1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Motor Resistance

  1. Apr 14, 2008 #1
    1. The problem statement, all variables and given/known data
    Here's another motor--this one operates on 54 V and does work at a rate of 140 W. How much resistance must be connected in series to this motor, if you wish to plug it in to a 120 V outlet?

    2. Relevant equations
    I'm not sure, but:
    P = (V^2)/R
    Where P is power, V is voltage, and R is resistance.

    3. The attempt at a solution
    Am I supposed to subtract the motor voltage from the outlet voltage?

    ((120-54)^2)/140 = 31.114 ohm

    I know that this is wrong...
  2. jcsd
  3. Apr 14, 2008 #2

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, but you're not supposed to just throw that difference into the power formula.

    Try to answer the following questions in order.

    1.) Given that you know the voltage and power ratings of the motor, what current will it draw?

    2.) Given that you know the voltage of the voltage rating of the motor, what voltage drop do you require across the resistor? (This is where you subtract the voltages).

    3.) Given that you know the current through the resistor and the voltage across it, what is the required resistance?
  4. Apr 14, 2008 #3
    thanks, i got it
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook