Options for monitoring generator output

Go To Last Post
7 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi All -

Looking for options for monitoring the output of a generator ... It puts out standard 240V with a center neutral for two legs of 120V. Would like to track both 120V legs (to monitor for load imbalances) and current draw on each leg, up to 78A (nominal 30A but burstable to 78A).

Also want it to be safe, and simple. This would be monitored by an AVR for display and logging.

Analog ADE7753 looks like it would do the trick. It would also do a whole lot of other tricks, lots more than needed for simple voltage and current monitoring. Not too expensive at around $6 I suppose.

It's a direct-to-120V connection for the voltage measurement, and uses a current-transformer for the current. Or a di/dt measuring device - a Hall-effect sensor ? The ADE7753 would do it all, and the AVR could simply read a pair of registers to get the voltage and current levels. I'll have to read the datasheet more carefully, there's a lot there.

Could use a Maxim MAX4081 for current measurement which would also use a CT, or a Sentron CSA-1VG which would use itself as the sensor. The CSA-1VG gets clamped directly over the main cable to be measured. Seems way cool, but needs a couple of extra boards for the chip, caps, connector etc.

This would mean doing voltage dividers to the 120V lines and manually doing the math to calculate RMS.

Surely someone else out there has done something similar. What did you use to measure 120V and current ?

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

I don't do voltage measurement, but I do manufacture a spiffy little 8-channel RMS load and ground fault current monitoring module for industrial use that speaks Modbus. It uses an Avr to directly and very accurately process the CT signals.

Tom Pappano
Tulsa, Oklahoma

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi All -

Just bubbling this up to see if anyone has any opinions or thoughts.

Tom - that sounds cool, but unfortunately it's a square peg to my round hole. If you care to share any insights on the CT hardware and/or coding, that would be tremendously welcome.

Dean 94TT
"Life is just one damn thing after another" Elbert Hubbard (1856 - 1915)

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

You can purchase load monitors. There are lots of them for 115VAC so I would assume that 230 versions are also out there. Try Google: AC Power Monitor

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

There is a low cost unit called the Kill-A-Watt, sometimes sell for about $20. It does not meet current measurement requirement, and is only single phase.

Model P4400
Operating Voltage: 115 VAC
Max Voltage: 125 VAC
Max Current: 15 A
Max Power: 1875 VA

With carefully engineered modifications, it might work.

It all starts with a mental vision.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Quote:

Analog ADE7753 looks like it would do the trick. It would also do a whole lot of other tricks, lots more than needed for simple voltage and current monitoring. Not too expensive at around $6 I suppose.


Yes, indeed, the Analog Devices ADE family (and similar families from other manufacturers) are a bargain for us low-volume users, as the pricing is driven by the high-volume energy meter market. I have a couple apps with the family.

The learning curve is quite high, with all the parameters. But remember that this is a precision instrument, with lots of calibrations.

OK, current is pretty safe when CTs are used. You can make your own circuit--they are readily available.

Voltage is another matter. If you drop it direct with resistors, as we do in production apps, then you need to be VERY careful about isolation. Not only to protect you, but your AVR. :twisted: (You can heal, but once you let the magic smoke out of the AVR...)

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Measuring 120V? Just use voltage dividers of standard resistors, let less than 1mA go through it. (Of course such circuit cannot be touched by hand when power is applied, just read the display :) ).

100A is a bit more difficult but the cheapest solution could be some shunt resistors, ie if using 100mV as fullscale: R=V/I=0.1/100=0.1milliOhm. And P=100*100*0.0001 = 1Watt.

10 1mOhm in paralell or the like.

Then use 6 opamps to buffer the signals and feed it into 3 AVR's sampling at some kHz rate. I believe you need more than one AVR to sample the signals fast enough if you want to do power calculations etc. If you only want to find the peak voltage you probably only need some simple processing and low pass filtering after sampling so you can compare the 3 phases. Hint: the sum of all three phase voltages should be zero if everything is OK.