Transformers

Question: When hooking up a transformer on a heating or air conditioning unit, the primary line going to the transformer is 230 V. If someone accidentally wired it up for 208 on the primary side of the transformer, would it still produce enough voltage for the secondary side if it were 24 V and 50 V amps?

Also, if primary voltage is 208 and the primary side of the transformer is wired for 230,could it cause a shortout over a period of time?

-Name withheld by request

Answer: A transformer changes voltage in proportion to the rating of its windings; in other words, a transformer having a 240 V primary and a 120 V secondary is a 2 to 1 transformer. It will deliver 120 V when connected to a 240 V supply. But it will also supply 115 V when connected to 230 V input. Likewise, it will put out 110 V when connected to a 220 V supply.

The transformer which you refer to evidently has a primary to secondary ratio of 10 to 1. If you apply 208 V to the primary, the secondary output would be slightly under 21 V. This should to satisfactory to operate the low voltage circuit unless it has some components which are very voltage sensitive. Many transformers have taps which may be used to compensate for input or output variations.

Maximum load current multiplied by the load voltage gives the volt amp capacity for single phase applications. For three phase applications, multiply load current times load voltage times 1.73. The transformer should have this minimum nameplate capacity in volt amps.

In your example of a 208 V primary transformer, it most likely would be a 9 to 1 ratio design. By applying 230 V to the 208 V primary, the secondary output would be 26 V. This increase of less than 10% should not cause a problem. The transformer primary temperature rise would increase slightly which should not be critical. It is recommended that a transformer with taps be used to compensate for these input and output variations.

-Richard Beard, consultant