Please Help me

Buy a three phase ammeter.

If, by "industrially", you mean lots and lots of amps - then you will have current transformers on the power lines and have the meter connected to those.

------------- You measure the current in each phase individually. Typically the incoming line currents are measured (as if the load was Y connected). You will need current transformers installed or, if the voltage is low enough and the leads are available you can use a clip-on ammeter.

There is no such thing as a 3-phase ammeter as suggested.

What is your specific problem and what exactly are you after? Current or power? Did you ever think to ask the plant electrician how he does it?

Your in deeper than your pay grade. If you want to live, find another form of employment, hire someone, or take a sufficient number of classes that teach how to avoid ending up smelling like breakfast bacon.

Bill

------------- I concede that it is true that it is called a 3 phase ammeter. 25-50 years ago the switching from phase to phase was done with a switch and peak readings weren't available Rather than electronic switching, there was a switch similar to those used for this purpose in panels- a make before break device designed to allow switching without actually opening a CT secondary (short secondary, move contacts to next phase (already shorted) then remove short from that phase). In any case, the measurement that actually occurs is that of one phase at any given time and the mode switching (see manual) is intended to allow this.-----so--in effect you have a single phase ammeter which can be switched from phase to phase. This device is simply (along with the auxiliary unit) a solid state equivalent of an ammeter and a transfer switch- with the extra bells and whistles that electronic circuitry can give you cheaply. The meter doesn't average the phase currents, nor sum them nor combine them in any other way but just allows the user to easily switch between phases for individual phase readings.

On the other hand, I would consider a "three phase" wattmeter to be a true 3 phase instrument in that it sums the power of the individual phases to display the total power.

One of the extra "bells and whistles" is that the CTs on the power lines only needs to have short, permanent, connections to the auxiliary unit. If the, usually remote, meter itself is accidently disconnected from the auxiliary unit, the results will be rather boring in comparison with open circuiting a power line CT wired directly to a meter.

The OP asked, "How could we measure 3 phase current industrially". I suggested the safest way that I knew to do it - using a 3 phase ammeter designed to do precisely what he appeared to want to do...

You were simply wrong when you stated that there was no such thing as a

3 phase ammeter. What it is called is what it is.

just use three ammetres at all the 3 lines. the current measured there is line current.(I line) you've o find out phase current (or curent per phase) (I phase)

if your coneection is delta one, then (I phase) = (I line) if your coneection is star one, then (I phase) = (sqrt 3)*(I line)

as you get (I phase) value, you can find the value for 3 phase current also. remember to use ac ammetre else a dc ammetre will provide a wrong reading.

if you dont have an ammetre, you can use 2 wattmetres and a voltmetre considering the load is balanced. if the load is unbalanced then one more wattmetre is required.] then,

(I phase) = (W) / ((sqrt 3)*(V line)*(power factor))

where W = sum of reading of 2 wattmetres V line = voltage measured ACROSS 2 lineS power factor = you've to find out else its usually = 0.8

-------------- "What it is" is one single phase ammeter which can be switched between phases to select from any of 3 currents. That is about as close as you can get to a 3 phase ammeter if you want to call it that. I don't care to do so. Why? maybe because I am an old fart and maybe because it is technically still a single phase meter measuring one current at a time (in spite of its name). I have done this before with an ammeter switch commercially available and a single good quality ammeter (Yes- it works with CT's and is a common practice).

Yes, the auxiliary unit can be left in place and the meter moved- that is useful and provides the important safety benefit that you mention but doesn't lend itself to taking measurements at different places unless you include an auxiliary unit at each site. If cheap enough that works. Otherwise you will, of necessity have to be reconnecting CT secondaries.

By the way, what is a 3 phase current or voltage? What if we had 3 independent single phase currents and measured them with this meter? Can the meter tell the difference? Am I being pedantic? (Yes)

The manufacturer appears to be located in Saskatoon and the company is probably started by U of Saskatoon graduates- I have a great deal of respect for the quality of the Uof S power engineering program from first hand knowledge of some of their profs and graduates.

-------------------------- Besides the errors that you have presented, confusing star and delta relationships between line and phase currents, there is the problem of "what is the 3 phase current?"

You aren't being pedantic.

You wrote "There is no such thing as a 3-phase ammeter as suggested.", without even the decency of asking me to justify my suggestion or even doing a simple Google search to check.

Why not just hold your hand up and saying that you were wrong and apologising? And then explain, at length if you will, why you think manufacturers shouldn't have called them "3 phase ammeters" but should call them "Single Phase Ammeters with integrated switch to allow inplace measurement of 3 phases".

