AC Motor Output Power

Discussion in 'General Electronics Chat' started by nradam123, Dec 19, 2013.

  1. nradam123

    Thread Starter New Member

    Dec 3, 2013
    I have to find the output power of an AC motor without using a torque sensor.

    My idea was to find the voltage and current through the terminals of the motor and then find V*I to get the input power. Then divide that power by the angular velocity of the motor from the shaft encoder to get the torque.

    But this method is wrong because there seems to be lot of losses such as heat losses that will reduce power output of the motor. Also, since it is an AC motor the phase keeps changing and my V-I measurement keeps fluctuating. So maybe i will have to use a rectifier, but wont that further reduce teh accuracy of my measurements??

    I dont want to buy a torque sensor. Do you guys know how i can accurately measure the output torque of a motor without using a torque sensor??

    Thanks in Advance. I will be forever greatful :)
  2. inwo

    Well-Known Member

    Nov 7, 2013
    You can get a ball park figure, for a small motor, with a home made sensor if your mechanically inclined.

    I've used a lift arm or pulley, lifting a known weight, and a wooden clutch.
    Two wooden pulleys. One free on the shaft for lifting. One locked to shaft.
    A lever and bearing pushing them together.
    Monitor current while tightening the clutch.
    Read lift lbs at full load current.
    Believe I've also used a spring scale and line on the lift pulley.
    A 1 foot arm would read lb foot etc.

    A known hp motor could be estimated mathematically at rated speed.
  3. wayneh


    Sep 9, 2010
    A little more context might be helpful. You're asking how to measure something ... without measuring it. Like taking temperature without a thermometer. It raises the obvious question, why?

    You could run the motor inside a calorimeter and measure heat production. The input power less the heat produced would leave you with an estimate of the shaft work.
    strantor likes this.
  4. MaxHeadRoom


    Jul 18, 2013
  5. crutschow


    Mar 14, 2008
    If you want a good measurement of the output power then you need to measure torque and rpm.

    If you want an approximation then you can measure the input power and multiply by an estimated efficiency factor based upon the motor size and type.

    Note that to measure input power you need the voltage, current, and the phase between the to determine the power factor. A wattmeter will measure the power accurately since it includes the power factor in its measurement.
  6. inwo

    Well-Known Member

    Nov 7, 2013
    Read op's post to mean, not "buy a torque sensor".

    There are ways to get ball park readings with simple equipment.

    Such as building a reactive mount for motor.
    Hinged mount with spring scale at a fixed point from pivot.
    Still need a way to load motor, but not too difficult with small motors.
    End to end generator or even a friction block.

    As far as watt meter. For small motor use Kilowatt (sp) meter.
  7. inwo

    Well-Known Member

    Nov 7, 2013
    Have one but haven't used it as much as I thought.
    Now local library has one to use.