by a step 505 to check to see if n is smaller than n 0 which represents a point in time for starting a transient correction for the damper 7 if this starting point is still not reached the correction clear step 502 is executed when the correction starting point is reached the angle correction data u03b8 c is calculated in a step 506 using a formula u03b8 c b n 0 n where b is a constant if n 0 is detected in the step 503 an 34 yes 34 decision route is taken to a step 507 the angle correction data u03b8 c is set to u03b8 0 which equals bn 0 it will be appreciated from the foregoing that as the steps of fig4 are recyclically executed the angle correction value u03b8 c is held at zero from time t 0 at which the compressor 5 is turned off to time t 1 which corresponds to the timer data n and the correction is started at time t 1 by gradually increasing u03b8 c as a function of time until it reaches u03b8 0 as seen in fig6 the correction value u03b8 c is held at u03b8 0 until the compressor 5 is turned on the interval between times t 0 and t 1 is selected to correspond to the time.