Page 107 - Louth Transformer Catalogue

LT DATA

WHAT IS A TRANSFORMER?

AN ANALOGY
A transformer could be considered as a ‘gearbox’ for voltage and current. The primary winding can be
considered as the input shaft and the secondary winding as the output shaft. Current is equivalent to
shaft speed and voltage to torque.

The ratio of the windings on the transformer are equivalent to the gear ratio. A step-up transformer
acts as a reduction gear in which power is transferred from a small, rapidly rotating gear to a larger,
slower gear, therefore increasing torque at the expense of speed. A transformer will do the same,
transferring power from a primary coil with less turns to a secondary coil with more turns, therefore
trading current for voltage. A step-down transformer works in the opposite direction. By transferring
power from the large, slowly rotating gear to the smaller, faster gear, greater speed is obtained at the
expense of torque. A transformer will do the same by transferring power from a primary coil with more
turns to a secondary coil with fewer turns, therefore lowering the voltage but obtaining higher current.

AUTO TRANSFORMERS

An auto transformer has a single tapped winding with a direct electrical connection between the input
and the output. When an AC voltage is applied to a section of the winding, a voltage is produced across
another section of the same winding. For example, if the transformer has a tap at the centre of the
winding and 230V is applied across the entire winding, 115V could be measured between one end and
the tap. The transformer can also, at least in theory be reversed so if 115V is applied between one end
and the tap, 230V could be measured across the entire winding, however losses mean that in practice,
slightly less voltage will be obtained. Auto transformers are usually smaller, lighter and cheaper than
the more commonly found isolating transformer which has two physically separated windings, giving
high resistance between them, even though they are closely coupled magnetically.

LOSSES
An ideal transformer would have no losses and therefore would be 100% efficient. In practice however,
energy is lost due to the resistance of the windings, (copper loss), and to magnetic effects in the core,
(iron loss). Transformers in general are very efficient, large power transformers may have an efficiency
as high as 99%. Small transformers, such as those used to power consumer electronics, may be less
than 85% efficient.

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