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Wattmeter with analog output

Discussion in 'General Electronics Discussion' started by chrisephoto, Mar 27, 2015.

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  1. chrisephoto


    Mar 27, 2015
    Hi all,
    I've come to you in a time of need, during my senior year of college
    working on our Capstone project. We are designing a lab with
    experiments for an internal combustion engine course, and need to
    measure the power being drawn by an AC motor. Looking online, I found
    many watt meters that plug into the wall and have an outlet on them for
    the actual plug, and feature a small LCD screen that displays the
    current/average power consumed. Unfortunately this is not appropriate
    for us, as we need to integrate the watt meters with a National
    Instruments DAQ. What I am looking for is a watt meter similar to this
    but with a 0-5v output, which is what the DAQ requires. If anyone has
    any suggestions or if this type of thing simply does not exist and I am
    completely bonkers, please let me know.
    Additional question, if we have a generator spinning but with no load on it, what exactly is happening?
    Thanks all!
  2. Gryd3


    Jun 25, 2014
    To determine wattage, you need to monitor amperage and voltage being produced.
    If you are spinning a generator with no load (ie. open circuit) then you will not be generating power and the generator will spin fairly freely. Once a load is connected electricity will flow and the generator will experience more resistance due to the back EMF generated in the coils.
    If you are wanting a 0-5v wattmeter, you may have to build your own...
    Luckily for you, most generators will attempt to keep the same voltage on the output, so you could cheat and simply measure the current. You can do this with a shunt resistor (which would be invasive and must be connected in-line with the load) or you can do this with a clamp on current sensor. You could then use an op-amp to buffer and multiply the output from the shunt or clamp on sensor to give you your 0-5V output.

    Anyone else have any ideas?
  3. JWHassler


    Dec 22, 2014
    I believe that the wattmeter you referred to was the "Kill-A-Watt,' whose schematic(the input part, anyway) is internetly available.
    If you have a single-phase motor and if you attend to the isolation/safety issues, this device could be modified to what you need.
  4. duke37


    Jan 9, 2011
    This is not easy, power is
    Volts * amps * power factor
    The volts and amps are easy, the power factor is not.
    Gryd3 and Arouse1973 like this.
  5. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    I'm confused. An internal combustion engine (ICE) produces shaft horsepower that could be used to turn the shaft of an AC alternator or DC generator providing electrical current to a load. Where does "power drawn by an AC motor" enter into this picture? Is this a single-phase or poly-phase AC motor? What is its voltage, current, and horsepower rating? Is this motor used to crank the ICE to start it?

    As @JWHassler noted, these are popular instruments used to monitor home electricity usage. Not particularly accurate for low power-factor measurements, i.e., lightly-loaded AC motors presenting highly reactive "power." And usually not available with a DC output for data acquisition purposes.

    There are several wattmeter devices available with analog outputs for data acquisition. See the attached file for a French version that is quite capable, but probably expensive. Google is your friend here, or you could also consider building your own wattmeter. Be aware that accounting for power factor, as @duke37 mentioned, is not a trivial problem, especially if non-sinusoidal waveforms are involved.

    One "simple" technique uses a Hall sensor to multiply the excitation voltage by the magnetic field produced by the load current in a coil coupled to the magnetic-sensitive axis of the Hall sensor. The Hall voltage output is DC for real power, but it also has an AC component that must be filtered out before sending the DC power signal to a data acquisition system. There could be problems with temperature compensation and fidelity of the magnetic field representing the load current, especially for non-sinusoidal currents or currents with harmonics. You should use a search engine to see if a commercial product exists.

    You are using some small amount of power to spin the generator shaft to make up for bearing friction and windage losses. There will also be a small electrical power loss to excite the field winding of the generator, unless it uses a permanent-magnet field.

    Attached Files:

    Gryd3 likes this.
  6. Gryd3


    Jun 25, 2014
    I'm glad you're on here @hevans1944 , you always seem to respond with a lengthy post full of directly useful detail with indirectly useful details sprinkled on top.
    Arouse1973 likes this.
  7. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Well, I try to be helpful, but the OP usually doesn't provide enough information for a definitive answer.
    Arouse1973 likes this.
  8. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    I think we all are. @hevans1944 . Hop has a great deal of experience we can all learn from.
  9. chrisephoto


    Mar 27, 2015
    The experiment is to demonstrate mechanical efficiency. So the engine is being turned over by the AC motor continuously. Parts are removed from the engine and the engine is spun again. The difference in power required to turn the AC motor is equal to the friction force generated by the now removed component.

    The AC motor is single-phase. I don't have the motor in front of me at the moment, but from memory: 120 VAC, 4 FLA, 0.25 HP.

    Good information, will look into some of these options a little more.

    That small amount of power (versus our baseline - no engine at all) is precisely what we are trying to measure and use to calculate efficiency in LabView.

  10. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    I thought I posted an alternative method of measuring your ICE efficiency, but it isn't here... perhaps I forgot to click on "Post Reply."

    What I would propose is not measuring the power input to the AC motor turning the ICE shaft but instead measuring the torque and rotational speed of the motor shaft. If the motor is suitably mounted so the case is free to rotate about the motor shaft, but the case is constrained from doing so by a load cell connected to the case via a moment arm, the load cell will produce a signal proportional to torque applied by the motor shaft to the ICE. The rotational speed of the shaft is easy to measure with a retro-reflective optical tranasducer and reflective markers applied to the shaft. The product of torque and shaft rotation speed is proportional to power input. Losses in the AC motor are not a factor in the calculation.
    duke37 likes this.
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