The Stroboscope

Author: E.E. Kimberly

The electric thermionic stroboscope is an instrument which emits accurately timed light flashes of extremely short duration. If the light flashes are caused to illuminate a line on the end of a motor shaft, as in Fig. 16-19, and the flashes are so synchronized with the motor speed that the interval between successive flashes is exactly one revolution or a whole number of revolutions, the line will appear to be stationary. Thus, if 3600 flashes occur every minute to illuminate a shaft running 3600 rpm, the line will appear as a radius as in Fig, 16-19. If the motor were running only 1800 rpm, the shaft would be illuminated two times in every revolution, and the line would appear as a diameter.

Fig. 16-19. End of Motor Shaft with Stroboscopic Spot

Fig. 16-20. Electric Stroboscope

If the speed were 1200 rpm, three radial lines displaced 120° from each other would appear. When the shaft is in a two-pole, 60-cycle induction motor, the spot fails to make a complete revolution between flashes because of the motor's slip; and it therefore appears to drift in a direction opposite to the direction of shaft rotation with a speed equal to the slip of the motor. The speed of an induction motor may be found very accurately by subtracting the speed of slip from the synchronous speed.

Sometimes, an electronic stroboscope is provided with a self-contained calibrated oscillator by which the flash frequency may be controlled throughout a wide range. Such an instrument is shown in Fig. 16-20. If a vibrating shaft be illuminated by such an instrument, the flash frequency may be adjusted to that of the shaft, and the shaft will be made to appear stationary. Such a procedure permits analysis of critical speeds, vibration amplitudes, and other phenomena of high-speed machines.