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Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
I made slide choke ballast to calm down my MOT 8-pack which not surprisingly wants to draw over 100 amps @ load. But here i'm limited to around 23amps RMS so i thought to made adjustable ballast.
I made coil like this: - wound on 50mm (2") PVC pipe - coil height is 280mm (11") - two AWG12 wires in parallel (i didn't want my inductive ballast to be heater element as well) - 37 turns per layer, 6 layers -> 222turns total
I did few tests with just steel axle (tight fit) inside the PVC pipe but results weren't that impressive. Load current dropped only to 45amps (insane arcs though) but it popped 16amp breaker in seconds. I think my problem has something to do with the core im using?
Registered Member #514
Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295
I presume you are using a solid core? It is just probably saturating. Ideally you should use a laminated core made of transformer grade iron in there, but things like welding rods seem to work well too.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Not quite correct, if a solid steel core saturates then probably a laminated one will too. Transformer steel does saturate at a higher flux density than 'normal' steel, but not much higher. The reason laminated steel is used is to reduce eddy-current losses/heating in the core, which are surprisingly large.
I haven't calculated it but 7220 amp.turns (222turns x 23Arms x SQRT(2)) does seem too high for a (guess) 1.5" diameter steel core.
If you post the diameter and length of the core I can give you an estimate of amp.turns for saturation.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I haven't got my notes to hand at the moment - I'll have a look tomorrow.
Meanwhile; I estimate that the core will saturate with about 60 Vrms at 60 Hz So if you measure the voltage across the inductor with the MOTs connected and it's less than 60 Vrms then it's probably not saturating.
The core will get hot due to eddy-current losses though, even if it's not saturating.
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
I just found this, here is about same size slide choke ballast as mine is, and it has 22A minimum setting. Does core material (or appereance) really effect that much?
EDIT:
Another thing came in my mind also, what if i just unwound that slide choke and wind that wire on quite large UI transformer core so that current is adjustable by airgap. I think that it would work much better.
Core is like this (i have 6 of these, one disassembled):
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
If you want to delay the popping of your breaker, while still drawing the high current through your choke and MOTs, then you could try power factor correcting the load. Motor run capacitors are fairly inexpensive, and available in much bigger capacity than MO caps.
Initially without the load connected, monitor the current as you add more and more capacitors in parallel across your supply. Keep adding caps until they draw nearly 16A, when your breaker should still be intact. They are actually drawing a current that leads the voltage waveform by 90 degrees, in contrast to your inductive load, which draws lagging current. As such, the current through the power factor caps increases the available load current and reduces the supply current. So if your load takes 45A, and the PFCs run 15A, that leaves only 30A, not 45A being supplied through the breaker, delaying its pop.
If you can add more ballast to get your load current down to 30A, then 15A PFC should allow 15A supply to run continuously.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
With a solid steel core, it will be more of a resistive ballast due to the eddy current losses. I'd suggest looking for welding rods or fence wire to cut up, and rewiring your two parallel windings in series.
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
If i make ballast using laminated UI transformer core, how big core i should use? I have 6 laminated UI cores @ 29.4cm^2 core area Im thinking to put 2 of those together to prevent saturation @ higher power. That would make UI core with 58.8cm^2 core area.
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