Extending wires between Current Transformer and Panel Meeter help

Discussion in 'The Projects Forum' started by diebog, Mar 22, 2015.

  1. diebog

    Thread Starter Member

    Mar 23, 2013
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    Hi everyone, I am working on a bridge saw that uses a CT which sends a signal through 2 wires to a AMP panel meter. I recently moved the control panel it is located in of the moving bridge and mounted it stationary to the floor via a steel 8x8 post. So I had to extend all the wires quite a bit to reach the cable wire chain track and then through the chain track and into the main control cabinet where all the relays, switches, computers, controllers and where the CT is located.

    The 2 wires that originally sent the signal from the CT to the panel meter were about 11-12 feet in length and around 20-22 AWG. I extended the 35 wires from this control panel to the cabinet around 50 feet with this wire. http://www.zoro.com/i/G1095306/

    So basically I went from 11 feet of 22 AWG to 50 feet of 18AWG and now the panel meeter reads way way low. I used my fluke amp meeter and around the same wire which passes through the CT and I get about 56 amps right at start up there but the panel meter shows only 20 or so amps. Then when its just running with no load the fluke showed about 7-8 amps and the panel meeter was on zero. I tried adjusting the screw on the front of the meter but that I found was just to hold the needle "zero" to the face place.

    I realize my issue was extending the length of the wire which caused more resistance and therefore lowers the signal by the time it gets to the meter. I tried adding 2 more loops through the CT (it had 3 loops originally) This elevated the readout a tiny bit but its not reading near what it should. I read a little on this site about https://electricalnotes.wordpress.com/2011/04/16/current-transformer/ Current Transformers but there is just so much info that is way over my head and would take me years to fully understand. But I did get a few Ideas on how to correct this. One way was to take the white secondary output wire that runs to the meeter and wrap it a certain amount of turns in the correct direction to get the right ratio so the meeter reads correctly. But the type and shielded/non shielded wire all changes the math on whats needed so I am lost. There was also a part that mentioned adding a second smaller CT to the equation to get the needed ratio, which again is over my head.

    The CT I have has allot of Chinese symbols and some numbers all which I goggled but didn't really get anywhere except for the "LM-0.5" and the "50/5" (50 is stamped in the metal) You can see this in these pictures. (there is also a picture of the panel meeter) http://s311.photobucket.com/user/diebog/library/Bridge saw

    Only thing I know is the 50/5 is the ratio of the CT. I think the 0.5 is the accuracy? There is a "7" stamped towards the right in the picture as well as the number 200 in the middle of the pic that is printed vs stamped inside the box. So I am hoping someone could perhaps tell me what the others mean? I don't even know if its important I understand them, I just need to fix the meter not reading correctly.

    So hopefully I gave all the info needed but If I missed something, just let me know. Id appreciate any and all's suggestion on how I can fix this current/signal loss going to my meter. Do I need to buy a better meter? A better or different size CT to accurately show the correct amps at the end of the 18AWG wire in the control panel? Something else?

    Another tid bit of info I thought I would share, before I reconfigured the control panel on this saw the meter worked as it should. I had a mark on the 20 amp line where the manufacture of the saw said to not go over while cutting (some granite/marble/quartz cut slower or faster which effect the amps) so you just cant always go by the same speed you use on the speed control. There are some stones out there that even with the best diamond blade made the stone is so hard and dense that it has to be cut so slow because the amps just jump right up. Other stones I can cut 4 times as fast as others and the amps barely go up. So this is something I use on a daily basis and needs to be as accurate as possible. The motor it is reading is a 3 phase 30 HP electric motor. In case your curious, here is a link to a similar saw, in fact you can see how the control panel is mounted on the bridge which moves back and fourth which is real annoying and hits you in the head constantly. http://www.ebay.com/itm/NEW-Bridge-...471?pt=LH_DefaultDomain_0&hash=item2303a8d48f

    It is so much nicer now that the control panel does not move with the bridge, I just gotta fix the amp signal issue.

    I know my post is somewhat lengthily which I apologize for, I was just hoping to answer any questions that may arise. And again, I welcome and appreciate any thoughts or suggestions you may have.

    Thanks,

    Jeremy
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    Yep, it sounds like you are loosing a lot of voltage in the IR drop of the extra wire. If you had used AWG #15 the resistance would be about the same on ;your 11 foot run of #22, but you already bought and installed the cable.

