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  1. #1
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    Default Device for measurinng amps drawn from 3 phase machines

    Hi guys, just wondering if there is anything on the market that can be wired in series with a lathe or milling machine to indicate the amps drawn when under cutting tool load. I guess it would be a simple matter if the machine was single phase. Ie. An amp meter in series, but I would imagine 3 phase would present problems. (Maybe 3 amp meters!).
    Thanks for any advice.
    Eric

  2. #2
    BobL is offline Member: Blue and white apron brigade
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    There's no need to put anything in series. All that you need is a clamp type ammeter that clamps over any of the active lines. Then you have to multiply the current by teh square root of 3 or about 1.7 to get the total current.

    Budget type clamp ammeters start at about $10 on ebay. If you need to measure currents from voltages anything other than 50Hz AC (eg from a VFD_)you have to be prepared to pay more.

    If you want something more permanent there are a myriad of small panel meters available on line that uses toroidal transformers that slips over any one of the active wires.
    Still need to multiply by square root of 3.
    Different types of meters can be powered from anywhere from 12VDC up to HV AC.
    Here's one that measures uses anywhere between 60-500V AC, Currents up to 100A and also displays power and frequency.
    https://www.ebay.com.au/itm/LCD-Digi..._5mho8vHxqqeFw
    The cheaper meters usually show only integer amps, some will display to 0.1A at low currents eg <10A.
    If you want more resolution than this you will need to pay more or use a higher end clamp meter.

  3. #3
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    That's great advice thanks Bob. I'll certainly get one of those meters.

  4. #4
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    indicate the amps drawn when under cutting tool load. I would imagine 3 phase would present problems.
    Realistically, you probably do not need to measure all 3 phases. Just measure a single phase: the other two will be very close.

    Cheers
    Roger

  5. #5
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    A lot of large heavy duty 3 phase machines incorporated a single AC ammeter on the headstock or some other conspicuous place. This monitored the current in a single phase of the motor supply and served as an indicator of how heavily the motor was being loaded. I haven't worked with a machine set up this way, but suspect that the meter did not need to be accurately calibrated in absolute terms, as a simple banded scale with safe and red danger zones would be adequate. I suspect that the intent was to give an operator a method to judge how hard to push depth of cut for max metal removal in production environments, without danger of overheating the motor. A simple banded scale would be better than a calibrated meter for situations where a large number of differing machine are in use and operators rotate through different machines as needs dictate, as the operators are just trying to stay below the red band, rather than hit specific currents that vary from machine to machine.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  6. #6
    BobL is offline Member: Blue and white apron brigade
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    From what I have seen of 3P current limiters inside 3P switch boxes most of them only monitor the current in one phase. When a VFD is used, current limiters are no longer needed as the VFD will do this.

    I have a (V/I) panel meters on the SP side of my VFD powered 4HP WW Band Saw and 4Hp Dust collector. There is also a meter on my SP 4HP compressor. To check out the currents drawn by other machines I use a short (1m) extension cord with a V/I meter installed in the middle of the cord. The meter displays are very instructive. Ie seeing how much the v drops and currents drawn under various loads.

  7. #7
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    If you are lucky enough to have an older electrical installation with a rotating disc kilowatt hour meter, you can get a fair idea of the consumption over a short period by counting the number of turns the disc makes in a period of time. The number of turns the disc makes per KWH is written on the meter. Count the turns of the disc during the cut - over a period of time - say 1 minute (while everything else in the house is turned off. Supply voltage would be nice too. There are some calcs to do, but no point mentioning it here if you don't have an old meter!

  8. #8
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by A J in WA View Post
    If you are lucky enough to have an older electrical installation with a rotating disc kilowatt hour meter, you can get a fair idea of the consumption over a short period by counting the number of turns the disc makes in a period of time. The number of turns the disc makes per KWH is written on the meter. Count the turns of the disc during the cut - over a period of time - say 1 minute (while everything else in the house is turned off. Supply voltage would be nice too. There are some calcs to do, but no point mentioning it here if you don't have an old meter!
    I think the OP was after something that can measure an instantaneous or peak current under high load. The disc spinner or most amp/hour meters will tend to average things out so it won't be possible to see the peaks. He also wants to be able to read the current while he's doing something specific, my guess is he wants to see what happens under various load/cutting conditions and probably not continually having to go back and forth to his meter box.

