 are seen labeled from #1' through #54' in the counterclockwise direction. With the AC-excited synchronous generator 1, power generation is performed, that is, armature current is caused to flow through the armature winding 13, by rotating the shaft 25 by rotational force parted thereto while exciting current is flowing through the field winding 23. The frequency of the generated armature current varies depending on the rotational speed of the rotor 20 and also on the frequency of the exciting current flowing through the field winding 23. Even in the case where a windmill drives the rotor shaft 25 as in a wind power generation systems and therefore the rotational speed of the rotor 20 continually changes, the frequency of the generated current can be maintained at a fixed value by controlling the frequency of the exciting current flowing through the field winding 23. [Three-phase Rotary Electric Machine] Now, description is made of how a three-phase rotary electric machine is designed. The article extracted from "Electric Machinery (I)" of Applied Electrical Engineering Treatise, by Sakutaro Nonaka, published by Morikita Publishing Co. Ltd., p. 227 (1973), recommends, in the design of a three-phase rotary electric machine, that the number Npp of slots per pole per phase in either of the stator and the rotor should be set within a range given by the following inequality (1). (Formula Removed) fig. 2 is a table listing various values indicating the number Ni of slots per two poles in stator minus the number N2 of slots per two poles in rotor (hereafter referred to as " value of Ni - N2") calculated when the number Npp of slots per pole per phase in either of the stator and the rotor lies within a range given by the above equality (1). It is understood from Fig. 2 that the maximum and minimum values of Ni - N2 calculated when Npp falls within the range given by the inequality (1), are +24 and -24, respectively. [Value of Ni - N2] Concrete description will now be made of the value of Ni - N2. It is recommended in the design of an induction machine to set the total number ni of slots in stator and the total number n2 of slots in rotor within a range given by the following inequality (2) which is described in " Transformers, Induction machines and AC commutator machines", Coramunicational Education Group of lEEJ, p.112, 1967. (Formula Removed) The value of Ni - N2 ranges between +6 and -6 when calculated under the condition given by the above inequality (2). In general, since AC-excited synchronous generators are almost the same in structure as wound-rotor type induction machines, they have been designed employing the above recommended value of Ni - N2. Namely, in the design of AC-excited synchronous generators, the value of Ni - Nz has been generally set between +6 and -6 in accordance with the condition given by the above inequality (2). [Derivation Of Expression For Giving Current Flowing Through Armature Winding] The expression for giving the armature current I flowing through the armature winding 13 will now be derived. Here, the expression for giving the armature current I is derived in simplified consideration of phase-related effects. First, let the magneto-motive force (mmf) AT associated with the frequency components of the armature current I flowing through the armature winding be represented by the following expression (3) . (Formula Removed) where x is fixed to the stator coordinate system and takes a value of 2n per two poles; cOo the angular frequency corresponding to synchronous speed; t the time; k the order of mmf; and fwk the stator windings ffactor for the k-th harmonic wave. The stator windings factor fwk is given by the following expressions (4), (5) and (6), (Formula Removed) - 11 - where fpk is the short-pitch winding factor for stator; fdk the distributed winding factor for stator; p the ratio of the armature winding pitch to the stator pole pitch; q the the number of phases in stator; and Nspp the number of slots per pole per phase in the stator. Let the spatial distribution P of permeance, which is the reciprocal of magnetic resistance, be represented by the following expression (7), with the spatial change in the magnetic resistance due to the slots in the rotor and stator taken into consideration. (Formula Removed) where Ki is the factor of pulsation of permeance due to stator slots, Kz the factor of pulsation of permeance due to rotor slots, and s the slip. The magnetic flux density B is proportional to the product of the magneto-motive force AT and the permeance P, and can be represented by the following expression (8). (Formula Removed) The magnetic flux density B is then spatially integrated, and if the short-pitch winding factor of the stator is taken into consideration, the magnetic flux O linking the armature winding 13 is assumed to be given by the following expression (9). (Formula Removed) where ke is the order of the spatial higher harmonic of the magnetic flux density B, and fpkB is the stator winding factor for the ke-th harmonic of the magnetic flux density B. The magnetic flux 0 is then temporally integrated, and the electro-motive force E induced in the armature winding 13 is assumed to be represented by the following expression (10). (Formula Removed) where VB is the order of the time-dependent higher harmonic of the magnetic flux density B. If the armature current I is assumed to be equal to (induced electro-motive force E)/(reactance) and if the reactance is assumed to be proportional to the order ve of the time-dependent higher harmonic of the magnetic flux density B, then the armature current I can be assumed to be given by the following expression (11).(Formula Removed) Fig. 3 shows the relationship between the quotient, i.e. (stator short-pitch winding factor fpke) / (order ke of higher harmonic) , and the order ke of higher harmonic. Since the armature current I is proportional to the quotient , as is seen from the expression (11), then it is considered from Fig. 3 that the armature current I increases as the value of the quotient increases. The relationship shown in Fig. 3 reveals that the armature current I generated when the order kB of higher harmonic is equal to unity, becomes so large that the armature current I generated when the order ke is other than unity is negligible. Therefore, only that higher harmonic component of the armature current which correspondeds to ke = 1, will be hereafter considered. The inventors of the present invention have derived the expressions given below, paying attention to higher harmonic components related to the value of N1-N2. If the expressions (3) and (7) given above are substituted into the last given expression (11) and if only the higher harmonic component with respect to the value of N1-N2 is considered, then, in the case where Ni > N2, the expression (11) can be modified into the expression (12) as follows. (Formula Removed) On the other hand, for N1