DC rotary machine
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a DC rotary machine. 
     2. Description of the Prior Art 
     There is well known and widely used in the art a type of DC rotary machine comprising a first coil assembly for a motor and a second coil assembly for a generator rotating together. Such DC rotary machine is used, for example, as a rotary machine for tape recorder, VTR etc. 
     In use, the rotational speed of the DC rotary machine is kept constant by a speed detection and control apparatus. A DC signal representative of the rotational speed of the motor element is derived from the generator element and the DC signal is applied to, for instance, the speed detection and control apparatus. In response to the signal, the speed detection and control apparatus controls the supply of current to the motor element so as to keep the rotational speed of the DC rotary machine constant. 
     In almost all of the conventional DC rotary machines of the above-mentioned type, the output rotary shaft of the motor is coupled with the input rotary shaft of the generator so that the two shafts may rotate together as a unit. Such coupling between the two rotary shafts brings about a mechanical resonance by which some reduction in accuracy of the DC rotary machine may be caused. 
     DC rotary machines of the type pertinent to the present invention are also used in detecting, for instance, the rotational speed of a motor and controlling the speed. For this application of DC rotary machine, it is desired that a ripple wave in the wave form of the generated voltage from the generator be reduced to a minimum to attain a high accuracy of operation. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a DC rotary machine in which mechanical resonance is reduced as compared with the conventional machine. 
     It is another object of the invention to provide a DC rotary machine in which coil assemblies for the motor and generator are constructed as coreless rotary coil assemblies so as to minimize the inertia thereof. 
     It is a further object of the invention to provide a DC rotary machine which permits reduction of the ripple wave in the generated voltage wave form to a minimum and to assure a high accuracy in servo operation. 
     Still a further object of the invention is to provide a DC rotary machine of the above-mentioned type which allows reduction of the number of parts necessary therefor to make the assembly thereof very easy. 
     Other and further objects, features and advantages of the invention will appear more fully from the following description taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal section of a DC rotary machine showing an embodiment of the present invention; 
     FIG. 2 is a cross-sectional view taken along the line II--II in FIG. 1; 
     FIG. 3 is a side elevation of the machine shown in FIG. 1 with some parts being removed for the sake of explanation of the manner of assembly; 
     FIG. 4 is a similar view to FIG. 1 showing the second embodiment of the invention; and 
     FIG. 5 is a cross-section taken along the line V--V in FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIGS. 1-3 showing the first embodiment of the invention, the reference numeral 1 designates a cylindrical outer field yoke which is made of non-magnetic material. The cylindrical outer field yoke 1 is stationary and constitutes a part of the casing of the machine. Two end openings of the outer yoke 1 are closed by cover plates (end brackets) 2 and 3 made of non-magnetic material respectively. Fixed to the cover plate 3 is a bearing metal 4 which is made of, for example, oil-containing sintered alloy and disposed coaxially with the outer yoke 1. Designated by 6 is an inner stationary field yoke in the form of a cylinder and is made of oil-containing sintered alloy of iron system. The inner cylindrical yoke 6 comprises a field member 6 1  and a bearing member 6 2  of a smaller outer diameter than that of the field member. The inner yoke is disposed coaxially with the outer yoke and fixed to the abovementioned cover plate 2. On the inner wall of the field member 6 1   there is provided a bearing seat 6 1  a to receive a bearing 7 and on the inner wall of the bearing member 6 2  there is formed a bearing surface 6 2  a. Rotatably supported by the bearing metal 4, bearing 7 and bearing surface 6 2  a is a rotary shaft 8. Two field permanent magnets 9 1  and 9 2  are secured on the circumferential surface of the inner yoke 6 at two different positions spaced by 180° from each other. The direction in which two permanent magnets 9 1  and 9 2  are magnetized is normal to the longitudinal axis of the inner yoke 6 but the polarity of the exposed pole surface of one permanent magnet 9 1  is opposite to that of the other one 9 2 . Designated by 10 1  is a coreless coil for the motor and 14 1  is a coreless coil for the generator. These two cylindrical coreless coils are rotatably disposed within a space defined by the inner wall surface of the outer yoke 1 and the outer surfaces of the permanent magnets 9 1 , 9 2 . The coreless coil 10 1  is fixed on the rotary shaft 8 at its one end through a coil supporting member 10 2 . The reference numeral 11 designates a cylindrical commutator having a number of commutator bars 11 1  provided on the circumferential surface. The commutator 11 is fixedly mounted on the rotary shaft 8 and is electrically connected with the coil 10 1 . The coil 10 1  and the coil supporting plate 10 2  are shaped into a cup to form a coil assembly for motor A by moulding of synthetic resin material as shown in, for example, U.S. Pat. No. 3,308,319. If necessary, the commutator 11 also may be formed integrally with the motor coil assembly A. The commutator 11 is always contacted by brush elements 12 1 , 12 2  having terminals 12 1  a, 12 2  a outside of the yoke 1 respectively. Between the bearing metal 4 and the commutator 12 the rotary shaft 8 has an oil drain board 13 mounted thereon. 
