Measuring Toruqe - With an oscilloscope?

Discussion in 'Homework Help' started by Yusif Nurizade, Feb 14, 2013.

  1. Yusif Nurizade

    Thread Starter New Member

    Jul 4, 2011
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    Hello,

    I wasn't sure if this belonged in the Projects or Homework section but since my trouble is with the actual assignment, I thought this would be more appropriate.

    The project I have been assigned requires me to do work in a lab that I am fairly new to. I am to use a induction machine an produce a given curve of torque versus slip. The torque is to be measured using an oscilloscope which is something we have never done in our work with the induction machine (we usually used DMMs).

    My question, silly as it is, is how would one go about measuring the torque of an induction machine with an oscilloscope? I've tried looking this up online and have not been able to find any leads that explain it.

    I would really appreciate any help you could give me,
    Yusif
     
  2. The Electrician

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    Oct 9, 2007
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    Are you saying you usually used DMMs to measure torque? How did that work? What voltage or current or resistance did you measure that was proportional to torque? How was that signal produced? Was there a transducer of some sort connected mechanically to the output shaft of the motor with a load also connected to the output shaft?
     
  3. Yusif Nurizade

    Thread Starter New Member

    Jul 4, 2011
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    Allow me to clarify.

    We did not use the DMMs to measure torque; we used DMMs in our induction machine labs to measure voltage, current, resistance, etc.

    What I mean to say was the work we did with the induction machine didn't include oscilloscope work so I am unsure how to measure torque with it and I could really use some direction.

    Hope to hear back,
    Yusif
     
  4. The Electrician

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    You answered only the very first question that I asked you. What is the answer to these questions:

    "What voltage or current or resistance did you measure that was proportional to torque? How was that signal produced? Was there a transducer of some sort connected mechanically to the output shaft of the motor with a load also connected to the output shaft? "
     
  5. Yusif Nurizade

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    Jul 4, 2011
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    I apologize. I will try to answer the rest of the questions as best I can, please bear in mind that we did this lab at the beginning of last semester so I am a little rusty. The reason I posted this question is that I was assigned to independently produce a certain torque versus slip behavior, with an oscilloscope which I have not used with an induction machine.

    What voltage or current or resistance did you measure that was proportional to torque?
    We measured current, voltage, resistance but I do not believe we were measuring torque. Admittedly this was half a year ago, but as memory serves, we were more concerned with power. I don't have the experiment's data handy but I can find it if necessary.

    How was that signal produced?
    We had a power source as well as a set up where we could connect wires, like an old switch board, for voltages, resistance and inductance. The lab documents indicated how to make the connections and we were supposed to observe/document the results. Once we wired the connections, we would slowly raise the voltage and the motor would begin to spin. At this point we would document voltage/current/resistance and rpms.

    Was there a transducer of some sort connected mechanically to the output shaft of the motor with a load also connected to the output shaft?
    The transmission lines ran from the power source to the "switch board" where we could make the connections to a motor. There were two horizontal cylinders and between them a spinning disk that was color black with a white point that allowed us to track rpm.


    I apologize if I am not able to give better descriptions of the setup. We did this lab exercise at the beginning of the semester before relevant material was covered in class. I'm in the process of tracking down the lab documents which should help with descriptions.

    Anyway I am still unsure how I can attach a oscilloscope to the motor to produce the torque versus slip so any direction in that regard would be greatly appreciated.

    Thanks,
    Yusif
     
  6. The Electrician

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    If you were not measuring torque then, it would seem that you won't be measuring torque now.

    An oscilloscope is just an instrument for showing what amounts to a graph of voltage on the vertical axis with (typically) time on the horizontal axis. You could do the equivalent by hand by taking a series of measurements with a DMM and then plotting them on graph paper.

    But, measuring the current and voltage taken by a motor won't give you torque. You need some way to mechanically measure the torque produced by the motor and converting that quantity into a voltage or current that you could display on an oscilloscope.

    You should do a Google search on the phrase:

    "Measuring motor torque"

    You might also find it instructive to have a look at:

    http://en.wikipedia.org/wiki/De_Prony_brake

    http://en.wikipedia.org/wiki/Dynamometer
     
  7. Yusif Nurizade

    Thread Starter New Member

    Jul 4, 2011
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    Thank you for the suggestions, I've looked through the links you posted and will be looking at them more in depth in the coming week.

    Meanwhile, a grad student suggestion another setup that I was considering prior to seeing the previous post. The idea was to use a large Fluke DMM which can give me voltage and current as well as a tachometer to track the rpm. Both are pretty easy to interface with a microcontroller that can do the necessary calculations and output a calculated torque in terms of voltage as the assignment is to get the torque to slip curve. Another idea was to go through the different load setups with the motor and then run it with just a tachometer and have the microcontroller run calculations and output the torque results to the oscilloscope just like in the previous idea.

    These ideas made sense to me when we were discussing them but after I realized that the oscilloscope wouldn't give me the curve unless I was actively changing the speed and even then it would be a nightmare to get the whole thing to fit. I'll be talking with the professor about tomorrow but in the meantime I thought I would ask if our thinking is way off base here?

    Thanks again,
    Yusif
     
  8. The Electrician

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    What voltage and current are you referring to (in red above)? How will these allow you to calculate the torque?

    You can't calculate torque by knowing the terminal voltage and current of the motor. You need some mechanical method of measuring torque independently of the electrical power delivered to the motor.

    Once you have determined torque by such a method, then you can also determine the slip and relate torque to slip.
     
  9. Yusif Nurizade

    Thread Starter New Member

    Jul 4, 2011
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    Electrician,

    Thanks for your continued help on this. I intended to measure torque by the method explained here:

    http://www.ehow.com/how_7810276_measure-electric-motor-torque.html

    I hope this answers the questions about what is being measured. I showed this to the grad student helping me and the professor overseeing the project and they were satisfied. I am currently configuring a code to control the Fluke device that will be giving me power and tachometer that will give me rpms.

    Thanks again,
    Yusif
     
  10. The Electrician

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    Yusif, you are using the wrong word to describe what you are planning to do.

    What is described at the eHow link you provided is NOT measuring the torque. It is CALCULATING the torque.

    Measuring is not the same as calculating.

    The procedure described at that link fails to take into account the frictional losses in the screwdriver, and the efficiency of the electric motor.

    I would have thought that the purpose of your experiment would be to show that the horsepower output of your motor is less than the electrical power input, and then to find out what the torque is by actual mechanical measurement, relating that to the slip angle and the electrical input power.

    All this is not to say that you can't make assumptions about the efficiency of your motor and calculate the torque based on the electrical power input, and the known relationship: 1 horsepower = 746 watts. You could even assume the efficiency is 100 % (which it isn't), and on that basis, measure the electrical input power and the RPM of the output shaft, calculating from that the hypothetical torque. But, doing this does not constitute "measuring" torque, and you should stop calling it that.
     
  11. Yusif Nurizade

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    Jul 4, 2011
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    I apologize, English is not my first language.

    Allow me to clarify; the torque will be calculated in the code using the measurements I am taking. It will then output to a graph and plot the curve as if it was measuring torque.

    I do have some leeway for assumptions because the purpose of this class is to get experience working in the laboratory. The second part of the course receives more scrutiny.

    Thanks again,
    Yusif
     
  12. The Electrician

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    Good luck with your project!
     
    Yusif Nurizade likes this.
  13. Yusif Nurizade

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    Jul 4, 2011
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    Thanks so much!
     
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