Transformer excitation current

:?: We have two transformer designs that are supposed to be the same. They each have the same number of turns in the secondary, the same number of turns in the primary, the same laminations (same size, stack height, and alignment), and the same size wire. The only difference in design is that one is hand wound and one is auto wound.

The auto wound coils utilize a slightly larger winding block (because they will be wound tighter).

The problem that we're having is that the excitation current in the auto wound transformers is higher than it should be (it was expected to be lower and more consistent).

Does anyone know what would cause the excitation current to be high?

Is there a gap in the core (intentional or unintentional)? That would increase the excitation current. Is the core material from the same supplier? There may be a difference in the alloy.

The gap was thought of and carefully scrutinized and then determined not to be the problem. The core is laminated and both sets of laminations come from the same box.

There may be a shorted turn. This will reduce the impedance of the winding.

Len

Answer is in the question

You mentioned that the winding block is larger for the autowound coil. The problem with all magnetic and electrostatic phenomena is that the parasitics get satisfied first. By this, I mean that your uncoupled (aka leakage) flux
between the winding and the actual laminations becomes greater the more space you have between them. Another way to say this is that the 'coupling coefficient' decreases with an increase in physical distance.

Leakage flux or magnetic lines of flux are the magnetic fields which do not pass through your magnetic core (aka laminations) before returning to their source. Their is always some leakage flux, you can't avoid it but you can minimize it by keeping the windings as intimate with the magnetizing core or laminations as much as possible.

Think of it as the price of entry. The more space you place between the windings and the laminations, the less intimate they are and the more energy will have to be displaced to overcome this limitation. All energy decreases over distance and magnetic fields certainly follow the inverse square law as well. The energy in the form of excite current is satisfying - first your air core and then secondly the laminations. Kind of like the govt. getting their tax money first before you get a chance to spend it and then taxing you again when you do spend it.

If your autowinding is on the same size and type of coilformer (or winding block) as a manual wind, you will likely see much less variation between the two methods.

Welcome to the forum.

Thanks for the interesting post but unfortunately this thread is five years old.