    From what you wrote and the photographs of the current transformer, it looks like the shunt is in the meter. If that is the case, you might obtain a more accurate reading by placing the shunt at the current transformer and using a voltmeter, or the current meter with its internal shunt removed, as the indicator. That would reduce the load on the wires and it should cause the voltage to increase.

    Of course the simplest solution is to do nothing and live with the (probably annoying) mis-calibration.
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Hum, that's weird. With a higher load resistance, i.e. additional resistance from extended wire, the CT should maintain the current ratio. This means the measured voltage directly on the CT output would be higher.

    The shunt resistor in the meter should then still read the same value, independently of the wire length. That's (in theory) is the good about the CT, it puts out current , not voltage.

    Question: disconnecting the CT, what resistance do you measure at the meter input?
     
  4. crutschow

    Expert

    Mar 14, 2008
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    I agree with praondevou. A current transformer is a current output device and a slightly higher resistance in the load should have little effect on that current, so the problem you are seeing is very strange.
    It sounds like there is a much higher resistance in the circuit then you'd expect from the added wire. Measure the resistance of the meter at the shunt with the shunt disconnected and directly at the meter, and see what the difference is.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Maybe I am missing something, but if you moved the panel from the bridge to the floor, I can see extending some cable such as power etc, but the way I see it you should not have to extend the CT wires if you move the CT to the new enclosure and monitor the motor power source there together with the meter.
    Or is this because you need to keep the meter on the gantry pendant ?
    The other solution is to try one of the Digital readouts, they can be had for $10.00 C/W C.T
    Max.
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    It is possible that a CT burden may become so high that the core saturates. Then errors are inevitable. That would seem unusual (counter intuitive) in this situation. It might also be worth measuring the CT terminal voltage and other voltages around the secondary loop to check that everything is consistent & explicable.
     
  7. diebog

    Thread Starter Member

    Mar 23, 2013
    195
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    The meter disconnected reads continuity. The digital mufti meter I have starts to read a slight resistance but goes down to zero. Same result if the two DMM prongs were touched together. Is this normal for a meter? Or should I be reading some sort of resistance from these the studs on the back of the panel meter? If there should be, I have no clue what changed as all I did was extend the wires and only loosened the nuts to remove wires to attach new wires. So what could I have messed up that would of changed the reading so much? And as I said above, I don't see any shunt/resistor inside this meter. Maybe its down inside where the 2 copper wires go into?

    Sorry if I wrote that confusingly, but there is a control panel where the controls and computer inputs are and then there is a main cabinet that houses all the relays, breakers, motor controllers, the CT, etc. This stayed on the bridge and moves back and fourth with the bridge. The control panel is what I relocated onto a metal 8x8 post so it did not move back and fourth with the bridge. I will take some pictures in the next few days to show you.

    So the CT in mounted inside the cabinet on the bridge and the meter is on the control panel that I moved which created the 50 foot length. As far as a good digital panel meter, what would be a good choice or place to buy from?


    I will check the voltages tomorrow and report back. What should the voltages be? Any pointers or suggestions would be much appreciated. Im not a total novice on electronics, but just want to make sure I am measuring at the correct places so I don't report back with incorrect values. I was thinking ground to K1, ground to K2. K1 to K2, (With panel meter disconnected) and then check the voltage on the wire that is running through the CT. There are 3 hots obviously, is it possible one leg is lower then another? I remember back before I had actual 3 phase power in my shop I had a 45 horse phase converter to generate that 3rd leg and we had to switch 2 of the legs to get the motor to turn in the correct direction and each leg was a little bit different then the others. But this was with a converter which is out of the picture now. Just a thought I had and figured I would ask to see if it might help running a different leg of the 3 phase through the CT.
     
  8. diebog

    Thread Starter Member

    Mar 23, 2013
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    Well there was no shunt that I removed from the CT to the meter, just the 2 wires labeled "K1" and "K2". I took some pictures of the inside of the meter which you can see here:http://s311.photobucket.com/user/diebog/library/Bridge saw/Saw Panel Meter The two studs with the nuts go directly into the windings inside this meter as you can see in the pictures. So is there supposed to be a shunt inline here somewhere? When I was scouting eBay for replacement meters it seemed like it was either a meter with a shunt or a meter with a CT, not with both. I don't understand what one does vs the other, this is just the way the manufacture set it up.
     
  9. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    Ok , there is no shunt. The output from the CT then goes directly through the coil of the meter, right? The meter wire resistance is very low that's why you can't measure resistance. The multimeter probably changes slowly because the measured signal is filtered. depends on the multimeter model. I have one that does the same, quite annoying.

