CT Ratio

GA to kvsridhar. If the OP really needs 120:3 ratio (40:1), then a 200:5 CT will provide the ratio needed. That is a commonly available rating. My suspicion, though, is that the OP either mis-read the ratio, or it is a typographical error in the meter's documentation and was actually meant to be 120:5.

Thanks Peter. Yes, 120:3 does look odd. Dear OP, please recheck the documentation.

I checked the face of the meter 120 : 3 is the reading. It is an old Weston 476. Old may be the problem...

Yes, a quick Google search turns up some literature from 1934, saying this is an antique radio instrumentation type of meter. That's probably the reason for its strange ratio.

http://www.stevenjohnson.com/weston/weston1934.htm

Anyway, if you intend to use it, the 200:5 CT should work fine for you.

Thank you! I really enjoyed the finding of the information and the insights!

Thank you for the insight.

That makes sense. I think I can do this.

hello,

ya it is possible to get 120:3 current transformer or if your rating of power is not to high then you have to make of its own by make sufficient turns over the core

As per IEC-60044-1

Clause nos. 4.1.1:

The Standard values of rated primary currents are:

10 - 12,5 - 15 - 20 - 25 - 30 - 40 - 50 - 60 - 75 A

The preferred values are those underlined.

Clause nos. 4.2:

The standard values of rated secondary currents are 1 A, 2 A and 5 A, but the preferred value

is 5 A.

Vinu_Answers Sure Answers

The generator puts out 62.5 amps at 120 Volts..

Vinu, what is the point of quoting preferred values when the primary current is outside the range of values?

200/5A makes sense as a 120/3A would be to special order and expensive!

I don't think the 120:3 CT is available. You can use 200:5 CT as already has been suggested. You also have other options such as the use of 120:1 CT but in this case multiply the reading of the meter by 3.

- MS

If the 120:1 CT is a donut or window type and the opening is large enough, you can wrap the primary lead through the window three times. Then 120 amperes in the primary circuit will give 3 amperes out of the CT's secondary.

Is it possible or safe to use a 120:1 CT w/3 loops on each 120 leg and route them to a switch? I read that an open CT can be hazardous. I was wondering about this in order to check the balance on the generator and to use a single meter..

Interesting.

If i understand correctly, you want to put one loop of wire from each of the three phases in the window of the (donut) CT. If this is what you want to do, you will see zero current on the secondary since the vector sum of the three phases is zero in a balanced system.

However, if you want to check the imbalance of the system this way, your meter is unlikely to be sensitive enough to read, unless the imbalance is quite high. If it is high, you will know it otherwise also, it can cause severe damage to the loads.

Thank you for the insights. Obviously, I am not an engineer, but I value your training!

Gary

You do not want to open the secondary of a CT when current is in the primary, as that can cause a high voltage spike in CT secondary. If you want to switch between phase leads in the primary of a CT, you need to use a switch or switches that will short across the leads of each of the individual phases to the CT primary that you do not want to measure. That way you can have one phase active to measure, with the other two live and shunted around the CT. Be sure the switches are rated for the amperes of the load. I'm sorry I can't supply a schematic, I hope my explanation is clear enough.

It is. Thanks very much.

Thank you! I see what you mean.

Thank you very much.

Thanks for the insight. I researched this, but lacked the knowledge to work effectively.

As far as you are concerned your application, simply to read the current out from a generator, a 120:3 and 40:1 and 200:5 are the same thing, the primary is the cable i.e. 1 turn and the secondary has 40 turns so you call it whatever you want. Personally I wouldn't buy one I'd modify and existing one to 40 turns and repaint.

To check your meter run 1 amp, 2 amp and 3 amp through it and verify % full scale deflection, i.e. at 3 amp you should see 100% FSD or 62.5 amps whatever it is intended to show.

Thank you for the information and advice.

A 120:3 CT is the same as a 200:5. This is a standard size.

Thanks very much!

You have a great deal of information here at your finger tips.

Please consider that what you do to the meter will affect its originality, value, and presentation qualities. All of us have a responsibility to preserve the products of times past to the best of our abilities. This is my belief and I imagine this belief is shared by many others.

Before replacing anything I would certainly conduct a bit of research and try to discover how new vs rewinding, what you have, will affect the value and preserve the quality of the meter. Remember products of this type appreciate with age and anything early radio related is highly collectable.

These things came from an era when two methods of mass communication existed, the very slow arriving news paper and the quickly distributed radio signal.

