What is an isolation transformer? Pretty much every transformer is an electrically isolating transformer. The only exceptions are what we call autotransformers where the SEC, or output, is taken from the same winding as the PRI, or input.
Now even though all transformers that have separated PRI and SEC windings are electrically isolating transformers the term “isolation transformer” has generally come to mean one who’s only function is electrical isolation.
These would be transformers that do not transform voltage, current nor impedance but rather simply isolate electrically the output from the input.
This means they have a 1:1 turns ratio. A perfect 1:1 turns ratio transformer outputs exactly what is put into it. Now nothing in life is perfect…all transformers lose some amount of energy from input to output.
These real loses can come from the core and the wire. In addition, there are “apparent loses”. These would be losses from imperfect coupling and capacitances. If some lines of flux do not couple to the SEC from the PRI but rather just expand and contract in space, not breaking over anything, they lose no source energy, but they also deliver no useful energy. They simply reduce the possible amount of throughput energy from PRI to SEC.
We refer to lines of flux that do not couple as intended as leakage flux and like all oscillating lines of flux induced by a current through some turns of wire, they will have an inductance associated with them. This we call leakage inductance.
Now if what you need is a nearly perfect isolation transformer that when you put in, say, 120VAC @ 2A outputs 120VAC @ 2A then you need a transformer that has purposely been designed and made to compensate for real and apparent loses.
Perhaps a 1:1.05 turns ratio transformer. The exact turns ratio for a nearly perfect isolation transformer is a function of what we call transformer regulation. For our example, with 2A in and desirably 2A out; these currents flow through copper wires dropping some small amount of voltage across these windings. This reduces the input voltage that drives the current that induces the flux, and it reduces the output voltage seen on the SEC side.
How much exactly is a function of the wire DCR and current through each winding. Furthermore, frequency can influence the “seen” or effective resistance to current flow; causing heating and larger voltage drops across the windings.
In addition, there will be losses in the core that further decrease core flux density to the SEC side which also decreases the output voltage. Imperfect coupling decreases this further.
Now all transformer designers and manufacturers know this, but few will take the time to do a full analysis to get you a nearly perfect isolation transformer as we will here at CET. They assume what they have to offer is likely “good enough” for you. Maybe it is, but maybe it is not, and it is those times when it is not that we can help!
From big to small, low frequency to high frequency if you are need of an isolation transformer that is as perfect as possible, let us here at CET design and manufacture that transformer for you!