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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Thanks. I just read the article and will work with it. Using a primary of 120v and 50VA power requirement I get an area of 1.2 sq inches for the core and I need 572 turns on the primary. Now I need to get a large E-lamination for experimentation.
EDIT: Does the removal of the I-lamination affect the flux-linkage and current draw through the primary?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, of course it does. Removing the I makes the unloaded primary current skyrocket and the transformer burn to death after about 30 seconds, as I found out by experiment once
If the I isn't tightly clamped in place with negligible airgap, the same effect will happen to a lesser degree: you'll have a noisy transformer that draws too much current unloaded.
Better quality transformers mix the E and I laminations during assembly to eliminate the air gap altogether.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Steve, I noticed that when I put the I lamination back on top the primary inductance almost doubled. As a general rule, can one say that the primary current draw under load will be about Vp/(2*pi*f*Lp)?
Another question if all of you don't mind: Assume I have a primary 120v/60VA and a secondary that is already wound for 120v and 1/2 A. Let's say that the primary has 1000 turns. As I remove turns from the secondary my secondary voltage will go up. Is there some general rule as to how many turns one can remove before the transformer fails to follow the turns-ratio? If I reduce the secondary turns to 500 I should have 240v on the secondary (assume the current under load is fixed at 1/2A). What if I reduce the turns to just one? Would I see 1000v on the secondary? Is core saturation affected by the secondary as well as the primary turns for a fixed core area?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I expect that equation for primary current would not hold in practical transformers, since the inductance is non-linear and the magnetizing current is highly distorted. (it appears as little spikes instead of a sine wave.)
You have to add extra turns to the secondary to make its voltage go up, or remove turns from the primary. I'll assume you meant the latter. But if it's a commercial transformer, the primary already has the minimum number of turns to work off 120V without saturation, as it would be a waste to use more. (If you did, the transformer would either have more copper losses, or require thicker wire and hence a bigger core to make room for it.) So I doubt you could increase the output voltage significantly by removing turns, without making the transformer saturate and overheat.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
Yes, I was distracted and said everything backwards. What I meant to say was as I remove turns from the secondary and the output voltage goes *down*, is there a limit to the minimum turns needed to achieve a result? For example, assuming the primary has the proper number of turns, will a transformer work with just one turn on the secondary to make it a 1000:1 reduction (assuming no significant losses)? Also assume that we won't exceed the current handling capacity of the wire.
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I've taken the laminations apart and I'm rebuilding the secondary winding. If I don't plan to exceed 70 rms volts do I need any material to separate the layers (there will be three)?
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Making one's own transformer
