Step Down

How much power is consumed by a step down transformer.?

Like many Americans associated with the US government, I live and work overseas, more specifically Europe. We use step down transformers to power our American (110 volt) appliances. There is an ongoing debate on how much power is consumed when the transformer is plugged in but the connected appliance is not turned on. Typically the transformers we buy come rated with capacities of 75, 300, 1000, 1600 and 2000 watts. I would hope there is a formula to compute the answer.

There are two components to transformer losses – winding losses and core losses. Winding losses come about when the transformer is under load – current in the windings causes heating (mostly resistive heating) that is proportional to the amount of load on a transformer. As transformers become larger, the inherent resistance of the transformer becomes proportionately smaller. Conversely, smaller transformers tend to be lossier than larger transformers. Transformer designers attempt to achieve an optimum compromise between losses and cost.

The other component is core losses. This is the energy required to excite the iron core of the transformer. Core losses are partially resistive, and partially reactive, but the total results in current that must pass through other system components giving rise to additional resistive losses.

On larger transformers, core losses can be quite small – a typical number for large power transformers is 0.05% of the rating of the transformer. I would expect smaller transformers to have higher core losses, but it’s unlikely that there is one formula that will give a precise number for any given transformer since core losses are a function of the proprietary design of the transformer.

There are two ways that core losses can be reduced. One is to understand (measure) the magnetic characteristics of the steel used to make the core, and then stack the core laminations in a way that will minimize the magnetizing requirements. The other is to use special ‘amorphous’ steel to make the core. Both approaches are expensive. The manufacturer with which I am most familiar used a modified version of intelligent core stacking to achieve reasonable efficiencies in large power transformers, but that process was too expensive to be applied to smaller transformers. Amorphous steel was offered as a premium-priced option for small power transformers (the size typically used in residential distribution – for example, on the pole down the street from your home), but few purchasers were attracted to that option. Purchasers of industrial-sized typically do loss evaluation studies that establish the upper limit of efficiency that they are willing to pay for.

So the point is that there are both technical and market reasons for transformer lossiness to increase as the size becomes smaller. It is possible for there to be regulatory constraints on losses at the lower end of the spectrum, and one could argue that society should insist on such constraints. But absent regulatory limits, if you extend the trends to the very small transformers you are asking about – the very lowest end of the economic scale – I would expect to find that transformers will have much higher core losses. I’m just guessing here, but numbers in the range of 1-5% of rating would not surprise me.

Sick Of It All – “Step Down” EastWest Records