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#11125 - 06/29/02 03:58 PM ct versus vt  
Eandrew  Offline
Joined: May 2002
Posts: 68
seattle, wa, usa
what is the difference between a ct (current transformer) and a vt (voltage transformer) I thought all transformers were basically voltage changers.

where is a "ct" used instead of a "vt"

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#11126 - 06/29/02 04:16 PM Re: ct versus vt  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England

These terms are really referring to the use to which a transformer is put rather than the way it operates. All transformers follow the same basic laws of physics and work the same way.

There are differences between them in construction though, dictated by the functions they need to perform. A xfmr designed as a CT to monitor current remotely has a primary which might be just a solid busbar passing through it -- basically a 1-turn primary.

On the other hand, a xfmr used to monitor voltage remotely on a line will have a primary with many hundreds or thousands of turns so as to keep the current it draws low. The secondary will then have fewer turns so as to provide a low voltage output.

If you take the xfmrs calculations you were looking at a week or two ago, you'll recall the basic formula:

Np / Ns = Ep / Es

In other words, the ratio of primary turns to secondary turns is the same as the ratio of primary voltage to secondary voltage (in a perfect xfmr!).

In general electrical usage, a CT will have Ns much greater than Np. A voltage xfmr, where used to remotely monitor high voltages, will have Ns much smaller than Np.

Remember though, that you can't change one without the other:

Np / Ns = Is / Ip

So a current xfmr used to reduce the current to a level suitable for measurement will also step-up the voltage. (We're not talking about the supply voltage level on the system being monitored, we're talking about just the very small voltage drop on the "single-turn" of the busbar as it passes through the CT).

Similarly, where a xfmr is used to reduce voltage to a safe level for remote monitoring, the current in the secondary (Is) will have to be correspondingly greater than that flowing in the primary (Ip).

[This message has been edited by pauluk (edited 06-29-2002).]

#11127 - 06/29/02 06:37 PM Re: ct versus vt  
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...

#11128 - 06/30/02 08:22 AM Re: ct versus vt  
Electricmanscott  Offline
Joined: Jan 2002
Posts: 1,457
Holden, MA USA
Oh what a dissapointment. I thought this was going to be a war between Connecticut and Vermont!

#11129 - 07/01/02 03:17 PM Re: ct versus vt  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Funn, that's what I thought at first when I saw the subject! Somehow the term VT just didn't register as Voltage Transformer.

While we're on the subject of trandformers still, remember that a xfmr will also transform impedance (unless it has a 1:1 ratio).

Impedance-matching xfmrs are commonly used in electronics work (although not quite so much these days since ICs have become so common).

An example: Virtually every old tube audio amplifier had a xfmr to couple the output stage to the loudspeaker. The xfmr was used to match the relatively high impedance of the output tubes (several thousand ohms) to the low impedance of the speaker (typically 3 to 16 ohms).

The impedance transformation follows the square of the turns ratio, i.e.

(Np / Ns) ^ 2 = Zp / Zs

Again, calling it an impedance-matching transformer is just a reference to its primary purpose. It also results in voltsge and current transformation in the usual way, so in this application Ep > Es and Ip < Is.

#11130 - 07/01/02 08:51 PM Re: ct versus vt  
Eandrew  Offline
Joined: May 2002
Posts: 68
seattle, wa, usa
Thanks paul. I also found some transformer basics in one of my books which states:"current transformers use the circuit conductors as the primary windings and step the current down for metering."

I'm working in this buddist temple which has a 120/208 system (no transformers in the building, just the utility transformer outside which is a "ct" . I guess there is not much mechanical in the building so they went with 120/208 instead of 277/480. There is a whole bunch of lights though. Thanks paul. -Erik

#11131 - 07/01/02 11:24 PM Re: ct versus vt  
Bjarney  Offline
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
Customs vary, but if the service entrance is rated at more than 200 amps, CTs may often be used for the utility’s watthour meter. {No VTs at 208V.}

#11132 - 07/02/02 07:07 AM Re: ct versus vt  
Redsy  Offline
Joined: Mar 2001
Posts: 2,056
Bucks County PA
In most cases, when a service exceeds 200 amps the utility company will require CTs in order to run a smaller current through their meters. In order to send a full 1,000 amps through a kWh meter, it would need to be so beefy as to be impractical. A CT with an out put of 0-5 amps can be used with a ratio proportional to the Primary current (1,000 amps) by the Secondary current (5 amps) in this case the ratio would be 200/1.
The meter would only need to handle 5 amps in order to represent 1,000 amps.


[This message has been edited by Redsy (edited 07-02-2002).]

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