Patent ID: 8222776

Claim:
A rotation detector detecting a rotational state of a direct-current motor having armature coils comprised of phase coils of at least three phases, a commutator having a plurality of commutator segments with which the armature coils are connected, and at least one pair of brushes supplying a current to each of the phase coils through the commutator, the rotation detector comprising: a power supplying means so configured that at least alternating current superimposed voltage obtained by superimposing alternating-current voltage on direct-current voltage can be applied to between at least the one pair of brushes of the direct-current motor; an energization detecting means detecting a motor current passed through the direct-current motor through the brushes or path voltage, or the voltage on an energization path through which the motor current flows; and a rotational state detecting means detecting at least any one of the rotation angle, rotation direction, and rotational speed as the rotational state of the direct-current motor based on the motor current or the alternating-current component contained in the path voltage detected by the energization detecting means, wherein the direct-current motor is so configured that any two commutator segments among the commutator segments are taken as one set and the section between commutator segments in at least one set has a capacitance value different from the capacitance values of the sections between commutator segments in the other sets; wherein the power supplying means includes: a direct-current power source applying the direct-current voltage to the direct-current motor; a direct current interrupting means interrupting the application of direct-current voltage from the direct-current power source to the direct-current motor; and an alternating-current voltage applying means applying the alternating-current voltage to the direct-current motor, and wherein the rotation detector further comprises; a direct current interruption controlling means which controls the direct-current interrupting means, and applies direct-current voltage from the direct-current power source to the direct-current motor so that the alternating current superimposed voltage is applied to the direct-current motor when the direct-current motor is rotated and interrupts the application of direct-current voltage from the direct-current power source when the direct-current motor is braked.