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  • analog volt/amp panel meter questions.

    Hello Group.
    I am starting to build my RPC panel and have some old analog meters that I want to use but need help as to how they are wired into a control panel.
    Here is a link ( http://hoytmeter.com/images/catalog/...trial-web5.pdf ) to the meters I have.

    The model number is 584 for both the AMP meter and the VOLT meter and i'm just not sure how they are to be wired in.
    Do I need a current transformer for the amp's and for 3* do I need one for each phase? I also have a selector switch to look at each phase and each leg of the amp's and voltage, I would like to know how they are connected.

    I probably haven't given all the info needed so let it fly and I will try and get the answers as they come up.

    TX Mr. fixit for the family
    Chris

  • #2
    I would email them and ask for the wiring diagrams?
    https://www.flickr.com/photos/csprecision

    Comment


    • #3
      Such a simple answer I did't even think of doing that, just figured it was something I would have t figure out myself.
      TX Legendboy I will try on Monday.

      Chris

      Comment


      • #4
        The meters of type 584 seem to be "Ac repulsion", which is fine, they are for AC and so that's all right.

        For the current meter, the current is to pass through the meter. According to the data sheet, up to 50A, there is no external sensor required. Therefore, the wire in which the current is to be measured would be cut, and the ends connected to the two contact screws on the back of the meter. All the current in the wire passes through the meter (assuming the meter is of suitable maximum rating).

        The meter must have a scale rating which is appropriate for the intended current. If your maximum current is 10 amps, obviously the meter must be rated for at least 10 amps, or it may burn out. If the meter is rated for a lot more current, such as 50 amps, it may give a poor reading at 10A, as it will use only 20% of the full scale, and may be hard to read. So the best rating is larger than required , but the smallest amount larger which it is possible to obtain. A 12 or 15 amp meter would be good for 10 amps maximum, but as noted a 50 amp meter would be poor, and even a 25 amp meter would be less than ideal.

        Then again, if there is a chance of a high fault current, the meter is again in danger unless there is a suitable fuse to prevent large currents.

        For three phase that is a rather inconvenient setup, requiring either a very complicated selector switch to connect the meter in series with any of the wires, but keep the un-measured wires connected, or you need three meters.

        So in that case, it would be much more convenient to have three current transformers, and one meter, with a selector switch. The switch would SHORT the output of the current transformers not being used, and connect the meter to the output of the one on the wire being measured. The wire is generally passed through the center hole of the transformer, and need not be cut in any way. (Some transformers are "split core" so that they may be closed around long unbroken wires.)

        Most commonly, the meter is 5 amp, and the scale is set by the 'ratio" of the current transformer. A 20:1 transformer allows a 5 ampere meter to read 100amps full scale, for instance. However, it is less common to have a low ratio transformer, such as would be required to read say 10 or 20 amps with a 5 amp meter. That would need a 2:1 or 4:1 transformer, which is uncommon.

        In such cases, it is more common to use series resistors of low value, and a low voltage AC voltmeter to read the voltage across the resistor. Usually the resistor is made to have 50 millivolts, or 100 millivolts across it at its full current, and the meter reads that. The resistor is usually termed a "meter shunt", although it is connected in series with the load, just as the meter would have been. Shunts are available at more ratings than current transformers are, with as low as 10A for rated voltage.

        For that method, three "shunts" would be used, and the meter switched between them, so as to read across whichever is selected. The resistors and meter are all "hot". With the transformer, the meter circuit is not in contact with the wire, and is not "hot".

        The voltmeter is easier. It is simply connected across the circuit from one wire to the other, and reads the voltage directly. For these voltages, there is generally no need for extra parts.

        The meter can be switched with a simple two-pole switch to read any number of circuits one at a time, and 3 phase poses no problem.

        The matter of a suitable scale is the same, since a 500V meter would not read 120V with good readability. A 150V meter would read 120V well, and a 250V meter would read 208V or even 230V decently. For 240V you would probably want a 300V meter.

        Picture:

        These are two current transformers, and one shunt resistor. All are for 100A. The transformers, as you may be able to read, are 100:5 ratio, so a 5A meter would read 100A. The shunt is 100mV at 100A, but a 10A shunt would look about the same..

        The meter connects to the posts on the transformers, and the small screws on the shunt. Wire goes through the hole of transformers, but for the shunt, the wire is cut and an end connected to each of the large cap screws of the shunt. For AC there is no polarity for a meter, so the wire connections are arbitrary (except for power meter setups).

        Last edited by J Tiers; 06-09-2013, 01:50 AM.
        1601

        Keep eye on ball.
        Hashim Khan

        Comment


        • #5
          Wow, I got dizzy just trying to read J's response. And I am probably the longest winded guy here. I am sure all he said is correct, but probably a bit hard for an electrical novice to follow.

