AC Ammeter Installation Question

Discussion in 'The Projects Forum' started by nedriv, May 26, 2013.

  1. nedriv

    Thread Starter New Member

    May 26, 2013
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    Hello all, this is my first post.

    I'm seeking help regarding an Ammeter I purchased and how to install it. I am not knowledgeable in "electrical" or electronic matters. I'm a woodworker trying to modify my 22" drum sander by installing an ammeter so I can monitor the load placed on the motor while in use. I will then be able to change machine settings while using the machine to reduce the load on the motor and prevent a stall-out/overload situation.

    The drum-sander has 110v AC motor to drive the sanding drum and a small DC motor to drive the conveyor belt that feeds the wood under the sanding drum. The sanding drum motor is usually what stalls when the machine settings are too aggressive.

    I purchased (on Ebay) an ammeter described as follows:
    "Quadrate 20A AC Analog Amp Panel Meter Ammeter & Shunt"
    I've attached a picture of the ammeter.

    The ammeter came with a shunt. I've also attached a picture of the shunt.

    Other woodworkers have done this modification on their drum sanders based on what I've found online. But, I haven't found much in the way of wiring information.

    I've developed a couple questions as I researched how to install the ammeter and shunt.

    QUESTIONS:
    1) I'm wondering now if this ammeter is really a DC ammeter as I haven't found info online on how to use a shunt on an AC ammeter. There are no markings on the back of the ammeter. How do I know if this is an AC or DC ammeter by looking at the markings on the front (see picture).

    2) I know the shunt/ammeter should be wired in series. How do I wire this particular shunt (picture) with the AC load line and the Ammeter? Which screws do I use for the 110v ac line and which screws do I use for the ammeter?

    Thanks for any help you can provide this novice.

    Regards,
    Ed V
     
  2. JohnInTX

    Moderator

    Jun 26, 2012
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    Put the shunt in series with the AC power to the motor- I'd use the neutral (white one) to keep on the low side of the line. Use the outer brass screws on the shunt.

    Connect the ammeter across the shunt, each lead of the meter to the remaining brass screw on the corresponding end of the shunt.

    Make your connections tight and lightly twist the wires going to the ammeter.
     
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  3. MrChips

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    Oct 2, 2009
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    Ammeter shunts are wired in parallel with the ammeter.
    Then you put the ammeter and its shunt in series (inline) with your AC motor.
    You can try it and see if it works. If it does then it is AC.

    You need not worry about the accuracy of the scale reading because all you are interested in is an overload condition, i.e. excessive current.

    If the meter is too sensitive or not sensitive enough you can make your own shunt.

    Edit: My comment refers to the OP. JohnInTX and I are saying the same thing: the shunt goes across the ammeter terminals.
     
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  4. nedriv

    Thread Starter New Member

    May 26, 2013
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    JohnInTX and Mr Chips, Thank you for your quick replies. To ensure I am understanding what you are saying, I drew a picture (attached) using the shunt pic as a starting point. Am I correctly understanding what you are telling me?

    JohnInTX, as far as "lightly twist the wires" goes, I should twist the two wires from the shunt to the ammeter together to form a kind of "twisted pair" (like phone wire)? I can do that. I'm curious as to why this will make a difference (a learning opportunity for me).

    Also, why is it preferable to use the neutral wire and not the load (black) wire?
    What size wire should I use for the shunt to ammeter connections?


    Thanks again for your help.

    Regards,
    Ed V
     
  5. nedriv

    Thread Starter New Member

    May 26, 2013
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    I just noticed something written on the face of my ammeter. You can see it in the lower left corner. Does the C.T. mean anything in particular? I've seen reference to CT on specs for ammeters.
    Just curious.

    Ed V
     
  6. MrChips

    Moderator

    Oct 2, 2009
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    Your drawing is correct. I don't know about live side vs neutral side.
    I would have put the ammeter on the live side (black wire). It may not matter but it is related to a concern of safety. Make sure all the connections are covered with insulation tape and no live metal is exposed.
     
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  7. JohnInTX

    Moderator

    Jun 26, 2012
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    I think it means Current Transformer?
    Here are some docs I found reading the meter face..

    SQ72 Ammeter datasheet
    A datasheet for a similar meter showing direct and current transformer hookups.

    GB/T7676-98 refers to a shunt standard so maybe you are OK with the shunt for direct hookup.

    Hmmm..

    The picture looks OK if the shunt is the correct one. I like to twist the wires to keep them in physical closeness to reduce the effects of induced magnetic fields by reducing the loop area. Either hot or neutralwire will work but as long as you are wiring, why not keep the number of high-side wires to a minimum for safety's sake? Any reasonable hookup wire should do it 20-22 AWG.
     
    Last edited: May 26, 2013
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  8. nedriv

    Thread Starter New Member

    May 26, 2013
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    JohnInTx,
    Those are the same specs I was referring to as having seen CT mentioned. I've also visited that same web site trying to find installation "how-to" info.

    So, I wired in the shunt and ammeter as you and MrChips suggested. I used the neutral wire and I also tried the hot wire. I used 14ga wire between the shunt and ammeter. When I power on the motor, the needle moves maybe 1/16" above zero and then settles back to zero with either the white or black wire. So does that indicate anything to you guys?

    How do I find a CT (current transformer) and how do I know what "size?" transformer to use (assuming they come in different sizes)?

    Thanks again for you expertise and help.

    Regards,
    Ed V
     
  9. JohnInTX

    Moderator

    Jun 26, 2012
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    Yup. It means that the meter is working but the shunt is too small a value i.e. its not causing enough current to run into the meter. Its shunting too much of the load current past the meter.

