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  1. #1
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    Default Single phase power monitor

    Since I'm going to be stretching the friendship with my power supply when I get the new lathe going, I thought it would be a good idea to have some sort of power/current monitor in the shed. Needs to handle roughly 50A

    I'm mostly interested in the total current draw at the shed switchboard, but it would be nice to have that info available remotely somehow rather than having to physically go to the switchboard to check.
    If I was bored for a project I'd probably set something up with an Arduino and remote display or similar. Although it would be fun, I have plenty of projects already - definitely don't need another one!!

    Any suggestions?

    Steve

  2. #2
    BobL is offline Member: Blue and white apron brigade
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    I know what you mean and have thought about setting up something like you describe but every time it comes up I seem to move on mainly because when I'm doing something in the shed, like welding, I can't usually be simultaneously watching a power meter.

    On a several occasions in the past I have got someone to watch the V/I meter on the switchboard while I perform a task, but more recently I just video the shed power meter using a mobile on a tripod I keep in the shed.

    Some of my machines like the WW BS below have their own V/I meters.
    The other machines that have this are the compressor and the Dust Extractor.
    Electronics2.jpg

    I also have two portable V/I meters.

    This one has a 10A plug, a twin 15A GPO outlet and a 10A breaker.
    PB1.jpg

    This one has a 15A plug, a twin 15A GPO outlet and a 20A breaker.
    It also has a power factor display.
    PB2c.jpg

  3. #3
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    Thanks Bob. Videoing the meter is a practical solution to capture whats happening.
    Those meters that you've used, are they anything special or just whatever stuff you can find on ebay etc?

    Steve

  4. #4
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by OxxAndBert View Post
    Thanks Bob. Videoing the meter is a practical solution to capture whats happening.
    Those meters that you've used, are they anything special or just whatever stuff you can find on ebay etc?
    They are all from ebay but there are some differences.

    All 3 of the power meters in the photo are plugged into the same outlets.
    A Digital multimeter in the same socket reported 249.6V
    Meters.JPG

    The other 3 B&W LCD power meters in the shed all read 249V (nothings drawing power )
    The other green and red panel on the Band Saw read 258V

    The B&W LCDs tend to be more accurate but over a few years they fade and I have replaced both of mine after about 5 years.

    The red and green display ones are really cheap and I've only had them for a couple of years but noticed teh one on the bandsaw is starting to consistently read the Voltage too high.

  5. #5
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    We got one of these included in our solar power system, it might do what you want - a current transformer is placed on one of the AC power leads and through a battery powered dongle transmits to a 'base station' with a LCD display - the display then give a reading in Kw of the power going through the the AC cable. For a solar system 2 current transformers can be used so the base station can work out how much energy is being fed back into the grid - in your case only one current transformer needs to be used as you are only monitoring the power being used in the shed or lathe - the current transformers simply clip on to one of the power cables either active or neutral much like an AC clamp meter, the base station is wifi enabled so you can download an app and monitor remotely via the internet on a mob phone, our transmitter dongle sits in the meter box and I have had the base station up to 30 meters away, at present it is only around 10 meters away, there are other brands available.


    https://www.ebay.com.au/itm/33327184...frcectupt=true

  6. #6
    BobL is offline Member: Blue and white apron brigade
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    Minor point, but because breakers and wiring are rated in terms of current, that is what I prefer to see on display rather than the power.

  7. #7
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    If it's just AC amps you need to check then a digital clamp meter will do it - the fancy ones have peak hold and even connect to a mobile phone for remote monitoring.

  8. #8
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by familyguy View Post
    If it's just AC amps you need to check then a digital clamp meter will do it - the fancy ones have peak hold and even connect to a mobile phone for remote monitoring.
    I have two current clamps but to use them they must be clamped around the active or neutral which can be a PITA. To make things easier I made up a short split cord extension cord to plug in line between an outlet and the machine, but some machine's plug/socket are not well placed to be able to see the meter easily while operating which is when I want to see the current. For about $10ea those V/I panel meters can be permanently mounted on a machine in an easily/safe readable location.

    As far a using a mobile in a workshop goes, while I use a number of mobile Apps (including SPL, light intensity, vibration, air flow, calculator) in the workshop, I generally don't like using a mobile phone in there especially while actually operating a machine so I only do this on occasional basis. The main issue I have are where to put it safely while using the machine and then remembering where I put it. To get around this I sometimes use my home mADE mobile cradle mounted on a photographic tripod but doing this for a quick current measurement is a PITA.

    MkIIV.jpg

    But given how cramped and cluttered my shed is, I don't want to be doing this on a regular basis. I really dislike the general fiddling involved with using a mobile, especially in a workshop, stopping to find the effing phone, has it got enough charge, fingerprint password not working due to dirty hands, getting the thing dirty etc.

  9. #9
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    I recently got one of these:
    https://stuff.iotawatt.com/product/a...v=6cc98ba2045f

    And a bunch of their 50A CTs and went to town on our switchboard, 3 CTs for the house, 3 for the workshop, one for the offpeak hot water and one for the solar.

    The goal is to try and optimise the use of solar generation using automation to switch on loads when there's excess power being exported to the grid.

    The unit has inbuilt graphing, and you see individual loads show up on the different phases, which, among other things, is useful for seeing if the phases are reasonably balanced - for example, I've made sure my plasma cutter is on a different phase to the air compressor.