    I wonder if the wire inductance has something to do with it. The motor current is 60Hz? Or is it driven from a VFD?
     
  10. ebeowulf17

    Active Member

    Aug 12, 2014
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    This may sound silly, but if the meter is fairly easy to remove and temporarily move (as it appears to be from your pictures,) would it be worth temporarily hooking it up loose with the original cables just to make sure that nothing else has changed? (I don't mean re-wiring everything of course, just the two wires to the meter.)

    We know nothing was deliberately changed, but any time you're modifying 35 wires, there's some small chance of mixing wires up, shorting wires, nicking insulation, making loose connections, or even damaging the meter if it got bumped, tugged, etc.

    If it definitely works like before when you simply change the two wires, that eliminates the other possible errors. Also, if that did the trick, you could try the 50ft of #15 wire mentioned above as having equivalent resistance. If that worked too, it might be worth the effort to re-run just those wires.
     
  11. Reloadron

    Active Member

    Jan 15, 2015
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    I suggest you read this, and then this. The lead resistance becomes critical when using a current transformer. A Google of Current Transformer Lead Length will bring up a dozen hits on the subject. The links I provided will show you seberal charts and formulas used to determine lead length. When you start getting into long runs as you just did the best method is to use a current transducer similar to this. Quality current transducers are the best way to go when you start having distances like what you are talking about. Current transformers like those you have were not designed or intended for long lead length. Then generally feed a meter in the same panel they are measuring current in.

    Ron
     
    praondevou and #12 like this.
  12. diebog

    Thread Starter Member

    Mar 23, 2013
    195
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    Yes the output from the CT goes directly through the coil of the meter. Ill see if I have a decent analog MM and I will test it again. And the motor current is 60Hz I believe. Its a single speed motor.

    Tha'ts some good ideas. Ill try that tomorrow if I get time. Hopefully it should rule a few things out. Im pretty sure all the wires are wired correctly because they all shrink markers on them and I just started with one wire at at time, removed it, wrote the number down and corresponding wire number in the 36 wire cable which were numbered 1 through 36 which made it easy to hook up at the other end. But there is still a chance of error, no ones perfect right?

    Thanks for those links I checked them out and I think both "this" links are the same thing. I read the one anyway. I understand a little bit more now about what could be causing it. I like the Current Transducer Idea. I think that would be a good fix if indeed it is the wire length causing this issue. It seems fairly simple to hook up, the computers are all 24v as well so I could just tap into the 24v output on the power supply board. So if I have to end up going this route, will I need a different gauge/meter? Or will this one still work? And where is the best place to buy a quality Current Transducer? Digikey, or Mouser?
     
  13. Reloadron

    Active Member

    Jan 15, 2015
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    I had a bad feeling I screwed up those links. Sorry as I intended a few links. A Google of Current Transformer Lead Length will bring up plenty.

    As to the current transducers I mentioned. I used them mostly to input a signal to data acquisition cards for computers. They come in all ranges and output several outputs like 4 to 20 mA, 0 to 5 Volts and the list goes on. I would generally buy direct from CR Magnetics but a dozen companies manufacture and distribute them. The merit to using them was for me I could monitor pump currents on pumps hundreds of feet away and set my current limits. They can also drive analog panel meters. You can also pass "loops" through them just like a regular CT. For example you have a 50:5 CT and you loop the primary 2X you now have a 25:5 CT. While they do make True RMS versions in most cases the standard versions work just fine. I see Digikey carries the CR4200 series from CR Magnetics. Also note that on units with a 4 to 20 mA output placing a 500 Ohm resistor in the current loop gives you 2 to 10 volts a 250 ohm resistor yields 1 to 5 volts.

    Anyway, something for you to read about and think about.

    Ron
     
  14. diebog

    Thread Starter Member

    Mar 23, 2013
    195
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    So today I just used 2 short pieces of wire and hooked up the meter inside the cabinet where the CT is. And sure enough it reads correctly again. So its definitely the extra wire causing the issue.

    A few measurements I took, the CT put out .810mv while the saw motor was running. On the three legs I got 208v, 209v, 212v. The wire with 209v is the one running through the CT.

