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Torque to turn a generator

  1. Dec 5, 2007 #1
    Hi everyone, I'm new here.

    I'm taking electronics engineering at exeter uni and I cant find an equation for my current project.

    Its to do with max efficiency from a Pelton wheel.

    I need to know how much torque it will take to turn a motor (used as a generator) from stationary (zero rpm).

    I think it depends on the load resistance and/or current, but im not too sure.
    I should be able to measure any other variables you state.

    Any help would be greatly appreciated.

    Thank you,
    Sean Coleman
  2. jcsd
  3. Dec 5, 2007 #2


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    Science Advisor

    The starting torque of a three phase induction motor is given by,

    Tstart = Tlr ∙ (Vline,rated / Vline,lr)^2


    Tstart = starting torque
    Tlr = locked roter torque
    Vline,rated = rated line voltage
    Vline,lr = locked rotor voltage

    Don't forget that the line voltage will depend on whether the motor stator windings are configured as delta or wye.
  4. Dec 5, 2007 #3
    For a DC motor/generator, you can measure the torque required to start turning the motor. This torque has to overcome the static friction and other nonlinear frictions like the Stribeck effect.

    Put a 1 Ohm high power rated resitor in series with the + terminal of the motor and the + terminal of the power supply. Apply your voltage to the other end of the resistor and - terminal of the supply to the - terminal of the motor, and measure the voltage across this resistor. Slowly increase this voltage just to the point before the motor starts turning. The voltage across the resistor is also the current going into the motor at this point. This is the maximum current draw right before the motor begins turning. Once you see the motor turning, the current will drop down to a lower level. Your motor should have a datasheet telling you its Kt value (torque constant) and it is expressed in terms of Torque/Current.

    Multiply the maximum current you measured by this Kt value, and it will tell you the Maximum torque you had to overcome to start the motor. Although you used the generator as a motor to measure that torque, the value still applies for when it is used as a generator also, since it is just the static friction of the motor. Also you will notice that it varies depending on the position of the shaft, and this is just because of inconsistencies in physical part (bumps and grooves, distorted shaft shape, brush contact, etc.) and so at different locations the static friction will vary slightly.
    Last edited: Dec 5, 2007
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