  10. #10
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    Thanks Rusty - thatís interesting. Looks like a very flexible piece of kit, and the price isnít over the top. Iím a sucker for relatively open systems too

    Iíve got a couple of cheap VI meters and transformers ordered and in the post somewhere which will get me started - but the IoTaWatt is the sort of thing I was originally thinking of.

    Steve

  11. #11
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    Hi Steve, Guys,

    A real easy way is to simply measure the voltage drop across the feed cable !

    Since a volt meter (Multimeter) draws essentially no current a suitably insulated thin wire could be used to connect the meter to either end and would be quite adequate if not particularly practical. Of course you would have to know the length and resistance of the feed cable, but that would give you an instant value as the load changed.

    A current transformer would do the same thing but also requires a load resistance to measure across and be safe. You would only put one on each feed wire in a three phase system or just the live wire in a single phase system. The advantage is that it can be put at either end or anywhere along the feed wire and the reading will be exactly the same.

    Does the VFD have the ability to display load current ?
    Best Regards:
    Baron J.

  12. #12
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by BaronJ View Post
    Hi Steve, Guys,

    A real easy way is to simply measure the voltage drop across the feed cable !

    Since a volt meter (Multimeter) draws essentially no current a suitably insulated thin wire could be used to connect the meter to either end and would be quite adequate if not particularly practical. Of course you would have to know the length and resistance of the feed cable, but that would give you an instant value as the load changed.
    Not sure I would call it easy, especially if it was required to be left insitu

    I have experimented this method and the main problem especially with short power leads in decent condition is the lead resistance is generally too low for basic multimeters to measure.

    A 1m length of 1.75 mm diameter Cu wire has a resistance of about 0.006Ω.
    For a 10A current and a 3m length of cable that's a V drop across the cable of ~0.2V.
    Most basic basic meters would measure 250V to no better than 0.1V so at a minimum there's a +/- of 50%
    The other issue when accurately measuring the resistance of low resistance cables is contact resistance.
    Unless screwed down terminals are use just poking a couple of meter probes at the bare cable is unlikely to produce accurate resistance measurements.

    To get semi consistent results I ended up using longer (eg >10M) cables.
    To do this accurately the cable has to be uncoiled or the inductance effect will dominate the voltage drop.
    This meant I ended up with lengths of cable all over the floor
    Using thinner cables is a possibility but that has its own problems.

    Going back to the contact resistance problem to further complicate things, when I tried it, I inserted "thru" plug at both ends of the cable to act as meter test points but the contact resistance of the plugs was not consistent until i polished them with some steel wool, Plugs are easy to polish but sockets are not.

    It was after mucking about with these methods I resorted to cheap panel meters.

  13. #13
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    Default

    BobL you sound like you are on the ball with all this, not sure if you are aware but while Chinese AC digital panel meters are cheap and easy to use they can be non linear, most seem to be based on a 200mV ac meter that has had shunts and or multipliers added so larger voltages and currents can be measured. I was going to use one in a project and decided to check out the linearity when compared to my Fluke 87, it was accurate enough from 200mV down to 20 mV when the meter started to read under, getting worse as the voltage dropped - at around 10mV the error was almost 30%, there are probably instances where this doesn't matter as the voltage/current being monitored never drops that low, but in my case it was a no go.
    I suspect the error comes from the panel meters using a precision rectifier to do the AC to DC conversion rather than a more expensive AC to DC true RMS dedicated chip, there probably are AC digital panel meters out there that are linear but I doubt they are the ones that abound on ebay for $15 or $20.

  14. #14
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    Quote Originally Posted by BaronJ View Post
    Hi Steve, Guys,

    A real easy way is to simply measure the voltage drop across the feed cable !

    Since a volt meter (Multimeter) draws essentially no current a suitably insulated thin wire could be used to connect the meter to either end and would be quite adequate if not particularly practical. Of course you would have to know the length and resistance of the feed cable, but that would give you an instant value as the load changed.

    A current transformer would do the same thing but also requires a load resistance to measure across and be safe. You would only put one on each feed wire in a three phase system or just the live wire in a single phase system. The advantage is that it can be put at either end or anywhere along the feed wire and the reading will be exactly the same.

    Does the VFD have the ability to display load current ?
    Thanks John.
    There's actually a shunt in the DSG lathe that's intended to run a ammeter, and the VFD can display current on its panel (manual says its output current but I'm not convinced). Likely its available to read via RS485 too.
    So its not that I don't have methods of getting the data, but looking for a more convenient way to monitor and display.

    Steve

  15. #15
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by familyguy View Post
    BobL you sound like you are on the ball with all this, not sure if you are aware but while Chinese AC digital panel meters are cheap and easy to use they can be non linear, most seem to be based on a 200mV ac meter that has had shunts and or multipliers added so larger voltages and currents can be measured. I was going to use one in a project and decided to check out the linearity when compared to my Fluke 87, it was accurate enough from 200mV down to 20 mV when the meter started to read under, getting worse as the voltage dropped - at around 10mV the error was almost 30%, there are probably instances where this doesn't matter as the voltage/current being monitored never drops that low, but in my case it was a no go.
    Yes that is more or less what I see as well but it does vary from meter to meter.

    Here are a few readings I quickly cobbled together.

    I usually question any readings less than 1A on these panel meters - if I need to measure below this I get out the Fluke.
    Meter A (blue dots - a DIN rail meter) is not too bad above about 0.5A but everything else is at best indicative at low values.
    I use the panel meters mainly to observe currents when machines are under load so this is usually above 5A

    Screen Shot 2021-09-09 at 10.31.52 am.png

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