I built a choke coil out of a MOT which was recommended in another post to reduce ripple voltage in my electric kiln project and was surprised by how LOUD it was. I have placed it on a piece of plywood with a rubber material between the coil and the wood to help reduce vibration. It's better than...
Yes, more turns -> higher inductance. You'll also fit more turns if you use wire with thinner insulation ,like magnet wire (you are using insulated wire aren't you?).
You should be able to cut down on audible noise by sealing the windings to the core with some kind kind of silicone or rubber sealant, though that won't help any that's magnetostriction of the core itself. You could also ensure the switching frequency is higher than audible frequencies. Ferrite cores tend to be quieter than iron cores (and molded ceramic inductors get both of the above benefits), but that would require a new core.
@DanielVanAntwerp I don't know anything about the circuit you're using this inductor in, so I can't tell you how to change the switching frequency. Increasing the switching frequency will also let you use a smaller inductor, though.
Anyone have a way of testing an inductor during the winding? I don't want to clip the wire if it needs more turns but how else can I get a good measurement? Splicing on a high voltage application seems not so great.
Can you provide more details of how you built this choke? Particularly, the size and location of the air gap(s), and how you are holding the core together. Pictures will really help. I assume you still have the inductor on the output of the bridge rectifier, before the 20,000 uF filter capacitors. You can get an idea of maximum inductance by winding a single turn and measuring it. That will give you L/sqrt(N), where N is number of turns. Max turns is area of opening / cross sectional area of wire.
@Hearth, I'm afraid the old 'can't hear inductors over 20-odd kHz' thing is a myth. Lower frequencies can be louder but higher frequencies don't give silence
@TonyM I never claimed it did. Just that it would cut down on audible noise. They can definitely still be audible, mainly in cases where the converter operates in some sort of burst or pulse-skipping mode.
@PSTechPaul I cut open the welds and separated the E from the I. Then I used two-sided 3M tape as a spacer and a large vise grip clamp to let this set up overnight. The gap looks fine at ~1mm. I bundled everything together with nylon zip ties. They are situated as to not pass through the core area to maximize the volume for wrapping. Then I started with some lighter wire (14 ga) and passed through about 10 times. This gave an inductance of 0.3mH on the LCR meter. I removed the 14 ga and proceeded with the 10 AWG coil. I'll post a picture soon.
@Neil_UK That's me! I guess my choice of a gap-fill (2-sided tape) may be a problem. It felt genius at the time ;) Also, my choice of winding wire seems to have been a bad one.
@Kartman Is there any way to tell if it's saturating? Besides just seeing if a choke with more windings and a higher mH value works better. Not trying to be a wise a&&...part of me would just love to put this project to bed. The other part really wants to see a near DC waveform as a result of all this effort.
Use your ‘scope to view the current waveform. If the choke is saturating, you’ll see sharp changes in the current as the choke will cease being an inductor. Measuring the current can be a challenge - make another winding on your inductor of a couple of turns. Put a 100 Ohm resistor across it and feed into your ‘scope. Poor man’s current transformer.
Pictures would help greatly, as well as a schematic of exactly what you have. Double stick tape is not very good for a gap, and is probably why you hear so much noise. Try something like plastic from a milk jug, and experiment with different thicknesses. A larger air gap will lower the inductance, but also reduce saturation. You can use smaller gauge magnet wire with turns in parallel to get the current capacity you need as well as a more efficient "fill factor". 30 turns should give you about 3 mH.
@DanielVanAntwerp The pictures you provided do not show enough details to be useful. You should show the completely disassembled MOT, and then each step of the rebuild. The location and size of the air gap is very important, and how the laminations are secured. Have the magnetic shunts been removed? Also, it is futile to continue this discussion until you fix or replace your scope. Otherwise we are flying blind.
Your "schematic" leaves much to be desired. It doesn't show the connection of the negative terminal of the capacitor to the other side of the bridge and the load. Please try to use the forum's schematic editor, or something like LTSpice.
FYI: offhand, a spacing of around 3-5mm is probably optimal, based on my experience with cores around this size. To get a more accurate value, I would need dimensions of the core. The core MUST be securely clamped; zip ties probably won't do it. Get a big C-clamp and crank it down. There's about half a tonne of alternating, compressive force pulling the two pieces together, under load. Make sure laminations aren't loose and vibrating; varnish or glue them down if so.
@DanielVanAntwerp it looks like there are holes punched in the laminations that you have used for tie-raps. You should use them with some long bolts or all-thread to compress the laminations. Putting them around the core does nothing useful.
@TimWilliams Thanks for that. I realize that my first attempt leaves much to be desired. It has some nice holes that run through but I just didn't have the hardware. I'm about an hour drive to a decent hardware store so I try to give things a run through before honing in on the details.
@PSTechPaul I think I misunderstood your install description for the choke. I did not connect anything to the negative side of the capacitor. I connected the 2 canisters (plus to minus) in series and then connected the bridge and heater at the "Top" capacitor on the positive.
@PSTechPaul What are magnetic shunts? Please advise. I told Amazon what happened to my scope and they actually refunded my money so I can get another one.
Discussion on question by Daniel Van …
Discussion on question by Daniel Van …