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Homework Help: How much current does the elevator consume?

  1. Feb 3, 2016 #1
    The problem statement, all variables and given/known data

    Energy conversion efficiency of the elevator is 80%, it is working from 220 volts. How much current does the electro motor consume while it is constantly lifting the 500 kg cabin at 2 m/s.

    The attempt at a solution

    I=P/U, so I figured I need to know the power. P=F*v. I know the velocity (2 m/s) so I need the force.
    F=m(g+a). Now this looks simple but for some reason I'm stuck. The acceleration = 0, so F must = mg?
    If so - F=500*9.81=4905. (I suppose kilograms and m/s shouldn't be converted to anything).
    P=9810W. If the efficiency is 80% - the whole power consumed is 9810/0.8=12262.5W.
    And then I=12262.5/220=55.7 amps.
    I'm not sure if it's too much or too little for an elevator, so I'm not really sure if I solved it correctly.
    What do you think? Thanks!
  2. jcsd
  3. Feb 3, 2016 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    You're somewhat careless with units.

    W = mg has units of kg-m/s2, which are otherwise known as newtons. Energy has units of newton-meters, and a power of 1 N-m/s is known as a watt. You should learn the definitions of these derived units in SI.
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