PURPOSE: To measure the r.p.m. of a DC shunt motor without requiring any r.p.m. sensor by analyzing the frequency of a voltage corresponding to the AC component of field current.

CONSTITUTION: When a DC shunt motor is controlled by a microcomputer 15, armature 1 of the DC shunt motor is fed with a voltage for generating an r.p.m. passing through a specified corresponding region shown on the drawing. More specifically, r.p.m. of the motor increases from nmot-μ to nmot-o and thereby the r.p.m. of machine increases from nma-μ to nma-o. The latter is detected by a pulse produced from an r.p.m. sensor 10. Input side 16 of computer 15 is activated only during this r.p.m. region. The r.p.m. region can detect all conversion ratio on one hand and selects a rotational region where a spectral line can be detected definitely on the other hand. According to the arrangement, r.p.m. can be measured without requiring the sensor 10.