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Cmartinez:
Re: Fabrication of pot-core reluctance gap spacers:
Re: Agents effective against 'Epoxy' adhesives:
Jpanhalt
Shortbus:
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Re: Ballast resistor selection
The Electrician:
Research/info in regards to inductor inrush mitigation and ripple filter considerations:
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Re: Reluctance-gap spacer fabrication:
Authors of content quoted here are urged to review their contributions such that any desired 'adjustments' may be affected in a timely fashion -- Please be advised that any and all alteration of content must be by its author only (thence its 'amendment' here via 're-quoting' of the author-edited content at its original location)...
Note also that author submitted requests for alteration or deletion of content from these entries (and, hence, reference from our tutorials) will be honored at any time) -- Content is 'anchored' here merely as a convenience - Authors retain full ownership of their content!
---Begin member feedback (Ordered alphabetically by user name)---
Cmartinez:
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Re: Fabrication of pot-core reluctance gap spacers:
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/////Here's an addition to my previous suggestion ... so as to "gild the lily" ...
Instead of drilling the sheet stock one by one, and using one of the standoffs as a template, a better idea would be to sandwich several layers (up to ten, for starts) of sheet material for the spacers between two pieces of wood, and tightly hold it together using a couple of c-clamps (or screws, or whatever you can find available for this purpose). The wood would be sacrificial, as you'd be drilling through the whole thing. But the wood, and the pressure applied, would prevent the sheet material from moving, and more importantly, it would produce a much cleaner, sharper, and rounder hole, since the wood's fibers will be pressing tightly against the edges of the hole as it is being drilled. Afterwards, you may remove the drilled material and clamp it between the two washers that I had previously suggested to trim its outside diameter.
I've used this sort of technique many times before, and it always works like a charm.
Re: Agents effective against 'Epoxy' adhesives:
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/////Ok ... after some thought regarding the PTFE spacers, the obvious answer has come to me.
I've owned a signage business for more than 20 years now, and have extensive experience regarding the cutting and machining of plastics. But I have very seldom (maybe once or twice) had the opportunity to work with PTFE. But now I remember.
Like all materials, PTFE is elastic and can be plastically deformed, but in this case it is especially so. When you cut through any material, the force exerted on its surface while the cut is taking place distorts the material in the inner layers. Think of it this way, how can you cut a perfectly orthogonal cube of jell-o using just a knife? The answer is that it's close to impossible.
What's happening is that the ordinary two-flute, spiral drill bit being used to drill through it is tearing the material upwards as the bit penetrates it. Now, drill bits are only really sharp at their lips. So once the material is drilled through, the circle's edges are not really being cut through by the lips, but are rater being teared off by the flute's heel's. That's why the finished hole comes out distorted.
See this diagram so you may have a clearer picture of what I'm talking about.
What we need to do is make sure that the hole's edges are effectively being cut instead of teared off.
The answer to that is a straight-flute cutting tool that has a chiseled tip. And there are many types of that sort of tool out there, but some of them are not easy to find. Here are the two types that I'd recommend:
https://www.homedepot.com/p/Diablo-1-2-in-Two-Flute-Straight-Bit-DR12102/204073486
And another option would be a step drill bit, which not only complies with our requirement of a chiseled-tip and straight cutting edge, but has the added advantage of having multiple diameters to choose from when cutting sheet material:
https://www.homedepot.com/p/Astro-Pneumatic-Step-Drill-Bit-Set-AST9445/301969411
Again, it is important to sandwich the sheet stock between two layers of sacrificial material to make sure that it remains firmly in place while the drilling operation is taking place. In this case I would try two layers of 1/4" hard marine plywood, or two 1/8" thick plates of aluminum. You'd have to test and see what works best for you. So buy yourself a couple of these tools, and test test test until you get what you want.
And yes, punching the material with a round sharpened tool would be an excellent way of doing what you want. All you'd have to do is apply pressure to the material (using a flat piece of wood as backup), using a bench press, and voilá! ... instant excellent results. But although fabricating said tool (which would also punch through the piece's inner diameter) would be a piece of cake for someone like me, for the amateur building this thing it would be not just hard but also a waste of time and money if other, easier to implement options are available.
/////halogen bulbs such as these have an impedance of between 3.8 and 4.0 ohms (I know, because I just measured it) and 5 of them (connected in series, of course) will cost you a bit short of $10.00 dlls in Amazon:
Of course, the impedance will change once they heat up a bit. But it might not matter, because you want them only for startup purposes, if I'm not mistaken.
