How do home power monitors work? - Page 1
Forum section: electronics / beginners

author: FlyingMoose
author_rank: Contributor
timestamp: December 23, 2019, 05:13:49 am
content: There are numerous home power monitors available on Amazon, etc. They use clamps around the mains wires to measure current but they don’t have any way to measure voltage. I was under the impression that voltage is required to calculate power. Does anyone know how these work? My friend thinks his landlord has rigged the meter to run fast and I want to suggest one of these to check, but don’t know how accurate they are. Do they just ask for the voltage (which may vary throughout the day)? The power is 3-phase delta which I measured at 227v across each pair of phases...

author: FlyingMoose
author_rank: Contributor
timestamp: December 23, 2019, 05:22:11 am
content: The one I’m considering is the 3 Phase Efergy Elite Power Monitor.

author: james_s
author_rank: Super Contributor
timestamp: December 23, 2019, 06:32:35 am
content: I have one that measures voltage using a small transformer that also powers the receiver.Most of them only monitor current though, they know the voltage because you tell them what the line voltage is when you configure them. The voltage does not vary much in most places so just setting it to 120V, 240V or whatever you get locally is generally considered close enough. These are low cost consumer devices, not precision instruments.

author: FlyingMoose
author_rank: Contributor
timestamp: December 23, 2019, 07:58:13 am
content: That one I was looking at says it will only work for a balanced load, which the system I’m looking at is not. There are different circuits across all of the phases.

author: james_s
author_rank: Super Contributor
timestamp: December 23, 2019, 07:13:10 pm
content: Mine is made by Efergy, the transmitter has sockets for two current transformers to handle the split phase panels we have in the USA. There is a spot on the board for a 3rd transformer though so presumably they make a 3 phase version.

author: FlyingMoose
author_rank: Contributor
timestamp: December 24, 2019, 12:57:32 am
content: I’m looking at a current transformer that says “Conductor Through: 3 T”. Does this mean I have to pass the load conductor through the current transformer 3 times? I don’t think this would be possible because the wire is so thick I can’t bend it. The transformer is a Morning Group CT-200/5A. You can see a picture on Amazon.

author: langwadt
author_rank: Super Contributor
timestamp: December 24, 2019, 01:07:11 am
content: instead check it with a known load like a space heater, sorta like this in reverse,https://youtu.be/0l4PpqcV4wU

author: FlyingMoose
author_rank: Contributor
timestamp: December 24, 2019, 04:58:47 am
content: Excellent idea... I have one of those plug-in watt meters and I will try that later today with a vacuum cleaner.I’m looking at a panel-mount multifunction meter that says it requires a power connection (to power itself) from Live-Neutral. The manufacturer says it works with 3-phase delta systems but specifically says not to power it from Live-Live. I don’t think I should power it from Live-Ground, so should I treat the phase I’m not measuring as Neutral? (It only has a current transformer and voltage connection on 2 of the 3 wires since that’s all that’s required on a 3-wire system)... If not, how should I power it? And why can’t it be connected Live-Live? It’s a Morning Group 3D3Y.Edit: Morning Group, not Morning Star

author: james_s
author_rank: Super Contributor
timestamp: December 24, 2019, 07:33:31 am
content: In the main panel, neutral and ground are the same thing, at least they are everywhere that I've been.

author: langwadt
author_rank: Super Contributor
timestamp: December 25, 2019, 01:29:49 am
content: Quote from: FlyingMoose on December 24, 2019, 04:58:47 amphase to phase is sqrt(3) times the voltage of phase to neutral

author: SG-1
author_rank: Regular Contributor
timestamp: December 25, 2019, 02:53:52 am
content: This is the meter you are considering:https://www.amazon.com/Morning-Group-Phase-LCD-Digital-Display-Current-Voltage-Multifunction-Energy-Power-Panel-Meter-RS485-Modbus-Communication-Distribution-Box/dp/B076VKRN4G/ref=cm_cr_arp_d_product_top?ie=UTF8&th=1Tampering with a revenue meter is a serious offence. You can check it without buying anything, by controlling your loads. Turn off all loads by unplugging them or with a breaker. Leave everything off for an hour, then read the meter. Now use a known load like a heater that can draw 1KW in an hour. Take the reading on the meter, run the heater for an hour then take the reading again.If there is a serious discrepancy call the Power Company.Your friend may have a malfunctioning appliance, like a water heater, which is probably his problem to find.