          If you can provide specific information, perhaps I could provide a specific wiring diagram for what you have and want to do. I hope this list does not get too long.

          1. What Voltage, current, and how many phases does your RPC use for the input power?
          2. What Voltage, current, and how many phases does your RPC have for the output power?
          3. Exactly what, of the above, do you want to meter?
          4. How many meters do you have?
          5. What are the individual Voltage and current scales of each of those meters? Include the values of the left and right hand sides of the scales.
          6. Confirm that these meters are for AC.
          7. I assume the frequency of the input power is 60 Hertz. What is the frequency of the output power?
          8. Do you want individual meters for each item that is metered or can some meters be switched?
          Paul A.

          Make it fit.
          You can't win and there is a penalty for trying!

          Comment


          • #6
            Hello Group and Paul,
            I have a 240v 60hz single phase service to my house. My RPC is a 15hp 3* 240v 60hz output. The meters were just something laying around at my old job and thought they would look neat in the main panel so they are realy just for show. I have a AMP meter that is 0-300amps AC (on the face is 5amp=FS) and a volt meter 0-300VAC with rotary phase and volt switches that would give a reading on all 3 phases and readings of voltage combinations L1,L2,L3, n1,n2,n3, L1L2,L2L3,L1L3
            I understood J's letter for the most part but would like a drawing as some of us work better from seeing things versus just reading .

            Paul I would greatly apprecieate something on how these would be wired in to the RPC. I am building a automated panel with a timer relay for the start cap and will have fuses for the idler motor and all three machines that i will be supporting. my drawing for the RPC is completed and well understood I just am not sure on how the meters and the switches incorperate into the big picture. Sorry I don't have a scanner to show my hand drawing of the RPC but it came from a design by a fellow, Matt Isserstedt in the metalwebbnews forum a great discription in my opinion.

            TX Mr fixit for the family
            Chris

            Comment


            • #7
              Well your info does change some things a bit.

              What will the actual loads be?

              You said the meters are mainly for decoration, but do you want to read input or output current? I know you will want output volts.

              The load will be, at single phase 230V, most likely about 50A to 75A. You never load an RPC to full output, and the RPC itself draws about 40% of full load current (at a horrible power factor). Assuming that, then the idler may draw around 20 to 25A no load, with another 15A for a 5HP load motor, and yet another 10A or so for the generated phase.

              That's a guess based on a 5HP max motor load.

              In that case, you would just squeak in at 50A, and your meter plus a 100:5 standard current transformer can be used. You'd need to change the dial marks to reflect 50A full scale instead of 300A.

              To get a 50A full scale, you would pass the wire through the transformer twice, which will change the 100A full scale to 50A. (two wires at 50A is a total of 100A through).

              The 300V voltmeter is fine.

              Picture wanted? Picture provided. It's a jpg of a hand sketch, since the site likes JPGs but not other file types. It's a little dim in a couple spots, I had to work on it to get the contrast this good.

              if your actual needs are different, some modification might be needed.



              If you want to read currents on the output, you need 3 current transformers, and we might need to change the number of turns through the hole.
              The switching for one meter and 3 CTs gets complicated, since you never "open" the meter side of a current transformer. It has to be shorted, or connected through a proper meter at all times when power is on. If that is not done, the voltage on the output terminals can get very large.
              Last edited by J Tiers; 06-09-2013, 10:11 PM.
              1601

              Keep eye on ball.
              Hashim Khan

              Comment


              • #8
                Hello Group and J Tiers,
                That is awsome TX for the drawing. I may just be asking for a lot here since this is just a RPC not rocketship launch controls. I think that just reading current on one leg of the none manufactured output of the RPC is just fine. I do have 3 loads that will be supported by this system, 1- 5hp 240v 3* lathe, 2- 10hp 240v 3* aircompressor, 3- 2hp 240v 3* milling machine. I know that I will not be running all at the same time so I think the 15hp idler should work for what I have. I will see about getting a current CT sized @ 100:5 if not too much money, otherwise I will just let it look good in the panel and just not hook it up, no great loss.

                TX Again
                Mr fixit for the family
                Chris

                Comment


                • #9
                  I was looking at this a bit tonight but have to get up early tomorrow morning so I must quit. I do have a partial wiring diagram and will post it later.

                  One thing I must mention after reading J's posts. He talks about switching the Voltmeter around the three phases. I would suggest this also, BUT there is one very important spec to look for when purchasing or finding this switch. Electronic and electrical switches, especially the rotary variety, come in two varieties: shorting and non-shorting. What this means if the wiper or moving contact in a rotary switch either makes contact with the next position BEFORE it looses contact with the present position or it breaks contact with the present position BEFORE it makes contact with the next one. The first type that makes contact before breaking contact is called a shorting switch because it shorts the two adjacent positions when it is rotated. This is good for a situation where you do not want the circuit broken, causing large jumps in the Voltage or current. But in this case, if you short the contacts in the Voltage measuring switch circuit, you will be shorting two of the three phases. The results would be spectacular arcs and probably a blown fuse or tripped breaker. You DEFINITELY want a non-shorting switch for this application.

