# Difference between current and voltage transformer

1. Apr 26, 2012

### d.sonali20

what is the difference between a current and voltage tranformer constructionally and functionally??

2. Apr 26, 2012

### psparky

I don't think there is any difference. Power in = power out in either case.

If you raise the voltage you lower the current. If you lower the voltage you raise the current.

I guess you can say that a current transformer typically has 1 winding in the primary....simply because you generally grab the current from 1 wire (primary). But it's still just acting on the basic transformer concept.

So a typical transformer has a primary and secondary. You hook your input to the primary and get a output at the secondary.

For you typical "clamp on" current transformer. The current transformer only has the secondary. The wire you are "clamping on" is the primary. So when you clamp around the wire, you now have a primary and secondary. There is a super small voltage in this process....some call it voltage from the magnetic field...back EMF or whatever. This is why you do not get a large voltage out the other end of a current transformer. Power in does equal power out.

Last edited: Apr 26, 2012
3. Apr 26, 2012

### Staff: Mentor

4. Apr 26, 2012

### psparky

And to add....and drive the point home.

If you are measuring current thru a wire with a clamp on meter (current transformer) and it reads 50 amps.....

Disconnect the wire and wrap it around your clamp on meter....5 times or 5 turns.
Your amp meter will now read 10 amps....(EDIT: corrected to 250 amps....see post 6 and 7.)

Last edited: Apr 26, 2012
5. Apr 26, 2012

### jim hardy

there was a lonnnggg thread on this a month or two ago

in theory there's no difference, both are cores with windings.

Current transformers however are operated in a different region of the magnetization curve. They operate at low flux so that the magnetizing current will be just a small fraction of the current to be measured. Power transformers operate at much higher flux.

To that end a current transformer is operated with a low-Z load on secondary so it won't ever be asked to develop much voltage. It's almost short circuited. That allows secondary amp-turns to cancel out nearly all of the primary amp-turns thereby keeping flux (hence induced voltage) low.
That means the primary current must be fixed by something else because the transformer won't develop significant counter-emf to oppose primary current at such low flux.
So your current transformer always goes in series with a load and reports how much current is allowed by that load, without causing much insertion loss. Just like an ammeter.

I hope that paints a word picture for you.
The key to understanding CT's is that trick of almost shorted secondary hence low flux.

Now - on a CT there may be two secondaries connected in parallel. That allows the manufacturer to effect non-integer turns ratios so he can account for magnetizing current. An ideal current transformer would have infinite inductance hence draw no magnetizing current but of course a real one must draw some. So a turns ratio of 1 to 9.8 might be arrived at by paralleling nine and ten turn secondaries with carefully selected resistances. [EDIT] That effective turns ratio would give an ideal current ratio of 49::5 , but since there's some magnetizing curent required , a real transformer would require closer to 50 primary amps to give 5 secondary amps. Which is what one would want in a 50::5 CT..[/EDIT]
And CT's use superior grade of core material so magnetizing current is minimized.

Go to Butlerwinding.com and find their paper on theory of current transformers for a more complete dissertation. They explain why you never let a CT operate with open secondary.

I hope this helps. old jim

Last edited: Apr 26, 2012
6. Apr 26, 2012

### Bob S

Really? Or is the measured current 250 amps?

7. Apr 26, 2012

### psparky

It was one or the other....just making sure you guys are paying attention ;)

But ya, as you add turns to primary you would actually lower the voltage on secondary......therefore raising amps!

8. Apr 27, 2012

### Bob S

One important difference between a current and voltage transformer is that if the secondary of a current transformer is unterminated, or terminated in too high a resistance, the transformer core can be saturated (volt-seconds too high), leading to a highly nonlineear response. In general, $$\int_{0}^{t}V\left(t \right)dt=-NBA\text{ volt seconds}$$ where N = number of turns, B = magnetic field in core, and A = area of core. By terminating the secondary in a low resistance, the induced current in the secondary (Faraday's law) will oppose the primary current and keep the core from saturating.