1. Field of the Invention
Forming the subject of the present invention are a control system and a control method for “sensorless” drives with a.c. motors.
Currently, vector-controlled electric drives with a.c. motors are widely used in various industrial applications, in machine tools, pumps, conveyor belts, and the like, where they are progressively supplanting traditional d.c. electric drives, thanks also to their greater sturdiness, their reduced maintenance requirements, and their lower production costs.
2. Description of the Prior Art
The principle of vector control is based on the independent regulation of the components of the stator-current vector defined with respect to a reference system synchronous with the rotor flux. The instantaneous position of the rotor flux, which is necessary for vector control, can be measured directly using Hall-effect flux sensors or, in the case of synchronous machines, by measuring the position of the rotor by means of electro-optical or magneto-electric position sensors. Installation of said sensors causes an increase in the overall dimensions of the drive, renders necessary supplementary wiring and contributes considerably to increasing the cost of the drive. Furthermore, in particular operating conditions, the delicacy of such sensors can adversely affect the reliability of the drive.
The disadvantages linked to installation of the aforesaid sensors have been an impulse to the development of a series of vector-control techniques, referred to as “sensorless” techniques, which are able to reconstruct the position of the rotor flux without using a position transducer.
The above vector-control techniques are based upon different principles. In simpler sensorless vector-control techniques, said reconstruction is obtained by measuring the induced electromotive force. The techniques based upon said principle can be implemented simply and at a low cost; however, they do not function at low or zero rotor speeds.
More sophisticated techniques of a sensorless type are based upon injection of appropriate reference signals and upon measurement of current and/or voltage harmonics. Said techniques enable a considerable reduction in the minimum speed enabled by the control. However, they also prove extremely expensive in terms of computing power required and/or in terms of processing times and, in many cases, do not provide a complete solution to the problem of vector control of a.c. electric machines at very low or zero speeds.
Control of a drive at a very low or zero speed is required in many applications of electric drives in the industrial framework and in the transport field and, in particular, in electric-driven means, in robotics and in many new-generation machine tools.