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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I'm facing an issue with a MOT core I'm trying to rewind.
First it would appear my primary works out to 1.1 volts per turn for 120 Vac input. id like 3 secondary's of 25 VDC each. so that means each secondary should be 18 Vac before rectification. (and this would mean 20 turns to get 18 Vac)
So my question is . . if the label said 1.2 kW, and all I want is 500 watts. should I drop the volts per turn to 0.9 ? would this make a noticeable drop in temp ?
Registered Member #39190
Joined: Sat Oct 26 2013, 09:15AM
Location: Boise National Forest
Posts: 65
If your utility power hovers around 120VAC RMS like mine does, it would make a useful difference to reduce V/turn by about 10%. Beyond that it probably wouldn't matter much.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Yes its about 122.5 VAC unloaded, and drops to 119 when loaded. So 10% is what ill start with. The exact primary math was 1.13 V/t but with significant figures I don't thing it matters to be so exact.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
It can be worth throwing a few extra primary turns on a MOT to drop the volts/turn, if you are going to run the thing at lower power, and especially if you will sometimes run it unloaded.
MO and MOT economics are such that the manufacturer has designed the perfect transformer for the application. He has economised on iron, so runs it well round the corner into saturation. However, the resulting high magnetisation current doesn't matter as a) it's in quadrature to a high load current, so is barely noticable, b) the customer is paying for it and c) there's a howling great fan pushing air over the core and windings.
An unloaded MOT will still draw the amp or five of magnetisation current, which will appear to you as lousy efficiency.
If you want to run it without the fan, you may find the MOT's rating of >1KVA drops to nearer your 500VA!
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
I would recommend putting two (identical MOTs) in series. Both the primary and the secondary. This will make it somewhere near a conservatively rated transformer, with a low magnetising current and you don't need to rewind the primary.
I have a 2S2P MOT configuration to allow about 2kW with several rectifiers at different points to give 15V, 24V, and 43V DC. Apart from my poor rewinding causing sizeable voltage drop, it works quite well.
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MOT Volts Per Turn, please check my math.