     The coreless coil for the generator 14 1  has a coil supporting plate 14 2  fixed to one end of the coil 14 1 . The bearing member 6 2  of the inner yoke 6 passes through a central bore 14 2  a in the coil supporting plate 14 2  which is rotatably supported by it. 15 is a cylindrical commutator having a number of commutator bars 15 1  provided on the circumference of the commutator. The inner diameter of the commutator 15 is larger than the outer diameter of the bearing member 6 2  so that the inner wall surface of the commutator can not contact the outer surface of the bearing member. The commutator 15 is fixed to the coil supporting plate 14 2  at its one end and is electrically connected with the coil 14 1 . The coil 14 1  and the coil supporting plate 14 2  with the central bore 14 2  a are shaped into a cup to form a coil assembly for generator B by moulding of synthetic resin material. Brush elements 16 1 , 16 2  are always kept in resilient contact with the commutator to form a brush for taking up the output. Outside of the outer yoke 1, the brush elements have terminals 16 1  a, 16 2  a respectively. 
     The motor coil assembly A and the generator coil assembly B are aligned with each other with the open end of the coil 10 1  opposite to the open end of the coil 14 1  and firmly connected with each other with the interposition of an insulating member 17 between the two coil 10 1  and 14 1 . In this manner, the two coil assemblies A and B together form a rotary coil unit which encloses the field member 6 1  of the inner yoke 6 and two permanent magnets 9 1  and 9 2  secured on the circumferential surface of the field member 6 1 . 
     The manner of assembly of the above described DC rotary machine will be described with reference to FIG. 3. 
     At first, employing the known mould forming technique of synthetic resin material, the motor coil assembly A and the generator coil assembly B are formed. In mould forming of the motor coil assembly A, the rotary shaft 8 is positioned concentrically with the coil supporting member 10 2  and during this step of mould forming of the assembly A the rotary shaft is fixed to the assembly A making use of synthetic resin material from which the coil supporting member 10 2  is to be formed. Thereafter, the inner yoke 6 with the bearing 7 and the permanent magnets 9 1 , 9 2  provided thereon is fitted to the coil assembly A. Then, the generator coil assembly B is bonded to the motor coil assembly A in the manner previously described with the interposition of the insulating member 17. Finally, the bearing member 6 2  of the inner yoke 6 is inserted into the central opening of the cover plate 2. 
     In the DC rotary machine with the above described structure according to the invention, the motor coil assembly A and the generator coil assembly B are bonded together through an insulating member 17 interposed between the two open ends so as to form a single rotary coil unit. The coil supporting portion of the generator coil assembly B is rotatably supported by the bearing member 6 2  of the inner yoke 6. This structure assures a stable rotation of the rotary coil unit even when the unit (A, B) is elongated in size in the axial direction. Moreover, as previously noted, the inner diameter of the commutator 15 is larger than the outer diameter of the bearing member 6 2  to provide a space therebetween. This space has an effect of an oil trap to prevent lubricating oil injected into the bearing member 6 2  from flowing into the commutator bars 15 1 , 15 2  . . . . Therefore, it is unnecessary to provide a particular oil draining means on the side of the generator coil asembly B. Because of this feature, the size in the axial direction of the rotary electric machine can be reduced substantially. On the other hand, the commutator 15 is allowed to have a larger outer diameter sufficient to provide thereon an increased number of commutator bars 15 1 , 15 2  . . . . By increasing the number of commutator bars in this manner, it becomes possible to minimize the ripple wave in the wave form of the generated voltage from the generator coil assembly and thereby a DC rotary machine can be obtained which performs servo operation with high accuracy. 
     FIGS. 4 and 5 show a second embodiment of DC rotary machine according to the invention designed by modifying the embodiment shown in FIGS. 1 to 3. In FIGS. 4 and 5, the same reference characters and numerals as used in FIGS. 1 to 3 indicate like and corresponding parts. 
     The second embodiment is different from the first one in that permanent magnets 9 1 , 9 2  are provided not on the outer cylindrical surface of the inner field yoke 6 but on the inner cylindrical surface of the outer field yoke 1 and that the bearing 7 is omitted and instead a portion 6 3  of the yoke 6 is used as a bearing. The second embodiment has an advantage in that since the bearing 7 is omitted, the number of parts necessary for the machine can be reduce. Another advantage obtainable from the second embodiment is that the outer diameter of the rotary coil unit (A, B) can be reduced by an amount corresponding to the thickness of magnets 9 1 , 9 2  for the same outer diameter of the outer field yoke 1 because the magnets are provided on the inner cylindrical surface of the outer yoke 1. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the spirit and scope of the invention.