| 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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