Question

A stroboscopic system is used for measuring the speed of a rotating shaft. The shaft has one target mark on it. The maximum strobe rate at which synchronism achieved is 2400 flashes per minute. The next lower flash rate at which synchronism achieved is 1800 flashes per minute. The speed of the shaft in rpm is

• A. 1800
• B. 600
• C. 2400
• D. 7200

Answer 1

The correct option is D 7200

$\text{The speed of shaft is given by,}$

$n=\frac{r_1{r}_2}{r_1-r_2}rpm$

$\text{where,}{r}_1=\text{highest flashing frequency}$

$r_2=\text{lowest flashing frequency}$

$n=\frac{2400\times 1800}{2400-1800}=7200rpm$