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Power measurement of a servo motor using simple instruments

  1. Aug 8, 2016 #1
    Hello all,
    I have a 1kw 3 phase 10 pole servo motor which is powered from 240 volt ac single phase supply via an invertek p2 controller/inverter. The motor is only operated for short high speed bursts and I would really like to be able to measure the power usage of the motor using formula and simple instruments such as a quality multimeter with peak hold rather than an expensive power analyser. I know the power usage of the controller on standby but can anyone offer me any suggestions for measuring the motor power consumption.
    Thank you.
  2. jcsd
  3. Aug 8, 2016 #2


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    is this for observation/measurement purposes or control purposes?
    Will this be a permanent fixture?
    How accurate does it have to be?
  4. Aug 9, 2016 #3
    It is for MEASUREMENT purposes as stated, as accurate as possible with basic instruments your questions are already answered in my question description above.
  5. Aug 9, 2016 #4


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    (1) You could estimate the power usage from information about the load configuration and it's motions .

    (2) The upper limit of power consumption is going to be approximately given by motor rated power multiplied by the ON time .

    (3) An electrical method could possibly be devised using a clamp meter to measure average current flowing in one of the phases during ON time .
  6. Aug 9, 2016 #5


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    no those are not adequate answers.

    You can measure it but are you going to do with the data? simply record it and use it as a general reference? use it to tune motor parameters? Do you need to give this number to a customer? are you going to use this data to size components in a controller? how long is this high speed burst?

    are you going to use it once so a little handheld unit is sufficient? will this be in a lab for 10 years?

    Accurate as possible again is not sufficient. it boils down to how you need to use it and how much money you can spend. We also need to know the expected current range to properly size the device (this can be derived from the power rating)
  7. Aug 9, 2016 #6
    There are relatively basic instruments that can read single phase power, but still need V and I input - so you can measure at the input, 240V Line. Other than that it will be very difficult to get a reading more accurate than the drive will give you itself using something like a DMM. Note that drive also records energy usage.

    If you are concerned about the accuracy of info the Drive is giving you, then I would compare single V and I readings to the data from the Drive. You can not get PF from a basic DMM.

    Lastly - manually measuring 240 VAC in the US is live line work and requires PPE and training.
  8. Aug 10, 2016 #7


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    There is a device called "kill-a-watt" that's popular in the US. It measures Volts, Amps, Watts amd accumulates Watts. There may be a similiar device or model available where you are for 240V mains. If not, you may be able to use an international voltage converter ( a step down transformer) and have an electrician wire it up correctly to measure current in one leg of the 240V line but supply the device with 120V. Then you would just double the Watt reading of the device to account for the double line voltage.
  9. Aug 10, 2016 #8


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    I took apart my US version kill-a-watt and noted that the current measurement is done on the Neutral wire, that's the wide opening of the socket. So that's the one that would have to be connected to one leg of the 240V line. The third, round Grounding, pin is not used internally, just fed thru from input to output.
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