    The initial reading shows the high startup current of the motor. When the lightly loaded motor runs, the current drops. Adding load should raise the reading.

    To know which to use, you'd have to know what current the meter draws to indicate full-scale then use the proper shunt to divide up the total motor current, running enough through the meter to get the range you want. You can measure motor and meter current with a clamp on ammeter to get the ratio you want for the desired indication.

    Same with a current xfmr but for higher load currents where you would not want the shunt resistance in line.
     
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  10. MrChips

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    Oct 2, 2009
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    Do you have a DMM with an ohmmeter range? Measure the resistance of the ammeter without the shunt connected. Of course, do this with the ammeter disconnected from the power line.
     
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  11. debe

    Well-Known Member

    Sep 21, 2010
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    This looks simmilar to your meter & has a built in shunt, & the resistance across the meter is .1ohm & doesnt require an external shunt. Its an AC ammeter the wavy sighn above biro point indicates its for AC use. The other is just for interest & is a CT type ammeter, the CT is threaded onto one lead (cost on Ebay was $8 delivered)
     
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  12. nedriv

    Thread Starter New Member

    May 26, 2013
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    Mr Chips, I measured the resistance as you suggested and the reading was 0.7 ohms.

    Thanks
    Ed
     
    Last edited: May 27, 2013
  13. nedriv

    Thread Starter New Member

    May 26, 2013
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    Debe, thanks for the confirmation that my ammeter is in fact, an AC meter.
    I'm not sure I understand the current transformer setup, or if I need one.

    Thanks for your help.

    Ed
     
  14. MrChips

    Moderator

    Oct 2, 2009
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    I would say it is safe to test your ammeter without the shunt supplied.
     
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  15. nedriv

    Thread Starter New Member

    May 26, 2013
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    OK, I'll give it a try and let you know how it goes.

    Ed
     
  16. nedriv

    Thread Starter New Member

    May 26, 2013
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    OK, I hooked the ammeter in series with the neutral wire. When I turned on the motor, the needle jumped to about 1/8" above the 20A line, but then dropped back down to just 1/32" above zero. When I turned off the power to motor, the needle settled back to zero.

    I've read from other users that their same model sander pulls just over 10A with NO LOAD.

    Here's the specs from the owners manual for the motor:
    1-1/2 HP;TEFC; 3450 RPM; 110 volts; 60HZ; single phase; 14 AMP

    Attached is a pic of the motor label if that will help.

    Could there be something wrong with the motor?

    Thanks again for all your help.

    EDIT: I tried again and this time I put the sanding drum under a load. The needle on the ammeter moved up the scale as I applied more load to the drum. So maybe it's working? I just don't understand why the ammeter registers so small a reading (almost zero) when the motor is not under a load. Any ideas?

    Ed
     
    Last edited: May 27, 2013
  17. JohnInTX

    Moderator

    Jun 26, 2012
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    It sounds like its working.. At 120V, a rule of thumb for single phase motors is 10amp/HP, borne out by the ratings plate value of 14 full load amps. By full load it means that the motor is generating 1.5HP at the shaft so it fits the rule of thumb.

    What you are seeing when the motor turns on is the start up current which is always many times the no load current (which is why motor control contactors on larger motors have time delays built in).

    What you are seeing when you add load to the motor looks right as well, you just aren't loading it to the point of requiring the full 1.5 HP. Keep in mind that your scale is non-linear, 5A indication is much less needle movement than 10A.

    The trends look right.

    Its not doing much work just spinning the drum so it does not require much power (or current). If possible, borrow a clamp on ammeter to verify the installation.

    Are they sure about that? Sounds high for a 1.5HP motor installation to idle at 2/3 power... but what do I know 'bout sanders?

    Have fun.
     
    Last edited: May 28, 2013
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  18. Blackbull

    Well-Known Member

    Jul 26, 2008
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    If anyone is interested I have found the meaning of those symbols on the meter at post #1:
    First obviously AC or DC.
    Second, moving iron.
    Third, % class accuracy.
    The inverted T means the meter must be used with the dial in the vertical position.
    The star with a 2, the meter has been tested to over 500V. If a blank star it has been tested below 500V, and a zero means it hasn't been tested at all.
    Most meters need 1mA to go to Full Scale Deflection, and need FSD to measure their Internal Resistance. Determining the meter IR with a DMM will not be very successful; for one it will not produce enough current, and I read that an analogue meter will produce too much and destroy the meter.
    I put 5V through a 5k pot in series with the meter, and turned the pot down till the meter was at FSD. Then 5000 - Rpot = Rmeter. The resistance was about 400 ohms, the resulting meter/shunt were surprisingly accurate.
     
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  19. nedriv

    Thread Starter New Member

    May 26, 2013
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    Blackbull,
    Thanks for the info on the meaning of the symbols. I was wondering what they all meant. Interesting stuff.

    Regarding the requirement for vertical installation:
    I have the meter in horizontal position and find it is workable for my purpose. However, the meter doesn't react the way I expect. For instance, as I gradually increase load on the motor, I would expect the meter to register the load and climb to 14A in a linear fashion before the motor stalls. What happens is the meter steadily increases to about 10A. As I continue to gradually increase load, the meter doesn't seem to register the gradually increasing load until the meter suddenly spikes to 20+ amps and the motor stalls. As I increase the load, I would expect a more gradual climb to 14A (max load for the motor) before the drum motor stalls.

    Could this be because the meter is horizontal and not vertical?

    Thanks for you help.

    Ed V
     
  20. Blackbull

    Well-Known Member

    Jul 26, 2008
    70
    6
    Yep probably is. The spring and mechanism are very delicate and will sustain damage in a horizontal position.
     
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