The following describes a conventional motor driver.
FIG. 14 shows the structure of a conventional motor driver.
In the figure, a rotor 1010 has a magnetic field unit achieved by permanent magnets, and generates a rotation force according to mutual action of windings 1011, 1012, and 1013. A power supplier 1020 is composed of three upper power transistors and three lower power transistors, and supplies power to the windings 1011, 1012, and 1013. A position detector 1030 compares each of terminal voltages V1, V2, and V3, which are each from one terminal of the windings 1011, 1012, and 1013, respectively, with a common voltage Vc, and outputs a detection pulse signal FG in accordance with the result of comparison. A command unit 1040 outputs a speed command signal EC for controlling speed of the rotor 1010. In accordance with the signal EC, the switching controller 1050 outputs a PWM signal Wp for having the upper power transistors of the power supplier 1020 perform PWM operation. In accordance with the detection signal pulse FG and the PWM signal Wp, a distribution controller 1060 outputs upper distribution control signals N1, N2, and N3 and lower distribution control signals M1, M2, and M3 for controlling power distribution to the windings 1011, 1012, 1013. Accordingly, the power supplier 1020 supplies power to the windings 1011, 1012, and 1013, and has the motor perform PWM sensorless driving.
A further structure is disclosed in Japanese Patent Application Publication No. 2001-346394 (p. 18, paragraph no. 0051), for having position detection performed stably in order to eliminate instability in accelerated turning operation caused by lag in position detection.
A problem exists in these conventional motor drivers in that startup failure occurs easily. Startup failure occurs because the rotor 1010 is unstable in terms of position and rotates slowly at the initial startup, and therefore the back EMF (electromagnetic force) voltage that is induced in the windings 1011, 1012, and 1013 is low. Consequently, the position sensor 1030, which detects position based on the comparison results of the terminal voltage V1, V2, and V3 of the windings 1011, 1012, and 1013 with the common voltage Vc, detects erroneously.
Particularly, when having the motor perform PWM driving, induced noise that is characteristic of PWM operation is superimposed on the terminal voltage in the detection phase. As a result, the probability of the position sensor 1030 detecting erroneously further increases due to this superimposed noise.
A further conventional technique that attempts to deal with this problem is a method that fixes the position of the rotor in startup according to magnetic pull in a specific phase. However, this method is problematic because the motor driver takes longer to start up due to the additional time required to fix the position of the rotor.
In view of the stated problems, the object of the present invention is to provide a motor driver that enables stable PWM sensorless startup in PWM sensorless driving, taking into consideration the effects of noise that is characteristic of PWM operation.