What is an Autotransformer?

An autotransformer is a transformer that shares one winding for both the primary and secondary coil. The “auto” prefix refers to the single coil acting alone, not that it is for automotive use or automatic operation. Since portions of the same winding act as both the primary winding and secondary winding, high efficiency, small footprint, and large power capabilities are easily obtained. An ordinary transformer has separate primary and secondary windings which are not connected to each other, except for magnetically.  The advantage to ordinary transformers is that they create an electrical isolation between the feed in and out (also known as a seperately derived system).

Autotransformer windings usually have at least three taps where electrical connections are made. Since part of the winding does “double duty”, autotransformers have the advantages of often being smaller, lighter, and cheaper than typical transformers, with separate primary and secondary windings. Other advantages of autotransformers include lower losses, lower idle current, and increased VA rating for a given size and mass.

SolarEdge SEAUTO Autotransformer

where are autotransformers used & How to connect them

Below are some examples of where an autotransformer can be great to use!  We also provide an explanation on how to properly connect things!

I need 240 volts but only have a 120 volt generator or inverter

This is a very common issue in RV’s, solar, and off-grid applications.  Inverters like SMA’s Sunny Island and Growatt only have 120 volt outputs!  How can they power a home that requires 240 volts? 

Some small generators like Honda EU2200I, and EU3000 can be paralleled for more watts but still only output 120 volts! How do I use them to power my battery charger that requires 240 volts?

For this example, we suggest using SolarEdge’s 5kva 120/240v Autotranformer that is coming soon to our store.

From your inverter or generator, you will have one live wire, let’s call that “L1”.  You also have a neutral wire, let’s call that “N”.  Simply connect the wire we called “L1” and “N” to the “L1” and “N” terminals on the autotransformer!  Now, to take power out, simply connect wires to the same “L1” and “N” terminal, as well as the other terminal marked “L2”.  When you provide power to the autotranformer and check with a voltage meter. You should now observe 120 volts from L1-N, 120 from L2-N, and 240v from L1-L2.  Make certain that you have a ground wire connected to both the line (inverter or generator) and to the load being powered.

Phase Balance: tried to load both phases evenly but inverter overloads!

So you learned that your inverter is rated for 8000 watts, but you can only draw 4000 watts per phase at 120 volts.  Now how do you keep your panels balanced?  What happens if two high-draw devices on the same leg turn on and overload the inverter?  How can I make sure I get the full capability of the system?

The answer is simple!  We suggest using SolarEdge’s 5kva 120/240v Autotransformer, coming to our store soon. All you would have to do is put a double-pole 25 amp breaker into your main panel, then connect one pole of that breaker to L1 of the Autotransformer, connect the other pole to L2, a wire from your neutral bar to the N terminal of the autotransformer and a ground wire to keep everything safe!  Now you will have phase balancing that will keep you from unevenly loading your inverter!

I have a 208 volt electrical system but need to run a 240 volt appliance!

In the US, the existence 120/208 volt, 3 phase, wye connected systems and 120/240 volt, 1 phase, “split phase” systems means that equipment must be designed for one voltage or another.  For example a 24 ohm heating element often found in clothes dryers, would draw 10 amps at 240 volts, and convert 2400 watts of electrical energy to heat.  The same 24 ohm resistor now connected to 208 volts would only draw 8.6 amps and convert 1802 watts of electrical energy into heat.

Autotransformers are here to save the day!  As long as the equipment does not have 120 volt electronics in addition to the 240 volt, two pair of  120/12v autotranformers can be connected to boost the L-N voltage to 132 volts, now resulting in 228v L-L, which would bring the voltage within spec of the equipment being powered!  It’s an easy compact solution that could get you out of a pinch!

Where can I buy an autotransformer?

We will soon be stocking SolarEdge 5kva 120/240 volt Autotransformers in our store soon!  Check them out now!

Another type of autotransformer: the variac

A variac is essentially an Autotransformer with a variable tap.  These are great for use in the lab because we can see what happens to equipment when they are fed too high of a voltage or too low of a voltage.  If we wanted a variable load to test an inverter, we could also use a variac paired with a heating element. As the voltage goes up, current draw from the heating element increases, therefore drawing more power from the source.