1. Field of the Invention
The present invention relates to a switched reluctance motor, and more particularly to a low cost switched reluctance motor which can be used as a general-purpose industrial motor. The invention relates also to such a motor which is useful as a high-speed motor in which a centrifugal force is a problem in view of a rotor's strength rather than a rotor is solid.
2. Description of the Related Art
In order to rotate the high-speed-rotation shaft of a machine tool in, for example, a machining center, a motor requires approximately 100 mm diameter rotor and at least 30,000 rpm.
In the described use, it has currently been common to employ an induction motor. In order to resist to the centrifugal force, the rotor slot is often kept closed and also the coil ends of the rotor are often reinforced.
However, these conventional systems are expensive and inevitably adopt the second best reinforced structure with some sacrifice of the motor characteristics.
Attempts have been made to improve the structures of such conventional reinforced motors. To this end, studies on switched reluctance motors relating to their possibilities for increased rotor strength are common with some of the resulting ideas being put into practice.
A typical example of these conventional switched reluctance motors is shown in FIG. 23 of the accompanying drawings. A drive algorithm for the motor is shown in FIG. 24. A rotor 2 is in the form of a simple laminate body composed of a plurality of axially arranged silicon steel disks. Because of the increased strength of the rotor 2, this conventional motor meets one handle for high-speed rotation.
A stator 1 of the motor of FIG. 23 has six salient poles 20 each having a width of approximately 30 degrees in terms of angle of rotation of the rotor. Six windings are mounted one round each stator salient pole 20. The rotor 2 has four salient poles 21 each having a width of approximately 30 degrees in terms of rotor's rotational angle.
In operation, for generating a counterclockwise rotational torque in FIG. 23, currents are supplied to flow in the windings indicated by TC1, TC2 and TF1, TF2 to attract the rotor salient poles. At that point, the currents to flow in the windings TC1, TC2 and those to flow in the windings TF1, TF2 are opposite in direction in such a manner that prospective magnetic fluxes pass through the rotor 2. Further, when the rotor salient poles have reached the stator salient poles associated with the windings TC1, TC2 as the rotor 2 is rotated counterclockwise, generation of the rotational torque will then terminate. At that point, those counterclockwise next to these rotor salient poles approach the stator salient poles associated with the windings indicated by TE1 TE2; if no current is supplied to flow in the windings TC1, TC2 and the currents are supplied to flow in the windings indicated by TE1, TE2 and TB1, TB2, then a counterclockwise rotational torque will be generated. Thus if suitable successive currents are supplied to flow in the individual stator windings one after another, successive rotational torques will be generated.
Likewise, for generating a clockwise rotational torque in FIG. 23, currents are supplied to flow in the windings indicated by TB1, TB2 to attract the rotor salient poles.
Variation of the torque to be generated depends on the current of each winding and the relative position of the stator and rotor, but does not in principle depend on the speed of rotation of the rotor.
A practical example of a power amplifier section of the drive system for the switched reluctance motor of FIG. 23 will be described with reference to FIG. 5 which shows the common circuit to be used in the present invention. A winding E corresponds to the windings TA1, TA2 of FIG. 23 and a winding WD corresponds to the windings TD1, TD2 of FIG. 23; these two windings WA, WD are opposite to each other in turning. The current IAD to flow in the winding WA, WD is controlled accurately with respect to the current command value by PWM control using the difference between a current command and the detected value of the current IAD like the very ordinary, non-illustratedmotor-current control. In a microscopic operation, a voltage is applied to the windings WA, WD by rendering transistors 8, 9 to assume the ON state, so the current IAD will be increased. If the transistors 8, 9 are rendered to assume the OFF state, magnetic energy and dynamic energy caused by the flowing current at that time are supplied back to DC power sources VS, VL via diodes 10, 11 to gradually reduce the current. As the foregoing operation is repeated, the average current of the currents IAD with respect to the current command value can be controlled. The current control for the remaining two phases is controlled in the same manner.
Of symbols used in FIGS. 5 and 23 to designate the windings and currents, alphabetical characters A, B, C, D, E, F indicate the individual stator magnetic poles of the motor.
The drive algorithm for the switched reluctance motor of FIG. 23 is shown in FIG. 24. The horizontal axis RA is the angular position of the rotor in FIG. 23. For generating a counterclockwise torque, the currents having the characteristics of FIGS. 24(a), 24(b), and 24(c) are supplied to flow in the associated windings. For generating a clockwise torque, the currents having characteristics of FIGS. 24(d), 24(e), and 24(f) are supplied to flow in the associated windings. In either case a larger current amplitude will result in a larger torque.
Advantages of conventional switched reluctance motors include: (1) the manufacturing cost is low because the motor structure is simple and, particularly, the structure of the stator windings is simple; (2) the entire length of the motor is relatively short because the coil ends of the stator windings can be shortened; (3) high-speed rotation can be realized in a physical manner because the rotor has sufficient strength; and (4) the drive circuit can be simplified because the drive algorithm is simple so that only one-way flow of the current is needed.
However, certain disadvantages accompany such conventional motors. For example, a control algorithm is needed to smooth the relationship between the electric energy supplied and the magnetic energy accumulated inside the motor and the mechanical output energy to eliminate large torque ripples. In an effort to solve this problem, a current control method has been proposed in which the current is compensated so as to compensate torque ripples and the compensated current is then supplied to flow in the associated winding. However, this proposed current control method creates additional problems. Further, the intermittent torque generated by the individual stator salient pole would, along with the torque ripples, contribute to stator deformation, thus increasing vibration and noise while the motor is driven. As can be seen from the characteristics of FIGS. 24(a) through 24(f), it is in fact possible for a torque opposite to that desired to be generated.
For high-speed rotation, a very-high-speed current switch is essential. Also, as supply and regeneration of the magnetic energy inside the motor must be carried out frequently, only a limited power factor can be achieved.