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Old 19th Jul 2021, 11:20 pm   #5
joebog1's Avatar
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,092
Default Re: What is a reasonable magnetising current for a mains transformer?

Also, Magnetising current can depend on many other things within the transformer.
To mention a few:
1. The physical size and type of core material.
2. The efficiency of the design.
3. The winding material. SOME modern stuff uses aluminium wire NOT copper.

To enlarge a little bit, The size and shape of the core has a huge amount to do with magnetising current. We all know that a modern toroid is more efficient than an older C core or double C core design. It will almost always be more efficient than an EI style transformer.
BUT !!! this may not always be the case, as some very fancy iron used in very fancy transformers can be very efficient. Frequency of operation also has a HUGE influence on the current. A 400 Hz transformer used in aircraft can be one tenth the size of one designed for 50 Hz. Even America with its 60 Hz mains means its slightly more efficient than our 50 Hz.

The flux density that the design is based on will also affect the current. If low flux densities are used, it generally means a very much lower current. If a design is made for a transformer calculated around a maximum flux density of 2 Teslas, thats a fairly large amount of magnetic current roaring around the core. If, the same sized core is operated
at 1 Tesla it will have much lower magnetising current, but may suffer from higher operating temperatures because there is so many more turns in a low flux density design, caused by the longer wire length and consequent higher resistance.

The resistance of the windings will also add to the current. Pure copper ( NOT audiophool grade ) has a conductivity of 1. Aluminium has a conductivity of 5.

The above is very short of a thorough explanation, but will do for now. My technical writing ability is not good enough, nor my aging brain being smart enough to go over all the elements required. I do make quite respectable transformers however.

Colours of wires varies by country and the designer so I will stay out that argument.
BUT, if you run into my transformers, EI will be brown and blue for primary with green/yellow for a screen. It will be designed for a nominal 240 volts.

For dual primaries, 120/240 volt operation they will be:
pri one, brown beginning, orange end.
pri two black beginning, white end.

These are operated in series for 240 volts, and parallel for 120 volts.
Screen will again be green/yellow, if fitted.

Why dont I provide multi taps for the primary?.
I make my transformers to be a generous design which are more than capable of operating on slightly higher or lower primary voltages. These voltages will of course change the magnetising current.

Hope that helps a little.

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