    Here are so more pictures of the saw for those who were confused on what I was doing. This is the panel in its new position on top of the 8x8 post. http://s311.photobucket.com/user/diebog/library/Bridge saw/Saw pics
     
  15. diebog

    Thread Starter Member

    Mar 23, 2013
    195
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    So I guess I am going to need a transducer. I found this one one ebay for a good price, do you think it would work? Its also a true RMS http://www.ebay.com/itm/CR-Magnetics-True-RMS-AC-Current-Transducer-CR4120-40-New-/360868125884?pt=LH_DefaultDomain_0&hash=item54056a9cbc Here is the info on it http://www.digikey.com/product-search/en?vendor=0&keywords=CR4120-40

    Here are some others on ebay in case that one doesn't work. http://www.ebay.com/sch/i.html?_odk...=+CR+Magnetics+transducer&ghostText=&_sacat=0
     
  16. ebeowulf17

    Active Member

    Aug 12, 2014
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    I'm not up on the math/physics enough to say this with confidence, but if you used large enough wire for just the two wires in question, I think you'd get the resistance (and corresponding voltage drop) low enough to be a non-issue again.

    I know AWG #15 was mentioned above, but just for the sake of experimentation you could probably try even larger diameter wire if you have some on hand. I'm betting if you connected 50' of #12 or #14 wire you'd be back in business (again as a quick experiment with the meter nearby so you don't have to run cables permanently until you know it works - just coil the 50ft temporarily and see if it behaves as expected.)

    I suppose there's some risk of actually making the readings on the meter too high if you manage to get the wire resistance too low. Either way the experiment would verify whether you can simply replace some wires and make the existing hardware work or if you'll need some new parts.

    Of course this information is only useful if it would be feasible to then replace those wires with #15 or larger wire. If that's not practical because of where and how the wires get run, then ignore these comments!

    Also, found this handy calculator in case you haven't already found one:
    https://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table
     
  17. Reloadron

    Active Member

    Jan 15, 2015
    963
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    Sounds like you have a 3 phase motor and they monitor a single phase current which is not unusual at all. For some reason I want to believe your original CT is a 50/5? Meaning with 50 Amps through the primary (the single wire through the hole) you would get 5 Amps out of the secondary when properly terminated into a burden resistance. Your analog meter serves as a burden resistance. You never want to run a CT into an open circuit. The CT needs to see a load on the secondary which we call the burden resistance. Think of it this way. A CT with a 50/5 is a transformer with a 10 to 1 ratio correct? The primary line through the doughnut hole is current but also a voltage. So if the voltage is let's say 200 volts then the CT secondary, open circuit, would have 2,000 volts without a burden resistance. My guess (and only a guess) is with the system shut down if you measure the input resistance to the current meter it will be about 0.200 Ohms. That is with nothing connected to the terminals, only your ohmmeter. So with 50 amps primary and 5 amps secondary through 0.2 Ohms the voltage will be 1.0 volt. The meter is likely a 0 to 1 volt meter scaled to read 0 to 50 Amps. The meter has an internal "shunt" which serves as a burden resistor.

    The problem with extending the leads of a CT are multiple. Most CTs like what you have are designed to work into a very low burden resistance. It doesn't take much lead length to upset the balance of the burden resistance and cause large errors. Additionally there is a matter of lead length contributing to causing the transformer to go into saturation. Something I have not mentioned is if you were to terminate the CT secondary at the source (in the panel or at the CT location) you may be able to run a light gauge twisted pair to a meter but the meter now would need a high input impedance.

    Here are your options with the current transducer. You are looking at a 0 to 40 Amp AC transducer. The output will be 4 to 20 mA on that model. So 0 to 40 Amps = 4 to 20 mA. You place a resistor (a good 1% resistor) in the current loop. Using a 500 Ohm resistor the voltage drop across the resistor will be 4 to 20 mA = 2 to 10 Volts and you get a scalable panel meter to read 2 to 10 volts = 0 to 40 Amps. That is how it is done.

    Ron
     
  18. MaxHeadRoom

    Expert

    Jul 18, 2013
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    In N.A. there is Red Lion Controls or the $10.00 version on ebay.
    Max.
     
  19. BR-549

    Well-Known Member

    Sep 22, 2013
    1,980
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    Your CT cable is a twisted pair, right?
     
  20. Reloadron

    Active Member

    Jan 15, 2015
    963
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    As to a scalable programmable panel meter? I agree with Max, a Red Lion Like This One will do fine. There is Red Lion as well as a dozen others. I can't speak for the Ebay stuff from China, as I never used them. I have used Omega and others.

    Ron
     
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