Thanks. I am old also. My first job in the Air Force was as a Ground Radio Repairman.

I like the round meters. This one is in good shape. I cleaned the glass and looked over the rest. It is of bakelight construction and was calibrated to be mounted on a steel plate. The inside is very clean and the inside housing is brown. I will be very careful with it. I have been reluctant to do anything until I learned something about the correct connection of the meter. I may change it out when I find another one.

I looked the meter over and looked at the box in which it was shipped.

TAG ON BOX

MOD.476 AM 3 AMPS (ORIGINAL BOX)

SC.0-120 AMPS

RL.31/2' BAKE. CASE

1-8" STEEL PANE MTG

S C ORD NO. 380-MPD-43

ITEM NO.1

WESTON ELECTEICAL INSTRUMENT CORP.

NEWARK, N.J. U.S.A.

_______________________

F1225-100M-6-43 MADE IN U.S.A.

On behalf of all who posted on this thread, i would like to compliment you on what i perceive as an old-world courtesy of thanking people for their help. Very few do so nowadays. Certainly somewhat rare on CR4 too May your tribe increase !

Mom made that (being thankful) part of our training. We were living in poverty for a long time, but she gave us classy values. She and dad valued education and it has afforded me a good life.

How true. Despite widely differing cultures in the many countries i had the privilege to visit in my life, the underpinning values are the same the world over.

The values you learn at your mother's knee are what you take to your grave, thinking at the last moments ... ah, while i may or may not have left my footprints on the sands of time, i have not hurt anyone knowingly.... that's what matters.

If your ammeter requires 5amps FSD, then why do you want a CT 120:3, which is ratio factor of 40. Remember this number..40!!

What is the max amps/load in the circuit?

It is standard practice to have a meter or amp chart recorder working at the mid point, that is working on 2.5amps into the meter/recorder and the scale would be to suit, if the FSD is 5amps. For example;

Load = 50 amps.

CT = 200:5

Amps(load) divided by CT ratio multiplied by No turns thru the CT

50amps/(200/5 =40) x No of turns

50/40x1 = 1.25 amps at the meter.

Now add another turn

50/40x2 = 2.50 amps at the meter.

So you don't need to find that special CT.. use any old 200:5 with two turns....which PeterT & kvsridhar have already stated

if your ammeter requires 5amps FSD, then why do you want a CT 120:3, which is ratio factor of 40. Remember this number..40!!

Er...where does it say that the FSD is 5A? i can see 3 A in whatever majoday says, which is why he is looking for a 3A secondary CT i thought.

A 200:5 CT will have the same ratio all right, but a perfectionist would question the aspects of knee-point voltage, burden, accuracy class, saturation to cater to cramped scale, etc. Which is why i qualifed "...in case you do not need a very high accuracy...." when i suggested using a 200:5 CT.

Hello! The amps measurement is not necessarily a precision one. The need is more of a "ballpark" measurement. Thanks to all for the input. nothing like a knowlegable engineer to make things work. I am impressed and grateful.

I am so pleased that you are perfectionist.. they are in short supply.. however in answer to your question "where does it say the FSD is 5A?".. I'm sure the OP stated that figure in his OP!

Lets looks shall we....

I have a meter that requires a CT of 120 : 3. Is it possible to find such a current transformer? I noticed in my research that the American standard is 5 amps for full scale. Thanks for considering my question...

and I have a question for you too... what is "knee-point voltage?"

and in my post I gave you credit for your suggestion..

however no-one has given the maths for the OP to work out what he needs or any sort of formula to assist him. I used 5 amps FSD as a base.. yes it can be changed.. but hell man.. give someone a fish, they eat for a day.. teach them how to fish they will never go hungry!!

Sorry to have been so late in responding...did not get the usual email. And sorry ... something in my post offended you i see... not my intention, i assure you. Maybe since English is not my native language, i must have made a gaffe somewhere.

Ah yes, the OP did say that he saw 5A as a standard, not the 3A that he requires. OK, since he required 3A from 120A primary, and not very accurately either, a 200:5 off-the-shelf made sense i thought.

Knee point voltage and other terms of CTs is well covered here:

http://www.kappaelectricals.com/technical.html

i hope you will find it informative.

This has been a really inspiring experience in learning. Thanks!

I picked the meter up and its cost was small. It has a 120:3 CT requirement. The FSD is 3 amps. I was not knowledgeable about the ratio issue. Explaining the ratio really helped and I can now see how to do this. Thanks!!