                  If you employ J's suggestion for switching one current meter between the three phases, you would have to very carefully consider what happens in the time domain for the several steps in switching. The present phase would have to have a short switched in parallel with the meter, then the meter would be switched off of that phase. Only after it is completely disconnected from one phase can it be connected to another and finally, the short on that phase would need to be switched off. ALL of this switching would have to be done with contacts rated for the full, maximum current that your RPC can deliver. I suspect it would be far cheaper to buy three meters and just leave them in circuit all the time.

                  More later.
                  Paul A.

                  Make it fit.
                  You can't win and there is a penalty for trying!

                  Comment


                  • #10
                    Originally posted by Paul Alciatore View Post

                    One thing I must mention after reading J's posts. He talks about switching the Voltmeter around the three phases. I would suggest this also, BUT there is one very important spec to look for when purchasing or finding this switch. Electronic and electrical switches, especially the rotary variety, come in two varieties: shorting and non-shorting.
                    Absolutely. Most switches are in fact non-shorting. You MUST use non-shorting for the voltmeter. For the ammeter, the switch MUST short the secondary of all CTs which do not have a meter on them, and must NOT ever leave the secondary unshorted even for a "split second".

                    Originally posted by Paul Alciatore View Post
                    If you employ J's suggestion for switching one current meter between the three phases, you would have to very carefully consider what happens in the time domain for the several steps in switching. The present phase would have to have a short switched in parallel with the meter, then the meter would be switched off of that phase. Only after it is completely disconnected from one phase can it be connected to another and finally, the short on that phase would need to be switched off. ALL of this switching would have to be done with contacts rated for the full, maximum current that your RPC can deliver. I suspect it would be far cheaper to buy three meters and just leave them in circuit all the time.

                    More later.
                    Actually, while I agree the ammeter switch is *much* more trouble than it is worth, the ammeter switch does NOT need to be rated for full current. It only needs to be rated for the current it will see, which is 5A, the meter rating. The switch is connected in the secondary of the current transformer, which should never see much more than 5A. If you see it "pegged", you should shut down the unit, and won't be switching it around looking at currents while the smoke is let out of the RPC...... not to mention that your overcurrent protection should already shut it down.

                    For the current measurement, if one *did* want to switch between phases, it would be far simpler, and likely cheaper, to use 3 "meter shunts", and a meter designed for use with them, as I mentioned in a previous post. The meter could be switched between the shunts in the same way as the voltmeter, using a simple type of switch, no complicated shorting needed.

                    While we are making comments about safety, etc, the switch for the voltmeter should be rated for the 230V mains voltage, minimum, and it would be helpful to have a reasonable current rating, such as 1 amp, even though it won't need to carry that. The rating serves to guarantee that the switch is suitable for the voltage, and not overly fragile.
                    1601

                    Keep eye on ball.
                    Hashim Khan

                    Comment


                    • #11
                      It took me a while to get back to this, but here is my circuit diagram as I promised.



                      This is not the only possible way, there certainly are others and perhaps some of them would be better.

                      As J agreed, the switch for the Voltmeter must be non shorting and rated for the full Voltage. As for the current rating, almost any switch that is rated for 240 Volts will be at least one Amp. so there is little worry here. Actually Voltmeters draw, at most, only a few milliamps of current so it really is the Voltage rating of this switch that is important. And the non shorting spec. If adjacent contacts (phases) are shorted, even for a split second, you will see a lot of smoke and probably the arc as well.

                      On the current metering side, I had to actually draw the circuit before my mind would wrap around the complete problem. I drew two alternate ways to monitor current with only one meter. The first, and the one I would recommend, places the meter with an appropriate shunt, permanently in only one phase. This will not show a complete picture of what is happening if something goes wrong, but the meter will show the current for most applications where the three phases are balanced. I placed it on one of the input phase lines because this is where the heaviest current is likely to be as any single phase control circuits would likely be wired there.

                      If you want to monitor all three currents, I would strongly recommend three meters and three shunts, all permanently wired to one phase each. This avoids many problems.

                      I did include a circuit that allows switching a single meter between three shunts on the three phases. In this particular circuit, the switch also MUST be of the non shorting variety, for the same reasons as the Voltmeter switch is. However, this is not a garden variety switch. It also MUST be capable of carrying the full scale current of the meter, which is 5 Amps in this example. AND, it also MUST have very low Ohmic resistance in the contacts. The 5 Amp. Ammeter will have an internal resistance measured in tenths of an Ohm or less. The meter current passes through two switch contacts in a series manner so the resistance of the contacts is added to that internal resistance of the meter. So we have two times the switch resistance added to the effective value and causing an error in the meter reading. So, for example, in order to keep this error below 10 percent the switch resistance must be less than 5% of the meter's internal resistance. This is in the order of 5 mOhms or less. This is not a common switch and it is not going to be inexpensive. Three meters may be less expensive. And that was only to maintain a 10% error figure. And on top of that, this switch also must be rated for the full line Voltage.

