Weston type synchroscope:
Working: It consists of three limbed transformer. The winding on one of the outer limbs is excited from bus- bars and that on other limb by incoming machine. The two fluxes produced by outer limbs are forced through the central limb. The resultant flux through central limb is equal to the Phasor sum of these fluxes. When bus-bar and incoming machine voltages are in phase, the emf induced in central limb winding is maximum hence lamp glows with maximum brightness. When bus-bar and incoming machine voltages are 180o out of phase, the emf induced in central limb is almost zero and lamp does not glow. When frequency of incoming machine is different than that of bus-bar, the lamp will flicker. The correct instant of synchronizing is when the lamp is flickering at a very much slow rate and it is at its maximum brightness.