1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

[Unit conversion] From r/min to period (msec)

  1. Nov 24, 2012 #1
    1. The problem statement, all variables and given/known data
    I would like to know how a 4000 r/min is converted to period of t=2.5mSec


    2. Relevant equations
    Speed of motor = 4000 r/min
    Build-up time of current = 2.5msec
    Frequency = 60Hz


    3. The attempt at a solution
    [itex]\frac{1}{4000}[/itex]=0.25msec not 2.5msec

    OR
    Assuming it means 4000RPM which is equivalent to 419 rad/sec we obtain:
    [itex]\frac{1}{419}[/itex]=2.39msec not 2.5msec

    OR
    If frequency is 60 Hz, and 1-phase is conducting for 60° we can take period as:
    t=[itex]\frac{1}{f}[/itex]=[itex]\frac{1}{60}[/itex]=16.67msec
    Conducting for [itex]\frac{1}{3}[/itex] of the cycle (60°), t = 5.55msec

    Where have I made a mistake ?
     
  2. jcsd
  3. Nov 24, 2012 #2

    berkeman

    User Avatar

    Staff: Mentor

    If T = 2.5ms, then the frequency is 400Hz. There is something wrong with your initial statement. Where did it come from?
     
  4. Nov 24, 2012 #3
    It comes from "Electric Machinery" by Mr. Fitzgerald

    14ybs6b.jpg
    At the very bottom of the screen a period, T, of build-up of current is defined as 2.5msec.
     
  5. Nov 24, 2012 #4

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    For N = 4000 rpm, then n = 4000/60 rps = 66.67 rev/s

    1 rev = 2 pi radians

    omega = 2 pi * 66.67 = 133.33 pi radians/s

    T = 1/omega = .0024 s = 2.4 msec
     
  6. Nov 24, 2012 #5

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Disregard previous post:

    For N = 4000 rpm, then frequency f = 4000 / 60 = 66.67 cycles / s

    T = 1/f = 0.015 s = 1.5 msec
     
  7. Nov 25, 2012 #6
    All right, thank you very much. I will use the very first formula, still it isn't exact 2.5msec.
     
  8. Nov 25, 2012 #7

    gneill

    User Avatar

    Staff: Mentor

    The solution gives the expression:

    ##\theta_m = -\frac{\pi}{3} + \omega_m t##

    Plug in your value for ##\omega_m## and solve for t that makes ##\theta_m = 0##.
     
  9. Nov 25, 2012 #8
    This is the final correct solution.

    Thanks, thread can be now closed.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: [Unit conversion] From r/min to period (msec)
  1. Units Conversion (Replies: 3)

Loading...