# Autotransformer and fan

#### Article By : John Dunn

Making a cooling fan's motor serve using two different line voltages (nominally 120V/240V) put a burden on a power transformer's magnetics for the higher line voltage.

Some years ago, I wrote an essay entitled “Using One 115V Fan On Two Line Voltages.” Its essence was that a fan motor rated for one line voltage could be switched to running at twice that line voltage as in 120V/240V service by having a power transformer’s primary windings serve as an autotransformer.

The gist of that essay is in these first two illustrations:

Figure 1 Direct power delivery.

Figure 2 Autotransformer power delivery.

What I omitted was a look at how much of the fan’s power would need to be delivered via the magnetics of transformer T1. A look at the following SPICE model addresses that issue.

My version of SPICE does not include an autotransformer. Therefore, I had to devise a model using two 1:1 turns ratio transformers. The left side, the yellow shaded windings, is where the autotransformer properties are exploited. The coupling between the two right side windings allows this model to function in step-up or step-down service as in the following two illustrations:

Figure 3 Autotransformer step-up.

Figure 4 Autotransformer step-down.

Putting this to work on a one-hundred watt load (an arbitrary wattage choice made here just for purposes of illustration) in step-down fashion, we can examine the power transfer through the magnetics by examining voltage and current in those two right-side coils as follows:

Figure 5 Magnetics power fraction.

Half of the load power is carried to that load via the magnetics.

I had once worked out this conclusion algebraically, but those notes are long gone. Still, it felt good to see that result confirmed in this way.