In the home electric appliance, industrial equipment, and automobile industries, electric motor drive units are used for controlling rotation speed, torque assist, and positioning of fans, pumps, compressors, conveyors, elevator machines and the like.
Small, high-efficiency synchronous motors equipped with permanent magnets have been widely used for the electric motor drive units in the aforementioned industries.
To drive a synchronous motor, position information of the motor rotor's magnetic pole is necessary. Therefore, a position sensor, such as a resolver or hall integrated circuit, is required to detect the position of the magnetic pole.
Recently, however, a technology called “sensorless control” has become popular whereby the number of rotations and torque of a synchronous motor are controlled without using a position sensor.
As the result of the sensorless control, it is possible to reduce the costs of installing a position sensor (the cost of the position sensor itself and the cost of wiring of the position sensor) as well as reducing the size of the device since a position sensor is not necessary; and also the use in poor environments becomes possible. Thus, significantly advantageous effects can be obtained.
Synchronous motor's sensorless control systems that are currently being adopted include the following: a system which directly detects an induced voltage (speed electromotive voltage) generated by the rotation of a rotor, uses the voltage as the rotor's position information, and drives the synchronous motor; and a position estimation system which performs estimated calculation of the rotor position based on the mathematical model of the targeted motor.
The sensorless control creates significantly advantageous effects in that the costs to install a position sensor can be reduced, the size of the device can also be reduced due to an unnecessary position sensor, and the use in poor environments is made possible. However, there is a problem with the sensorless control in its position detection method executed during a low-speed driving process.
Specifically, since most of the currently and practically applied sensorless controls are based on the induced voltage (speed electromotive voltage) generated by a synchronous motor, sensitivity to the detection of an induced voltage decreases in the zero-speed vicinity (halting state) and the low-speed zone where an induced voltage is small. Consequently, there is a problem in that position information is drowned out by noise.
To address such a problem, for example, Japanese Patent Laid-Open No. 2009-189176 (patent literature (PTL) 1) proposes a sensorless control system in the low-speed zone based on the 120-degree energization control of synchronous motors. Thus, synchronous motors can also be controlled in the low-speed zone where the induced voltage is small.
Furthermore, sensorless control systems based on the induced voltage (speed electromotive voltage) generated by a synchronous motor include the following: a system based on the zero crossing of the induced voltage as described in Japanese Patent Laid-Open No. Hei 11(1999)-341869 (PTL 2), and a system described in Japanese Patent Laid-Open No. Hei 7(1995)-123773 (PTL 3) wherein a value of the induced voltage is directly read by an AD converter and the rotor position is estimated according to the rate of change.