1. Field of the Invention
This invention relates to a method of inspecting the counter-electromotive force of a motor, or in particular to a method of correcting the measurement value of the counter-electromotive force generated across coils during the inertial rotation of the motor to the value of the counter-electromotive force generated at the time of the rotation at a predetermined rotational speed.
2. Description of the Related Art
The motor is an indispensable energy converter in the current society, which makes possible the mutual conversion between the electric energy and the kinetic rotational energy. The motor of various sizes are available ranging from as small as several millimeters to as large as more than one meter in diameter and finds wide applications. In the field of information technology, for example, a miniature motor is used to rotate a discal recording medium.
The motor is configured of a permanent magnet and coils wound in opposed relation to the magnetic poles of the permanent magnet. The permanent magnet makes up one of a rotor and a stator, and the coils the other. The rotor is supported rotatably on the stator. Upon energization of the coils, an induction magnetic field is generated, and the rotor is rotated by the magnetic interaction with the magnetic field generated by the permanent magnet.
An important index of the motor characteristics is a torque constant. The magnetic field of the permanent magnet can be regarded to be constant with respect to the time and the rotation of the rotor. The torque generated by the motor is proportional to the current supplied to the coils, and the proportionality constant thereof is the torque constant. In similar fashion, the counter-electromotive force generated in the coils and the rotational speed of the rotor are proportional to each other, and the proportionality constant thereof is the counter-electromotive force constant. The torque constant and the counter-electromotive force constant are physically identical with each other, and the torque constant can be determined by measuring the counter-electromotive force constant.
The counter-electromotive force as referred to herein is an induced electromotive force generated in the coils by the magnetic field of the permanent magnet, and determined by measuring the potential difference across the ends of the coils or between the corresponding positions. The value of the induced electromotive force thus generated is known to be varied periodically with the relative rotational angular positions of the rotor and the stator.
In recent years, energy saving has been the demand in every field, and the need of lower power consumption of the motor has increased more and more. As described above, the torque constant (i.e. the counter-electromotive force constant) is a value indicating the energy conversion efficiency of the motor, and accurate measurement of the counter-electromotive force is crucial to accurately grasp the torque constant.
The counter-electromotive force is measured roughly by one of two methods. In one method, the motor is forcibly rotated at a predetermined rotational speed by an external drive unit, and the counter-electromotive force generated by the motor is measured. According to this method, the counter-electromotive force measured has a predetermined period, waveform and amplitude. By increasing the rotational accuracy of forcible rotation, therefore, the counter-electromotive force can be measured with a very high accuracy. In this measuring method, however, the forcible rotation of the motor by an external drive requires that the external drive unit is required to be mounted on the rotor of the motor or a rotary member on which it is mounted. In the process, fine damages, distortion or dust is liable to occur, thereby leading to the disadvantage that this measuring method cannot be used for the motor for precision equipments or the rotor fabricated of a fragile material.
The other method consists in stopping the input of the motor in steady rotation by the input current into the inertial rotation and measuring the counter-electromotive force generated in the motor by the inertial rotation. In the case where the moment of inertia around the rotational axis of the rotor and the rotary member mounted on the rotor is sufficiently large, the reduction in the rotational energy is small and therefore the reduction in the rotational speed during the inertial rotation is also so small that the motor rotates at substantially a constant rotational speed for a short length of time. The counter-electromotive force measured in the process has a substantially constant period, waveform and amplitude. In the case where the moment of inertia around the rotational axis is small, however, a large ratio of the effects of various losses immediately changes the rotational speed, and therefore the counter-electromotive force dependent to a large measure on the rotational speed and the rotational angular position cannot be accurately measured. In an extreme case, the motor is stopped before the required data is collected, and the measurement is impossible. For utilization of this method, therefore, the moment of inertia around the rotational axis is required to be sufficiently large, and this method is not applicable to the measurement of lightweight, miniature motors.
In the latter method, the reduction in rotational speed during measurement is substantially unavoidable. Therefore, the development of a method in which the change in rotational speed during measurement is properly evaluated and the counter-electromotive force is measured with high accuracy for a short length of time has been required.