                      I do not include a current transformer option. My understanding is that current transformers are usually designed to work with a specific meter and I do not know how to be sure that one that you would buy would work with your particular meter. Probably OK to match the meter's full scale reading and internal resistance, but I do not know. But even with the proper current transformer, you will probably be far better off to use three meters and three transformers if you need to monitor all three phases. From the sound of things you swap the current requirements of switching with shunts for the Voltage requirements of switching with current transformers. I would need to do more research on this topic before saying or recommending anything definite.
                      Paul A.

                      Make it fit.
                      You can't win and there is a penalty for trying!

                      Comment


                      • #12
                        I think 3 current transformers with individual burden (load) resistors and a switched voltmeter to read the resulting voltages would be easy enough. You could tie one end of all resistors together and just use a single pole switch.

                        In Paul's schematic, I feel that a separate RPC switch is not a good idea. I can't think of any reason for energizing L1 and L2 without the converter being on.
                        Don Young

                        Comment


                        • #13
                          You "can use" the meter shunts with a 5A meter, which means they take all the current which is above 5A. You need the shunt which is specifically made for that particular meter, assuming there actually is one. Most 5A meters are made for CTs.

                          However, another idea, and one which uses standard shunts, is to measure the voltage across the shunt, in which case you don't need a heavy current switch. Standard shunts have 50 or 100 millivolts "drop" across them for full current. You can then translate teh voltage to a current reading.

                          Usually, you use a milliamp AC meter, set up with a series resistor to calibrate it. That makes a low impedance voltmeter, and will work fine with standard switches. The small error due to the meter current is well below the accuracy of reading the dial.

                          However, if I had a 5A meter , I would surely use it with a current transformer (often available from surplus sources) and not try to use with a shunt. That goes double if it's an old meter and I might not ever find the right shunt, assuming any was ever made for it, which is not a guarantee.

                          For one thing, it is isolated from the mains. For another, you can change the effective scale by winding more than one turn through the hole.
                          1601

                          Keep eye on ball.
                          Hashim Khan

                          Comment


                          • #14
                            In a small shop you may need to use the same circuit for both three phase and single phase devices. The RPC switch would allow you to turn the third phase off when using a single phase machine on it. This would save any losses in the RPC and also prolong it's life. I did say it was optional. One, the other, both, neither: there are reasons for installing any combination of the two power switches I drew.



                            Originally posted by Don Young View Post
                            I think 3 current transformers with individual burden (load) resistors and a switched voltmeter to read the resulting voltages would be easy enough. You could tie one end of all resistors together and just use a single pole switch.

                            In Paul's schematic, I feel that a separate RPC switch is not a good idea. I can't think of any reason for energizing L1 and L2 without the converter being on.
                            J is likely right on the current transformer thing. I was trying to show only one way to do this and illustrate some of the concerns.
                            Last edited by Paul Alciatore; 06-13-2013, 02:38 AM.
                            Paul A.

                            Make it fit.
                            You can't win and there is a penalty for trying!

                            Comment


                            • #15
                              Originally posted by Don Young View Post
                              I think 3 current transformers with individual burden (load) resistors and a switched voltmeter to read the resulting voltages would be easy enough. You could tie one end of all resistors together and just use a single pole switch.
                              Yes, that is a very good method also, which takes good advantage of the isolation provided by the transformer. It has another excellent advantage also.*

                              However, the OP already HAS a 5A meter (with a 300A scale on it). So it isn't totally about how to do this in the ideal case, but rather a case of "how do I use this thing which I have?". For that the CT is the best method, since the 5A meter is the standard type used with a CT.

                              A little-considered point about CTs is that whatever the burden resistor you use is, it is "reflected to" and shows up in, the primary side (the wire you are measuring in). So if you had a 1 ohm burden resistor, and a 100:5 transformer, the primary side would show the effect of that 1 ohm resistor as a 0.05 ohm series resistance (just an easy example, the actual resistances will be lower). You might need to consider that effect in some instances.
                              .
                              .
                              * As for the resistor and voltmeter method, the other advantage is that the resistor value can be adjusted to compensate for the offset cause by the CT losses. It takes some energy to actually set up the magnetic field in the core, and that is subtracted from the meter reading, causing an offset in the reading. An adjustment to the burden resistor can minimize the error, although in many cases the error is somewhat non-linear at the low end of the scale.
                              1601

                              Keep eye on ball.
                              Hashim Khan

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