Discussion in 'Homework Help' started by rick, Apr 19, 2005.

  1. rick

    Thread Starter New Member

    Apr 16, 2005
    Can you help me with this homework question as I cant find any formula for it. The resistance of a shunt field winding of a motor is 20 ohms at 15 degrees c. After some time of running the resistance increases to 22 ohms. Calculate the final temp of the winding given that a is 0.004 per degree celsius? Thanks.
  2. Brandon

    Senior Member

    Dec 14, 2004
    No formula. Just basic Algebra.

    20 ohms = f(15)
    1 Celcius = 0.004 ohms

    You have 1 point and you have the slope therefor you have a line. Since you have a point and the slope, you can get the exact formula for the line and solve for any point on it.

    point = (15,20)
    slope = 0.004 Ohms/Celcius


    20 Ohms = 0.004 Ohms/Celcius * 15 Celcius + Constant

    From this we solve for Constant and we get 19.4 ohms

    Now we have the next point we need to solve for 22 ohms,

    22 Ohms = 0.004 Ohms/Celcius * X + 19.4 Ohms

    X = 650 Celcius

    There ya go
  3. dragan733

    Senior Member

    Dec 12, 2004
    The variation of the resistance of the resistor by the temperature is gave by the formula:
    R=R0[1+ α (T-T0)]
    from here:
    T=T0+(R/R0-1)/ α