Sim's Electrical Companion
Transformer Construction, Types & Calculations
Single Phase Transformers
A transformer is a simple device that changes a primary voltage / current into a secondary voltage / current. The method of achieving this is so simple, a primary source conductor current produces a magnetic fields and is wound around an iron core to increase its subsequent magnetic field. The lines of flux cut across a secondary winding which induces an emf in the secondary circuit. The secondary voltage will be dependant upon the ratio of windings primary and secondary. If there are an equal amount of windings on both sides the transformers output and input will be the same (excluding loses). If the secondary windings are lower the transformer is considered to be a step down transformer. If the secondary windings are greater than the primary windings it is a step up transformer.
Transformers have a ferrous core which increases the magnetic field produced by the primary coil. There are three common types of core construction Shell, Core and Toroid (pictured right). The toroid has the secondary windings wound over/under the primary windings. Shell and core types the windings are clearly separated.
Illuminance and Luminous Intensity
Illumination is the prime way in which electricity has transformed the modern world. Before electric lighting the only way we knew how to extend our useful working day into the night was to burn things like gas or candles.
Electric light is created in a great many ways, some types and methods are covered in the design section later in this book. For the most part electric light is created through thermal or chemical reactions.
Inverse Square Law
It makes sense that the further an observer is from a light source the dimmer it appears. This is a saving grace or we would all be blinded by the brilliance of all the light sources in the universe at their full intensity!
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A way of mathematically describing this by use of the inverse square law. This shows very neatly why light is dimmer further away, much the same as sound or gravity.
Pick an appropriate unit of distance measurement e.g. a metre, a kilometre, a light year etc. In the illustration I have chosen a metre and four beams of light. The amount of beams per square metre rapidly reduces over the distance of 3m. Expressed mathematically it is clear to see how much the light at the point of measurement has been reduced compared to its source.
Cosign Law
When light is not being directed perpendicular to the lit surface the cosine rule comes into effect. The hypotenuse of the lights angle needs to be determined before the Illuminance at the point of utilisation is calculated.
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E = Illuminance at the point of utilisation measured in Lux (lx)
I = Luminous Intensity measured in Candella (cd) aka candle power
h = Hypotenuse in metres (m)
