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Homework Statement
I would like to know how a 4000 r/min is converted to period of t=2.5mSec
Homework Equations
Speed of motor = 4000 r/min
Build-up time of current = 2.5msec
Frequency = 60Hz
The Attempt at a Solution
\frac{1}{4000}=0.25msec not 2.5msec
OR
Assuming it means 4000RPM which is equivalent to 419 rad/sec we obtain:
\frac{1}{419}=2.39msec not 2.5msec
OR
If frequency is 60 Hz, and 1-phase is conducting for 60° we can take period as:
t=\frac{1}{f}=\frac{1}{60}=16.67msec
Conducting for \frac{1}{3} of the cycle (60°), t = 5.55msec
Where have I made a mistake ?