A current transformer is a device that “transforms” or “steps down” the current input on the “primary winding” to an alternating current of equal proportion on its “secondary winding,” or output. In this way current transformers can convert a potentially dangerous current to one that is more manageable and easier to work with. Because the output current is proportional to the input, it’s ideal for power monitoring, controlling devices, etc. because we can know what the actual current is on the primary conductor by measuring the corresponding current on the secondary output.
True current transformers are passive devices, meaning that they do not require external power. Rather, they use electromagnetic principles to function. More specifically, they typically contain a laminated core of low-loss magnetic material. Next, a wire is wound around the laminated core. The number of windings, or “turns,” is inversely proportional to the current desired on the secondary winding, as expressed by this equation:
(Secondary Current) = (Primary Current) * (Number of turns on the primary conductor / Number of turns on the secondary conductor). We abbreviate this as Is = Ip * (Np/Ns)
In most situations with power monitoring current transformers the number of turns on the primary conductor = 1, that is to say, the conductor is simply passed through the center hole of the transformer, so in this situation we get:
Is = Ip * (1 / Ns), or Is = Ip / Ns.
The most common “true” current transformer used for power monitoring and power controls has a 5 Amp AC current output, but 1 Amp AC currents also exist. Having said this, many current sensors in use today use a large number of windings, resulting in a very low current output. Many industries are preferring this type of output because it’s easier to work with. Instead, what they often do is add a “burden” resistor to the secondary winding to create voltage. Voltage is defined by this equation:
Voltage = Current * Resistance, abbreviated V = I * R
Using this formula, let’s come up with a hypothetical current sensor. Let’s say that we want to produce 333mV when 1000 Amps are “sensed” on the primary conductor, which in our scenario will be a bus bar passing through the center. If the current sensor has 7500 turns, we would expect 1000/7500 Amps, or 133 mA of current if no burden resistor existed. But in our case we want 333 mV of output, so we can divide 333 mV / 133 mA (or .333 V/ .133 A) and we find that the needed burden resistor should be 2.5 Ohms. Once burdened in this way, we can ignore the amperage output (it’s pretty small after all) and consider this a “voltage output” device. Because the current output is alternating current (AC) the output voltage is also alternating, abbreviated Vac.
Current transformers with a 1A or 5A output should not be left open-circuited or operated without a load when current flows on the primary conductor. Instead, one should short-circuit the secondary terminals to avoid the risk of shock. A device called a shorting block exists for this very purpose. When installing a 1A or 5A current transformer one needs to first short the secondary terminals (typically via the shorting block mentioned), and after the secondary terminals are connected to their load the short-circuit (shorting block) is removed.
Current sensors with a 333 mV output don’t have this risk because the current output is extremely low.
Current sensors that change the output type are called current transducers. The hypothetical current sensor described earlier would be most accurately called a current transducer, yet they often simply get called current transformers because they operate using the same basic principles as a current transformer.