Calculate power

I want to calculate power - I know amps, voltage but not power factor - on a single phase max 50A circuit. There is a mechanical KW meter at the in-feed, could I log data for a month say, compare and work out an average PF...? Yes it will vary, but this is for fun (home automation) not industrial. Or, are there PF meters like this: https://www.laurels.com/transmitter-phase.php that would work as a PLC input?

Secondly, what is the best way to totalise; in the PLC or SCADA (Ignition)?

For home use? Could you get something like this?

I already have something like that (a samsungs smart-things socket) which shows me watts used when device is powered on.

I don't want it for one device. I want it for the whole house, main incomer, 50A.

Ah got ya.

I have been eye-balling this thing, not sure it reports power factor tho. Running HASS for home automation myself .

You can always track your RMS V and I then use the KWH on your bill, which *should* be the real component of power to determine your actual power factor. I would have to sit down and look at the math. Perhaps I'll try to do that for you this weekend.

Basically your kWh on your bill (or from your meter) should be the real wattage used. Then the measured (or RMS) values of V * I should give you your |S| or apparent power.

So you would basically take your readings for a month.

Avg(kWh from utility) / [Avg(Irms)*Avg(Vrms)] = avg pf

This should hold relative to your changing loads. Also, someone want to check that math. I just took a test on this today and have not gotten the results back yet hehe. Bring those results back and we will see what else we can pull off with that pf.

Your laurel is only good for cos Phi, not for the PF, In the old days this was enough, however with all these chargers, and electronics the waveshape is altered. So you will need a special device or module for this, The current is no problem as you can use Current transformers they make from 50 Amp a nice signal of 1 A, or even 4-20 mA.

Have a look at arduino, it is cheaper, and you can make it yourself.

This will give you all the values you want, but you will need to grab the data using modbus RTU over an RS485 connection. It is direct connected so no CTs required but you would have to break in to your supply cables somewhere, hopefully just in the breaker box. You can find similar but requiring CTs but they end up costing extra.
http://www.smartprocess.co.uk/mid-a...-pulse-modbus-smartrail-x100?search=SMARTRAIL

There is a three phase version as well, though for domestic use in the UK you are probably single phase.
http://www.smartprocess.co.uk/mid-a...tricity-meter-pulse-modbus-smartrail-x835-100

Does your utility actually track power factor? None that I know of do for residential users in the US.

I don't know about residential, but in commercial buildings the power company would sell to us and install very inexpensively a pulse relay that would give a pulse every X kWh. You might want give your electric utility a call - they generally want to aid energy conservation efforts.

For a high-speed PLC, you can just multiply instantaneous I & V sensor readings to calculate instantaneous power. I did that at my last company, but with National Instruments hardware. But, you need at least 1 ms sample period. Some PLC's (Beckhoff etherCAT and B&R Automation) can do that easily, indeed Beckhoff has power modules which handle it on-module. The biggest issue is measuring the instantaneous current waveform with no phase lag. Many clamp-on AC current probes have large and erratic phase shifts. For 3-phase, 3-wire power (delta), you can use 2 I and 2 dV sensors (search "two wattmeter method"). For 3-phase, 4-wire (wye), measure power on each 3 supply wire.

BryanG said:

This will give you all the values you want, but you will need to grab the data using modbus RTU over an RS485 connection. It is direct connected so no CTs required but you would have to break in to your supply cables somewhere, hopefully just in the breaker box. You can find similar but requiring CTs but they end up costing extra.
http://www.smartprocess.co.uk/mid-a...-pulse-modbus-smartrail-x100?search=SMARTRAIL

There is a three phase version as well, though for domestic use in the UK you are probably single phase.
http://www.smartprocess.co.uk/mid-a...tricity-meter-pulse-modbus-smartrail-x835-100

Click to expand...

Looks exactly what I need, thanks! Single phase for now, 3 three is planned for future. It is a 100A supply but I got (already have) a 50A 4-20mA out CT, as I reckon even with all the high loads on at once (which, in reality never are on at once) it wouldn't peak 50A. I could go the modbus route, the PLC is a CPX L32E which (with a bit of work) can do modbus over the serial port I belive.... however to get started would counting pulses not be simpler?

rdrast said:

Does your utility actually track power factor? None that I know of do for residential users in the US.

Click to expand...

I don't get billed for it like commercial do, but I thought (assumed) that it would be integrated as a measured component in the existing meter... if not then I can by comparing my results to theirs find out what constant they are using?

Tom Jenkins said:

I don't know about residential, but in commercial buildings the power company would sell to us and install very inexpensively a pulse relay that would give a pulse every X kWh. You might want give your electric utility a call - they generally want to aid energy conservation efforts.

Click to expand...

Good idea, thanks.

RocketTester said:

For a high-speed PLC, you can just multiply instantaneous I & V sensor readings to calculate instantaneous power. I did that at my last company, but with National Instruments hardware. But, you need at least 1 ms sample period. Some PLC's (Beckhoff etherCAT and B&R Automation) can do that easily, indeed Beckhoff has power modules which handle it on-module. The biggest issue is measuring the instantaneous current waveform with no phase lag. Many clamp-on AC current probes have large and erratic phase shifts. For 3-phase, 3-wire power (delta), you can use 2 I and 2 dV sensors (search "two wattmeter method"). For 3-phase, 4-wire (wye), measure power on each 3 supply wire.

Click to expand...

I do have NI hardware in my office, unfortunately currently no analog inputs and NI hardware is £££.

Tom Jenkins said:

I don't know about residential, but in commercial buildings the power company would sell to us and install very inexpensively a pulse relay that would give a pulse every X kWh. You might want give your electric utility a call - they generally want to aid energy conservation efforts.

Click to expand...

Second this...sometimes utility company even installs a device to take care of your questions.