Patent ID: 8390277

Claim:
A rotational angle detector for detecting a rotational angle of a rotor arranged in a motor, the rotational angle detector comprising: a circular rotary plate integrally rotatable with the rotor and including a circumferential surface having a plurality of marks spaced apart from one another by different angular intervals in a circumferential direction; and a rotational angle detection unit arranged along the circumferential direction of the rotary plate to detect a rotational angle of each of the plurality of marks, which are rotated together with the rotary plate, in a predetermined detection zone; wherein the rotational angle detector determines, when adjacent ones of the plurality of marks are located in the predetermined detection zone, angular intervals between the adjacent ones of the marks based on the rotational angle of each mark detected by the rotational angle detection unit, specifies from the plurality of marks a mark that is located in the predetermined detection zone based on the determined angular intervals, and detects the rotational angle of the rotor based on the rotational angle of the specified mark rotating in the predetermined detection zone.