Transformers VT relationship with error?

Question

In a transformer I apply a low percentage of the rated primary voltage and get a secondary output of the primary divided by the VT ratio. On many transformers this doesn't hold true, and I get errors over 15%. However, when applying higher voltages, the error isn't noticeable. Can someone explain the relation of VT and error?

Thanks.

E

jvdberg@ieee.org · Accepted Answer

What you noticed is not an error. In transformers there are losses, due to the ohmic resistance of the wire. (there are also magnetic losses) To meet the specifications, the ratio of turns is not equal to the ratio of the voltages. When loaded with a resistive load at the specified voltage and current, the voltage ratio will meet the label values. At a low output current, the secondary voltage will be higher. With a pulsed load, like a bridge rectifier and capacitor, the secondary voltage will be lower. For calculations a transformer will act as an ideal transformer with a resistor in series. Very small transformers and small transformers with an extra isolation between primary and secondary have more losses than big transformers. In small transformers a relative large amount of available space in the core is used for insulation. So thinner wire must be used to fit into the available space. Small transformers have a big surface relative to their power so they can easily dissipate the heat caused by the losses. When a transformer is used at a much lower voltage than rated, those losses are higher than necessary. A lower number of turns could have been used, leaving space to use thicker wire