A "3 phase ammeter" isn't what *I* chose to call these devices. It is what manufacturers choose to call such devices. They are now commonplace. Crompton, amongst others, make them, eg The Integra 500. However, they all seem to think that "Three Phase Ammmeter" is better as a description than "Three, Single Phase, Ammeters in a single unit".

No one doubts your expertise and years of experience. By "3 phase ammeter", you had assumed that I meant something akin to a 3 phase wattmeter, that presented some single figure representative of the 3 currents. You were quite correct in that no such thing, as far as you or I know, exists. Personally, I cannot even think of what that single figure should mathematically represent, for it to be of any great and general use.

Even "old farts" can make wrong assumptions about what is written in a post. If we were chatting about this, in a group, you would possibly have simply raised an eyebrow at that point and I would have explained further what I meant. I would suggest, in a usenet thread, the equivalent is simply to query the statement before pronouncing it wrong.

How could we measure 3 phase current industrially.

Sir, The company has a 3hp motor for a screw conveyor. And there is a siemens PLC with a MicroMaster 440 Drive along with it to control speed of conveyor. Conveyor speed is varied according to process requirements. Only one analog input is alloted for the current measurement for motor in the PLC. I know that we can use 3 CTs and average it out in PLC.

But they want a transducer which can give us the required PLC input (4 to 20 mA) using a single CT. Can u suggest certain companies which can give me such a product or another way to measure the current for this project?

Sir, The company has a 3hp motor for a screw conveyor. And there is a siemens PLC with a MicroMaster 440 Drive along with it to control speed of conveyor. Conveyor speed is varied according to process requirements. Only one analog input is alloted for the current measurement for motor in the PLC. I know that we can use 3 CTs and average it out in PLC.

But they want a transducer which can give us the required PLC input (4 to 20 mA) using a single CT. Can u suggest certain companies which can give me such a product or another way to measure the current for this project?

How could we measure 3 phase current industrially.

Simply ask your electrical equipment supplier for a "Current Transformer with 4-20 mA Converter" with an appropriate rating for the load being monitored.

It is possible to buy an integrated unit, eg a CT with 4-20 mA DC output, although some may prefer to call it a "CT with built-in converter" eg

You need to check with the system designer what the current measurement is used for. I would assume (always dangerous!) that it is used for i2t detection of motor overheating because direct measurement of motor temperature is impractical. Of this is the case, then measurement of a single phase current should be fine - provided fault protection measures are in place to detect highly unequal phase loads. The controller will be averaging the CT input over a relatively long period, so the effect of the inverter harmonics should be minimal.

If, however, they are using this input for something else than i2t (unlikely) then a single CT with converter may not be appropriate.

-- Sue

It is a 3 phase motor.

Current in phases will be near enough equal.

You only need to measure 1 phase...1 CT, 1 Analogue input

See here;

Get a manual for the MicroMaster 440 drive...there are 2 Analogue outputs

If you look at the Analogue output options, you can get the drive output current

Just wire the drive to the PLC

The manual is here;

That should get you going

But they want a transducer which can give us the required PLC input (4 to 20 mA) using a single CT.

Can the sample be DC? if so 3CT and a simple 6 diode rectifier. This will help take into account voltage sags on one or more legs vs. sampling a single leg.

I would suggest you look at SSAC (or even call and talk to an application engineer)

---------------------------------

You have made your point very effectively. "Good on you" as would be said in NZ. I do sincerely apologise for not querying this point with you in a less confrontational way or doing a google check (and realizing that whatever the manufacturers call it, the users should know what it is-which the originator of the thread apparently didn't).

Sir, The company has a 3hp motor for a screw conveyor. And there is a siemens PLC with a MicroMaster 440 Drive along with it to control speed of conveyor. Conveyor speed is varied according to process requirements. Only one analog input is alloted for the current measurement for motor in the PLC. I know that we can use 3 CTs and average it out in PLC.

But they want a transducer which can give us the required PLC input (4 to 20 mA) using a single CT. Can u suggest certain companies which can give me such a product or another way to measure the current for this project?

The answers that Palindrome and Fulliautomatix have already given should point you in the right direction. I can't add to them.

In other words you are suggesting use of two more costly (in general) meters and a voltmeter, along with a guess as to power factor, to replace an ammeter. Why?

In addition, the expression that you give will result in line current rather than phase current so it is in error. Please check your textbooks.