Measuring Current in 3 Phase Equipment

If the motor is a Delta, there will never be current thru the "neutral" this wire is actally a ground.

But if your motor is a Star then you should have 5 wires: 3 phases, 1 ground and one neutral, and all three currents return to it, so, you could use just one transducer.

Regards

The lowest hardware cost will be to fabricate all of the test equipment yourself. The software cost to be able to achieve the lowest hardware cost will involve paying for a lot of schooling and to hit the books yourself. The considerable advantage of doing things this way will be that you will know and can prove with certainty that your results are valid.

The lowest total cost will likely be for you to hire somebody that is already certified in this schooling (an electrician or electrical engineer) to install a properly scaled power monitoring system to your existing equipment. The electrician will know what safety standards your installation must meet for your location and which parts are readily available for your system and location. They will also be able to certify that the data you obtain is your actual power usage and not some completely erroneous measurement.

What will probably happen though is that you will get some advice here on parts you cannot find for your area and people will try to explain how to wire this up. You'll find different parts with similar names and install them as you think we told you. You won't know how to test your installation so you won't know if the installation and/or components you found are working properly.

You will be indignant to me for my honesty, but what did you expect for free.

I find the best way to learn is by actually doing projects, and having my own reputation and funds on the line. I did go to an actual school for architecture a while back and have self taught on most elements of small building (residential and commercial) systems, by being a builder - developer. In my first projects, I did attempt to hire on the talent I needed to do things that were not the norm and outside of my direct expertise (renewable energy systems primarily, but other things as well) and as far as I could tell it was an expensive exercise in getting nowhere.

So I`ve been doing various projects as design build, and have some failures and some successes, but at least this way I`m learning more than just how to over pay an engineer to figure it out, then paying him more to produce useable plans and argue with a contractor!

For this project, I am planning on using the Onset Data loggers - which are inexpensive and fairly straightforward to use.

I simply did not have a compressor handy to go out and test with my multi meter, hence the question on where the current is flowing when the system is running...

I do appreciate the advice though, and don`t feel indignant about you taking the time to respond... We all have our own opinions and biases, and I`m not going to let someone elses ruin my day - all the best!

If you just want to log if the compressor is running or not a simple auxiliary contact on the main contactor would suffice.

Or is that to simple?

to too

If the motor is healthy, there will be no current in the neutral. Only in the event of unbalanced system will the neutral have any current. Anyway, if it is delta-connected when running, there will be no neutral, as has already been said.

The cheapest way is to use a 'Tong tester' on each phase in turn (one at a time) when you need to measure.

Since you only want to track run time, the extra contact is the simplest, and usually available on contactors. Or use any terminated current transformer over one phase wire. It will provide a voltage for logging. The same is with a clamp-on.

You mention add on current transducers. If the motor is in good condition the current draw on all three phases should be equal. This can be easily verified with an amp clamp meter. If they are equal, you can proceed to use a single current transducer on a single phase and do the math. I am assuming that the load varies. This is why you are interested in measuring current. If the load is very constant, I would measure on time by data logging voltage at "hour meter" if so equipped. If not measure at A1 on contactor coil. Side note: one of the best ways to keep a handle on motor health is to keep an eye on the current draw for each phase.

A wattmeter like the power company uses should do what you want. It will record the watt-hours your three phase motor uses. All you have to do is read it each day.

Your compressor most likely has a low-pressure start, high-pressure stop, switch.

This switch is a contactor. It probably moves. Mount a micro switch near it so that also moves, so that the microswitch closes when the contactor turns on the compressor.

Wire a clock through the micro switch.

Set the clock to 12:00.

Plug it in.

Unless your compressor runs more than 12 hours every 12 hours, you should be able to read the amount of time it has run in the last 12 hours.

If you don't want to check it that often, you can also get data loggers that have longer than 12 hour scales, but I was trying to offer what could be the lowest cost option.

How cheap is your labor? Got a bored guy with OCD and a stop watch?

If you just want to know the time the compressor is running then a simple hours-run meter fed by a signal from an 'on' contact from the motor starter will do. You make a note of the meter reading and time you start and stop the test. That enables you calculate the 'on/off' ratio for the period. Choose a meter that can be reset to zero if you like - otherwise they go on counting.

The load on a 3-phase compressor motor is balanced which means there is no neutral current. But being balanced you only need to know the power in one phase. The power itself can easily be measured cheaply by connecting a single phase 'watt-hours' meter (the sort you use at home) in one line and neutral wire. If power cables are too large or you cant re-route them, then, as you say, you will need a CT type meter (but much more expensive). It's a balance between cost and convenience. Either way, you multiply the reading by a factor of 3 to get the total power consumed in the period.

What are you trying to achieve by this exercise. Please define your goal clearly.

As usual we have no response from the OP.

Thanks to everyone for the suggestions.

Our requirement is to measure the on-off in the same manner as a trend log in a Building Automation System might. So we need to see when a compressor comes on and when it goes off in relation to other timed events (occupancy vs over nights, what the other equipment is doing, space temperature, weather etc). The building is too small to justify a full BAS, so that is why we are exploring the use of an add on data logging system.

In this case, we are leaning towards the HOBO Data Loggers from Onset http://www.onsetcomp.com/ since they are locally available, relatively affordable and straightforward to use.

We will look into the option of using the motor contact - thanks for that suggestion.

We are also trying to determine if the equipment (chillers) will have a 24VDC dry contact option available, but that is not clear right now. Looking into this with the various manufacturers.