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 the operation of the motor by generating 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 grooves 4, which constitute a secondary circuit. FIG. 1 is a schematic view in which only a portion of 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 higher) can be removed. However, for example, the 5th and 7th harmonic components, which are largely responsible for adverse effects, 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 manufacturing 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.