Electrical Engineering ⇒ Topic : Stroboscopic Method

Stroboscopic Method

In Fig.(a) a black disc with white sectors, equal in number to the number of poles of

                                              Fig.(a) Slip measurement by stroboscopic method.

the induction motor, is attached to the induction motor shaft. It is observed through another disc having an equal number of sector-shaped slits and carried on the shaft of small self-starting synchronous motor, in turn fitted with a revolution counter which can be thrown in and out of gear at will.

                           If n = number of passages of the sectors, then

where            n_s = number of sectors, and

                       n_r = number of revolutions recorded by the counter during the interval of observation.

For large values of slip the observations can be simplified by using only one sector; then

                         n = slip in revolutions.

With a synchronous light source to illuminate the target on the induction-motor shaft,the synchronous motor is no longer necessary. An arc lamp connected across the A.C. supply may be used, but the carbons must be readjusted from time to time. A neon lamp makes a satisfactory source of light when the general illumination is not too bright.