An autotransformer is just like a “regular” transformer in that it transforms one voltage, or current, or impedance to another….
The main difference is it does so without electrical isolation…the transformers most people are used to electrically isolate the SEC from the PRI…electrical energy from the PRI is converted into magnetic energy which flows through the core breaking upon the SEC side inducing a voltage and current (if a load is attached)….
In an autotransformer both the SEC and PRI share the same winding without electrical isolation….
The main advantage of this is size reduction….let’s say for some transformer the PRI turns are 100T’s and the SEC are 200T’s…that’s a total of 300T’s and while the wire sizes almost certainly are different it is still two separate windings…
An autotransformer can have just a single 200T winding and the PRI would be tapped at 100T’s….and indeed the wire size certainly can also differ….meaning you are saving space due to having less turns….
Let’s use a simple example….let’s say we have 120VAC/60Hz and we want 240VAC at a power rating of 240VA…..
In this blog we will ignore small changes to turns which are usually needed to account for voltage regulation and instead will concentrate just on determining turns and wire size….
In the above diagram Vp is the PRI voltage which is 120VAC/60Hz….
Turns, N2, are found as follows:
Now V is 120Vrms, t = (1/4.44)*(1/60), B is the MAX flux density you want to see in the core and A is the core area of the core….
Now let’s make this easy and just set Np to 100T’s….
If you want 240VAC out this makes N1 = 200T’s….
Now let’s look at the currents….load current IL is just 240VA/240VAC = 1A
This is the same as I2 above….now the source current must be 2A above because the power is 240VA and the PRI voltage is 120VAC….
This means I1 above must be 1A in order for the sum of the currents into and out of that node to be zero….
Both N1 and N2 see 1A of current flow so they can each use the same wire size and it should be sized for 1A….if this were a traditional isolation transformer the SEC would be the same 200T’s and use the same size wire having 1A flowing through it….in addition we’d need a PRI with 100T’s with wire that has twice the area to handle to 2A….
Making this transformer an autotransformer has certainly saved us space; making the part smaller….the entire PRI winding of 100T’s using larger wire is not needed in this autotransformer design….
There are other considerations as mentioned previously…there is loss within the core itself plus current flowing within the wires with DCR voltage drops across those windings reducing further the useful voltage….turns should be adjusted in order to compensate for core and DCR losses if regulation is of importance…
We here at CET are experts at this and are happy to help design and manufacture pretty much any autotransformer you may need!
Feel free to reach out and see what we can do for you!