1. Field of the Invention
This invention is directed to a drive circuit for an alternate phase brushless DC motor, and more particularly relates to a drive circuit arranged to minimize torque ripple.
2. Brief Description of the Prior Art
It has become common to reduce wow and flutter in VTRs cassette tape recorders, and the like, by incorporating therein a "direct drive system" including a superslow speed brushless DC motor directly connected to a reel mount for directly driving the latter. The so-called "direct drive system" also makes it possible to improve the operability and reliability of the system and to reduce the size of the system. However, a brushless motor having a remarkably small torque ripple has to be used to ensure that the wow and flutter are reduced. For this purpose, a brushless motor having an exceptionally high performance quality is necessary, but the narrow tolerances required to achieve such performance quality for that motor raise the production cost of the brushless DC motor to an unrealistically high level, and thereby make it difficult to put such a motor into practical use.
Therefore, with a view to reducing torque ripple, previous proposals have been made, including using non-conventional techniques for winding the armature coils, forming armature coils into non-conventional shapes, and altering the magnetizing pattern of the rotor magnets. However, while these proposals have had some success in reducing the torque ripple, such success has been limited and insufficient.
The following types of DC brushless motors are well known, and each has its own well known intrinsic operating characteristics and its own peculiar drawbacks:
(1) Three-Phase Unidirectional Energizing Type PA0 (2) Three-Phase Bidirectional Energizing Type PA0 (3) Sine-Wave Drive Type PA0 (4) Alternate-Phase Switching Type
In this type motor, three-phase coils are successively supplied with current in one direction. The motor of this type has a relatively simple circuit construction but has a drawback in that the torque ripple is quite high.
In this type motor, a selected two out of three three-phase coils are consecutively supplied with current in a selected direction, with the third coil being supplied with current in a constant direction, thereby to effect bidirectional operation. The motor of this type has its torque ripple reduced more than that of the type (1) but has a drawback in that its drive circuit construction is rather complex.
In motors of either of the aforementioned types (1) and (2), since the current change-over timing and the pole inverting timing overlap with each other, they each have a common drawback in that the torque drops upon current changeover.
In this type motor, a sinusoidal drive current, having a strength that varies in proportion to the linking magnetic fluxes of the respective alternate coils, is supplied to these coils so that the sum of the torques generated by the drive currents and the linking magnetic fluxes is a constant independent of the rotational angle of the rotor. The motor of this type can suppress the torque ripple to a rather low level, but requires extremely accurate adjustment of all circuit parameters. Furthermore, because it requires a double power source, which cannot easily be constructed to be efficient and small in size, this motor type is unsuitable for use where a small-sized motor is demanded.
In this type motor, a unidirectional torque is generated in each phase of first and second alternate stator coils over an angle of more than 180 electrical degrees, and the first and second stator coils are alternately switched and supplied with power each 180 electrical degrees, thereby generating a continuous rotational torque. A constant, continuous rotational torque is provided except in the neighborhood of the current change-over times of the first and second stator coils.
However, since in such an alternate-phase brushless DC motor the torque drops somewhat upon a current change-over (or switching) operation, as shown in FIG. 2A below, it has a drawback in that unacceptable torque ripple results. Moreover, if this type of brushless DC motor is used, for example, as the capstan driving motor of a VTR, the wow and flutter that can result from this torque ripple, which has a significant high-frequency component, can result in jitter.