There is something strange in very old transformers. Normally transformer unloaded is open circuit in common types, but in old ones they used forgotten method of "multiple arc connection" when secondary of transformer is closed circuit always, even unloaded . Think about it ....I think it is related to JackNoSkills experiments...

There is something strange in very old transformers. Normally transformer unloaded is open circuit in common types, but in old ones they used forgotten method of "multiple arc connection" when secondary of transformer is closed circuit always, even unloaded . Think about it ....I think it is related to JackNoSkills experiments...

Posted by Colin Dedman on 20 October, 2011 - 11:53 pm

-The current transformer secondary is short circuited during the operation.
  What is the reason, and explain with details.

Most replies answered by pointing out that destructive voltages will be generated if the secondary is open-circuited. While this is true, it does not exactly answer the question.

A current xformer will still operate safely over a wide range of secondary load resistances, so why is a very low secondary load resistance (= approximately a short circuit) the best choice?

The reason is that the fidelity of the current transformer is highest when the secondary is short circuited.

The purpose of a current xformer is to provide a scaled-down version of the primary current with the highest possible fidelity, that is, the secondary current should be a faithful replica of the primary current. In many applications it is not just the magnitude of the measured current that is important, but also faithful reproduction of the phase and high-order harmonics.

Thus, the question becomes, why is the fidelity highest when the secondary is shorted? At first this seems counter-intuitive, for the fidelity of the more familiar voltage transformer is most definitely NOT optimised by shorting the secondary! Indeed, as a voltage transformer is loaded more heavily by reducing the secondary load resistance, the secondary voltage 'sags', phase error is increased, and frequency response decreased, all leading to a loss of fidelity of the secondary voltage waveform. This occurs mainly due to winding resistance and leakage inductance, which would both be zero in an 'ideal' xformer. For a voltage transformer, the fidelity of the secondary voltage is highest with a high secondary load resistance, which draws very little current.

However, with current xformers, just about everything turns out to be reversed compared to voltage xformers, which can be confusing. As with all xformers, the secondary load resistance is reflected back to the primary side, scaled by the square of the turns ratio. Therefore, if we short the secondary, then we also short the primary. Note that the primary current is fixed and constant, being set by the external circuit. Therefore, from V=IR, the voltage developed across the primary winding is approximately zero when the secondary is shorted. Of course, the secondary voltage is also zero when shorted. But why does this improve the current fidelity? Without going into too much detail, the 'magnetising current' of an xformer depends on the winding inductance and the winding voltage. The inductance is set by the number of turns and the core material and geometry, which are fixed for any given current xformer. However, by reducing the winding voltages, by shorting the secondary, the magnetising current is also reduced to near zero. That is a GOOD THING, because the magnetising current is an error, representing a proportion of the primary current that DOES NOT end up being reflected in the secondary current. The magnetising current results in an error in both the magnitude and phase of the secondary current, definitely not a good thing where high fidelity is required. To say the same thing in a different way, shorting the secondary permits larger currents to be measured with acceptable fidelity, and without core saturation.

The conclusion is a beautiful inverse of the situation for a voltage transformer. In summary:

For a voltage xformer, the primary voltage is fixed, and the current is set by secondary load resistance. Reducing the secondary load resistance increases the current, increases the 'loading', increases the power delivered to the load, and lowers the secondary voltage fidelity. A high value of secondary load resistance is desirable for best voltage fidelity. Shorting the secondary will generate dangerous currents.

For a current xformer, the primary CURRENT is fixed, and the VOLTAGE is set by the secondary load resistance. INCREASING the secondary load resistance increases the VOLTAGE, increases the loading, increases the power delivered to the load, and lowers the secondary CURRENT fidelity. A LOW value of secondary load resistance is desirable for best fidelity. OPEN-CIRCUITING the secondary will generate dangerous VOLTAGES.

Source:  http://www.control.com/thread/1026235833 (about halfway down page) Check out some of the other answers on this page.

I WILL JUST LEAVE THE FOREST UNTOUCH TO MAKE IT IN VIRGIN STATE.