Announcement

Collapse
No announcement yet.

Hollow Grinder! Electrical question...

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Hollow Grinder! Electrical question...

    Ok, I scored an old hollow-grinder from a company not too far from here. It was used to make knife-blades in production.

    I too plan to make knife-blades in production... Acquiring this machine has been part of my overall "plan."

    Both grinding stones are driven by separate motors. Load the stone, flip the speed and both motors spin up.

    Problem is... I'm used to having an amp-meter or % Load meter to guide my setups when grinding.

    Motors are driven off of 220, there are two of them... no drive no regulators no controls. Just ON and OFF.


    So what's involved in wiring in amp-meters on something like this? Is this a case of opening the covers on the wiring box on each motor and attaching a few leads? Or are we talking epic-scale electroengineering?
    "The Administration does not support blowing up planets." --- Finally some SENSIBLE policy from the Gov!

  • #2
    GB,
    You should be able to find some AC ammeters from an online source, for cheap. If I have it pictured correctly, that machine has relatively small motors, so they'll draw light current, probably less than 10 amps. Wire the ammeters in one leg of each motor.

    You'll find both "direct" ammeters that need to be wired in series with a motor lead and "indirect" meters that use a Current Transformer. Again, that will measure just one leg, which will be physically routed through the coil.

    Comment


    • #3
      Ok I know a bit more than I did before, thanks for the information. I can use this to generate more useful questions, after dinner.
      "The Administration does not support blowing up planets." --- Finally some SENSIBLE policy from the Gov!

      Comment


      • #4
        There are two basic types of ammeters for this purpose. One is the in-line, which takes one line of the electrical circuit in and passes it out to its normal connection, sending a signal out to a voltmeter. The other type is a wrap-around which clamps around one leg and sends a signal to a voltmeter. The wrap-around can be had as just a lump with a hole, through which the main line passes, with a pair of leads to wire to the voltmeter.

        The in-line is usually the more expensive of the two types, as it has to be able to pass the entire full load current. The wrap-around type can be fabricated with an inexpensive multimeter (in your case, two such.) The multimeter will have a removable clamp, which you then wire to the meter. The one I used for several years, when I was installing center-pivot irrigation machines and heavy-duty water pumps was a Triplett. They no longer build the one with the detatchable clamp on head, such as I had.

        When we needed a meter in a control panel for a number of pumps, I made the pick-up coils by wrapping a few turns of wire around one lead, connecting the two ends to a bridge rectifier and that off to a panel meter. We didn't need to have a reading of true amps, but only a relative reading of what current was being used compared to what it was when installed new. I installed one for each pump (7 ea. 1K horsepower, 3 ea. 500 horsepower, 1 ea. 250 horsepower and 3 ea. 100 horse power.) I also attached one to each of the 1330 VAC, 3-phase lines, to monitor balance among the lines. I got teased a bit when the power company electricians came around, but they had to admit that it worked.

        I'm reasonably sure you can find ready made instruments which do the same, without the extra thought, design and fabrication you might have to do with a home-brew. I have no idea, at this time, of the cost differential.

        This is a real quick'n'dirty look at what I'm talking about:
        http://bluesea.com/files/resources/i...tions/9975.pdf

        Pops

        Comment


        • #5
          If you use the current transformer approach, you can run a lead from each motor through the doughnut, and the reading on the meter will be the sum of the two currents (provided you don't have one running in the opposite direction of the other, in which case you'll get the difference of the two).

          With the in-line approach, an ammeter in the line before it splits to the two motors will show total current for both.
          For just a little more, you can do it yourself!

          Comment


          • #6
            Originally posted by ed_h
            If you use the current transformer approach, you can run a lead from each motor through the doughnut, and the reading on the meter will be the sum of the two currents (provided you don't have one running in the opposite direction of the other, in which case you'll get the difference of the two).

