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Old 20th Jul 2021, 10:03 am   #9
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 6,223
Default Re: What is a reasonable magnetising current for a mains transformer?

A reasonable rule of thumb is that for transformers up to 50W or so, current taken off-load could be 20% of the full-load primary current. (Because they're in quadrature, this does not mean that on-load current is 20% higher than what you'd expect from simple watts calculations.)

Transformers for higher powers generally have magnetising currents of a smaller percentage.

Just a couple of point's I'd query about Joe's comprehensive post:

Originally Posted by joebog1 View Post
Also, Magnetising current can depend on many other things within the transformer.

3. The winding material. SOME modern stuff uses aluminium wire NOT copper.


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.
First, conductor material should make hardly any difference to magnetising current. Conductor resistance is low, otherwise there'd be a significant voltage drop on load; and as such the low winding resistance contributes hardly anything to the off-load impedance, which is dominated by primary inductance.

Secondly, if copper has a conductivity of 1, aluminium has conductivity 0.6, not 5!! It's actually a worse conductor, not better. Ratio of resistivities is about 1 : 1.6, the table in the link agrees

Copper 1.68E−8
Aluminum 2.82E−8

The advantage of aluminium is it is considerably less dense, so even after using beefier wire gauges in a necessarily larger transformer, relative to copper, you can still end up with something not as heavy which is a plus for aircraft transformers (been there, done it...). But it's a pain to terminate, and minimum order quantities for aluminium wire is huge...
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