author: Gregg
author_rank: Super Contributor
timestamp: December 25, 2019, 03:14:54 am
content: The forum has been down for most of the day, so I might be late to the party.I personally wouldn’t buy this meter unless I could download a full featured instruction manual and made absolutely sure this device would work for the given situation. Either my Google-fu has failed or there just isn’t this information available in any language I understand. I did find a connection diagram that seems to indicate it may not work on split phase or single phase power, but there may be settings to allow it.The maximum AC voltage seems to be 265V which may or may not work for you. It definitely needs line voltage to calculate power as well as input power for the electronics.If you do end up using this, make sure you get the current transformers to put around the line wires to sense amperage.Make sure you fuse it to the line connections with proper fuses

author: FlyingMoose
author_rank: Contributor
timestamp: December 25, 2019, 04:09:43 am
content: Quote from: SG-1 on December 25, 2019, 02:53:52 amI plan to do that (load testing).This is in the Philippines. Here, for example, almost all taxi meters in Manila are tampered and no one does anything about it, despite it being a “serious offense”. There are people whose career is tampering various types of meters. It’s very common to illegally tap electricity from your neighbor through a $20 meter from the hardware store. Grounding is pretty much unheard-of. I’ve only seen a handful of grounded outlets and I doubt the ground prong is hooked up to anything.My friend purchased and had a meter installed but then the landlord said it was broken and switched it out with one he provided. The landlord also wants to add a 30% surcharge which isn’t in the lease. This is only the problem with the electrical system; there are many other sleazy things going on.If there is a discrepancy it will need to be handled diplomatically since if you get someone in trouble with the law or the courts you might end up shooting yourself in the back of the head a few times, especially if you’re a foreigner in a dispute with a local, and the police won’t really do anything. If they can get away with murder, meter tampering doesn’t rate that high.I measured phase-to-phase at 227v. This is in a commercial space, not residential (it’s a disco). There are five 8HP air conditioners so I think that is the majority of the power.The work done by the supposedly certified electricians here is a dangerous mess. The main panel has an extra circuit breaker attached below the rest (which are on bus bars) and is tapped off from before the main breaker in the panel. There are also wires for 2 sub-panels attached on the bus side of two other breakers. I can see all of this because the cover for the panel is long gone so someone could easily poke their finger into the bus bars. Thankfully the panel is behind the bar and not where the customers have access to it.

author: SG-1
author_rank: Regular Contributor
timestamp: December 25, 2019, 03:13:42 pm
content: Good hunting Flyingmoose.Is there any chance the service is a center tapped delta, aka high leg delta. One transformer is usually larger than the other two.Then a neutral would be available for the meters power supply.

author: FlyingMoose
author_rank: Contributor
timestamp: December 26, 2019, 03:52:09 am
content: Quote from: SG-1 on December 25, 2019, 03:13:42 pmI don’t think so, only 3 wires and equal voltages across each pair.I wonder if I should run it from an isolation transformer...

author: SG-1
author_rank: Regular Contributor
timestamp: December 26, 2019, 03:01:55 pm
content: If the delta is corner grounded, that phase could supply the ground reference the meter power supply is asking for.

author: MiDi
author_rank: Frequent Contributor
timestamp: December 26, 2019, 08:27:36 pm
content: If there is no measurement of voltage, this is just an AC Current Clamp Meter with a calculator.The (apparent) power values displayed (in VA) may be way out of what you are paying for (real power/energy) - this is just a toy and nothing serious.For serious results you cannot come accross a (certified) Energy Meter.

author: SG-1
author_rank: Regular Contributor
timestamp: December 27, 2019, 03:15:15 am
content: Looking closer at the meters connections in Post 11 it shows L & N or 1 & 2, so no neutral needed.