Numerical - Maximum & minimum number of blades strokes per minute if UJT controls the motor of an automobile

Q.  What will be the requirement of maximum & minimum number of blades strokes per minute if UJT controls the motor of an automobile with stand-off ratio of about 0.7, capacitance of 40 μF and the charging resistances of 40 k & 480 k potentiometer connected in series combination?
- Published on 20 Oct 15

a. 24 & 2.2 seconds respectively
b. 28 & 2.4 seconds respectively
c. 30 & 2.7 seconds respectively
d. 32 & 2.9 seconds respectively

ANSWER: 30 & 2.7 seconds respectively
 
According to the given data,

Time constant has a least value of about 50000 x 40 x10-6 = 2 seconds

Considering the utilization of an entire potentiometer, the maximum value of this time constant is given by,
= (40 + 500) x 103 x 40 x 10-6
= 21.6 seconds

Maximum number of blade strokes per minute = 60 / 2 = 30 seconds

Minimum number of blade strokes per minute = 60 / 21.6 = 2.7 seconds

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