            With the in-line approach, an ammeter in the line before it splits to the two motors will show total current for both.
            The electricity in the wire doesn't know which way a motor is turning, so you will get the sum of current.
            Amount of experience is in direct proportion to the value of broken equipment.

            Comment


            • #7
              Sorry, I didn't express that very clearly. I meant if one wire is running through the core "wrong way", it's field will be out of phase with the other wire and cancel.
              Last edited by ed_h; 04-19-2012, 02:16 AM.
              For just a little more, you can do it yourself!

              Comment


              • #8
                It's always the sum, but one of the numbers may be negative if the wire is reversed...... which would effectively "subtract".

                So BOTH of you are correct.

                The problem is if you need to know what each is drawing, and not just the sum (perhaps for an overload sensor, load meter, etc). Then with both wires through a single current transformer ("CT"), the current variations in one wire may be masked by a larger current in the other. You cannot separate them unless you know one current, in which case you don't need it in the CT to begin with

                If , as in this case, you figure the motor currents are related, you can put both through, but your uncertainty of current in either individual motor will be approximately the difference between full load and idle current. You won't be able to separate the loads on each,. you only know the total. So one at full load and the other idling, could look similar to both at a decent load, you can't tell the difference.

                If you NEED to know the difference, two meters are needed.

                Or, you can listen to the darn thing and see if it SOUNDS heavily loaded.... or watch the spark volume, etc, etc.
                Last edited by J Tiers; 04-19-2012, 09:35 AM.
                1601

                Keep eye on ball.
                Hashim Khan

                Comment


                • #9
                  its AC so no direction it goes pos and neg

                  with DC it matters think of kircoffs right hand rule


                  Stuart

                  Comment


                  • #10
                    GB,
                    I did a quick search on Bing for "AC Ammeter" and had a whole bunch of suitable sites pop up. Vendors on eBay were represented, as were many others. Prices for instruments that will work for you seemed to be less than $50.00; you need 2, of course.

                    You'll probably find the "direct" type a bit cheaper, and they are perfectly fine for your application. The "indirect" sorts are usually used on larger motors, say above 5Hp.

                    Comment


                    • #11
                      Armedandsafe brings up a valid point. Do you need to know actual amps, or just a relative number? If just a relative number, wraps of wire around one leg going to a couple of Horrible Freight volt meters should do it. The more wraps, the more sensitive, just make sure both setups have the same number of wraps.
                      Definition: Racecar - a device that turns money into noise.

                      Comment


                      • #12
                        Originally posted by Blackadder
                        its AC so no direction it goes pos and neg

                        with DC it matters think of kircoffs right hand rule


                        Stuart
                        That response is exactly why I have about quit responding to electrical
                        questions. :-)
                        Blackadder, If you run the wire through the pickup and then bring it back through the opposite direction do you get twice the reading?
                        Sheeh.
                        ...Lew...

                        Comment


                        • #13
                          Originally posted by Lew Hartswick
                          That response is exactly why I have about quit responding to electrical
                          questions. :-)
                          Blackadder, If you run the wire through the pickup and then bring it back through the opposite direction do you get twice the reading?
                          Sheeh.
                          ...Lew...
                          no if it dc nill reading if ac twice

                          fo ac are you talking about a CT then if AC the twice

                          Stuart

                          Comment


                          • #14
                            WAY more detail than I need, guys!

                            Thanks for the information, I found a pair of indirect meters that I can wire in and mount in the "dashboard" that I am constructing.

                            Amps, hydro pressure, air pressure, coolant temp, and hydro temp. All information that is very important to me when I'm trying to repeat a setup on precision grinds.

                            Often time its .01% science and the rest being feel, sound and luck. With the numbers from the dashboard I'm hoping to make a much more repeatable process without buying excessively complex modern machinery.
                            "The Administration does not support blowing up planets." --- Finally some SENSIBLE policy from the Gov!

                            Comment

                            Working...
                            X