  9. #9
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    Quote Originally Posted by malb View Post
    A lot of large heavy duty 3 phase machines incorporated a single AC ammeter on the headstock or some other conspicuous place. This monitored the current in a single phase of the motor supply and served as an indicator of how heavily the motor was being loaded. I haven't worked with a machine set up this way, but suspect that the meter did not need to be accurately calibrated in absolute terms, as a simple banded scale with safe and red danger zones would be adequate. I suspect that the intent was to give an operator a method to judge how hard to push depth of cut for max metal removal in production environments, without danger of overheating the motor. A simple banded scale would be better than a calibrated meter for situations where a large number of differing machine are in use and operators rotate through different machines as needs dictate, as the operators are just trying to stay below the red band, rather than hit specific currents that vary from machine to machine.
    Yes I remember the Repco Training Center in Carlton (vic) had machines with those amp metres on. There was one German instructor there that just loved loading up the lathe cuts just to watch the amp meter peak out. Can't remember his name, but he was pretty obsessed with heavy cuts. I'm sure he'd be dead now, that was in 72 ( I think).

  10. #10
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    Quote Originally Posted by BobL View Post
    I think the OP was after something that can measure an instantaneous or peak current under high load. The disc spinner or most amp/hour meters will tend to average things out so it won't be possible to see the peaks. He also wants to be able to read the current while he's doing something specific, my guess is he wants to see what happens under various load/cutting conditions and probably not continually having to go back and forth to his meter box.
    Yes that's correct Bob. I was approached recently about setting up comparison tests for various cutting lubes. Everything being equal as far as the tests go, the winner would be the brand that draws the least current from the same cutting load action.. I'm taking all the variables out from the test. Ie. New drill, new test piece, etc.

  11. #11
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by Gestalt View Post
    Yes that's correct Bob. I was approached recently about setting up comparison tests for various cutting lubes. Everything being equal as far as the tests go, the winner would be the brand that draws the least current from the same cutting load action.. I'm taking all the variables out from the test. Ie. New drill, new test piece, etc.
    I doubt you will see any difference using a cheap clamp meter as these typically have a resolution of 0.1A - you might need to go for something a bit better to get a 10mA res.

  12. #12
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    Quote Originally Posted by BobL View Post
    I doubt you will see any difference using a cheap clamp meter as these typically have a resolution of 0.1A - you might need to go for something a bit better to get a 10mA res.
    Yes I did consider the sensitivity. Do you have any recommendations Bob?

  13. #13
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by Gestalt View Post
    Yes I did consider the sensitivity. Do you have any recommendations Bob?
    When I wanted something with 10mA res I bought a Fluke clamp meter because all the budget level meters were 100mA but just looking now online I see even a $30 clamp meter will do that these days. The other reason was the Fluke was true RMS across a wide frequency range but this is not needed for what you are doing.

    The Flukes are a bit more accurate but all you want to be able to do is set up a loaded cut run it dry for a few seconds and then turn the lube on and see what happens so you should be able to spot say a 0.02A difference with even a cheap clamp meter.

    Would you like me to do a dummy run - I have only got a Hercus 9 to play with but if I can see something with this then you would probably see it clearly with a lbigger machine.

  14. #14
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    Quote Originally Posted by BobL View Post
    When I wanted something with 10mA res I bought a Fluke clamp meter because all the budget level meters were 100mA but just looking now online I see even a $30 clamp meter will do that these days. The other reason was the Fluke was true RMS across a wide frequency range but this is not needed for what you are doing.

    The Flukes are a bit more accurate but all you want to be able to do is set up a loaded cut run it dry for a few seconds and then turn the lube on and see what happens so you should be able to spot say a 0.02A difference with even a cheap clamp meter.

    Would you like me to do a dummy run - I have only got a Hercus 9 to play with but if I can see something with this then you would probably see it clearly with a lbigger machine.
    Thanks a lot for that info and the offer Bob. I don't want you to go to any trouble, but it would be an interesting exercise.

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