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A current transformer is a type of transformer that is usually placed in the main circuit to step down a high current circuit to drive a low current device, usually a low current meter or resistor. It is also very useful in measuring or monitoring high current, high voltage and high power circuits. Current transformer (CT) applications can be found everywhere, such as,

1
In over-current or under-current protection circuits: The current transformer can drive an over-current relay as a switch, so when the main current exceeds the specification, the output voltage of the current transformer activates the relay and the protective circuit.
2
In high current monitoring circuit: A current transformer can step down the current for a low current meter to monitor the high current circuit. Or, a resistor can be connected in series with the current transformer output winding, when the stepped down current passing through this load resistor, results in a voltage over the resistor. This voltage can be used to monitor the high current circuit.
3
In high voltage current monitoring circuits: A current transformer can be used to step down the current, as well as used as an isolation transformer. It is a lot safer to use a current transformer to monitor the high voltage current circuit.
4
In remote circuit current status monitoring system: A current transformer can also be loaded with an LED as a discrete current indicator to monitor the circuit condition in a remote location. When the remote circuit current is flowing, the current transformer will generate enough current to light an LED to indicate the current flowing. When the remote current cease to flow, the LED will be turned off. This method can be used to indicate a stalled motor (unusual high current being generated) in a remote location.


There are two windings in a current transformer, one of them is a high current primary winding and the other is a low current secondary winding. Unlike in other transformers, the primary winding current in a current transformer is independent of the secondary winding load. The primary winding current depends only on the circuit into which the primary winding is connected. The primary winding is designed to have very low impedance (it often will only have one turn in the winding) and hence has negligible effect on the main current. Therefore, regardless of what change may be made to the secondary winding load, the primary winding current is always the same as that of the main circuit. One may argue that if the load on the secondary winding is changed, something in the primary winding has to change. This is partially true. The change is in the relationships between the several components of the primary winding current. The primary winding current is essentially made up of three components; the loss current that supplies iron and copper loss, the magnetizing current that establishes the flux in the core and the load current. However, these currents are not in phase with each other; there are differences in the phase angles and these angles change when the load changes. The primary winding current, as always, is the vector sum of these currents.

The design of the current transformer has to satisfy the equation: IinNin = IoutNout if the magnetizing and loss currents are not taken into account. If accuracy is a concern, then, the magnetizing and loss currents must be kept small. The value Iin is determined by the circuit into which the primary winding is connected and Iout is by the load that is to be supplied. Iin verses Iout is the current ratio, which is the same as Nout verses Nin the turn ratio.

As mentioned earlier, the primary current consists of loss current, the secondary load and the magnetizing current. If high accuracy current transformer is needed, the loss and magnetizing current need to be kept as low as possible so that the secondary current will truly reflect the primary current which is also the current to be measured or sensed. As the result, a low-loss and high permeability core material shall be chosen.

The toroidal core is the most readily used core geometry for current transformer application. There is no air gap in the core so that the magnetizing current will always be small. Toroidal core geometry fits perfectly with flux flow in the core, therefore the core material can be utilized efficiently giving you a small core and small core loss as a result.

Caution:

A current transformer should never be open-circuited while main current is passing through the primary winding. If the load is removed from the secondary winding while the main circuit current is flowing, most of the primary winding current becomes magnetizing current, but the vector angles change in such a way as to keep the total current in the primary the same as before. Because the main circuit is now mostly magnetizing current, the flux in the core shoots up to a high level and a very high voltage appears across the secondary. Due to the high turn ratio usually found in these transformers, the voltage in this condition can reach a dangerously high level, which can break down the insulation. It also becomes a hazard to personnel. The high flux can saturate the core and result in strong residual magnetism left in the core, thereby increasing magnetization current and introducing error in the transformation ratio. We strongly recommend that one put a short on the secondary winding before removing the secondary load while the main current is flowing through the primary winding.

 


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