Jpanhalt
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Max Headroom:
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Shortbus:
As promised a simplified sketch of the mounting plan. The dimensions are based on a 4 inch meter, since I don't know the size, I guessed.
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/////Now for the core disassembly part.
The first picture shows how to measure for the hole positions in the parts to be made.
The second shows the individual parts to be made. While I show one way of making the screws, out of allthread rod, wing nuts, hex nuts, and lock washers, that is not the only way of doing it. An eye bolt that is long enough and with threads the full length of it's shank would also work. I show the "U" type clip nut because most people won't be able to tap the holes for the screws. It has to be the "U" type clip nut shown, there is another type that is just sheet metal without the thread tower that won't give enough strength or threads for this purpose. All of the things shown in this picture are available at McMaster-Carr, most Ace hardware stores will also carry them, or the ones in my area do.
The third and fourth pictures show the assembly of the whole. In use the angle iron pieces would be clamped to the core as shown. Then the screws would be snugged up against the bottom angle irons and a slight pressure put on the assembly with them. This will then start to pull the two part of the core apart. The angle iron keeps the core from twisting and supports it through the process. After the assembly of this "puller" it's into the oven to bring the whole up to heat. After letting it heat for a while it would be removed from the oven and both puller screws tightened a little more, it may take a few times of the heat and tighten cycle to get the job done. The big thing is making sure both screws are turned to keep the pressure equal on both sides.
This 'puller' is similar to how a gear puller works, it's just a glorified gear puller made to fit the job at hand.
Re: Ballast resistor selection
The Electrician:
Research/info in regards to inductor inrush mitigation and ripple filter considerations:
∨I received the AN-10435 transformer from Antek today: http://www.antekinc.com/an-10435-1000va-35v-transformer/ and http://www.antekinc.com/content/AN-10435.pdf
My first measurement is of the turn-on surge. Using a standard shunt to sense primary current and displaying applied voltage and resulting current on an oscilloscope, here's the result. The secondary is open circuited and 120 VAC is repeatedly applied to the primary until it happens that the voltage is applied just as the sine wave is crossing zero in the positive direction.
The yellow trace is applied voltage and the blue trace is resulting current. Notice the scale on the blue trace--50 amps per division! A peak current of 300 amps results. The lights in the room flicker! The applied voltage (yellow) never reaches its normal peak; the high surge current drops a considerable voltage in the resistance of the house wiring! However, subsequent current pulses are much smaller; the second is only about 20 amps peak.
Next, the same test but with a 10 ohm resistor in series with the primary of the transformer. Notice that the current scale has changed to 10 amps per division.
Finally, with a 5 ohm resistor in series with the primary. Here the maximum surge current reaches 30 amps peak. This would be perfectly safe, without risk of damage to the on-off switch.
∨Continuing the saga. Getting involved in all this has now given me an excuse to buy a bigger variac which I've been wanting for a while.
I got one of these: http://www.mpja.com/2KVA-0-130VAC-Variable-Power-Transformer/productinfo/15163+TR/
First thing I did was to measure the maximum turn-on surge as I did with the transformer; here's the result:
The peak current is about 275 amps; this is only a little less than the 300 amps peak for the Antek transformer. I don't show a scope capture, but adding a 5 to 10 ohm ballast resistor decreases the surge to a safe value just as with the Antek transformer.
For the benefit of non-EE readers: regarding the value of these peak currents I should mention that the DC resistance of the primary of the Antek AN-10435 transformer is .218 ohms. Since the peak voltage of the 120 VAC line is 170 volts, one might expect that the peak current ought to be 170/.218 = 780 amps. So why is it only 300 amps? Because the resistance of the house wiring is added to the resistance of the primary in determining the value of the peak current. When the core of the transformer (or variac) saturates, the inductance of the primary drops to a negligible value, and it's only the total resistance of the circuit that determines the peak currrent.
The surge that occurs when the AN-10435 is connected to the output of the variac, and the variac wiper is set to the position on the winding just where the line input is connect is essentially not much different than the transformer alone.
∨The next thing I wanted to measure is the surge due to the charging of the big electrolytic capacitors following the bridge rectifier. I obtained a couple of 34,000 uF 50 volt capacitors and a heavy duty bridge rectifier. I connected everything in the manner of this post: https://forum.allaboutcircuits.com/...-and-construction.113504/page-69#post-1248540 except for some of the details such as the line filter and the over voltage protection devices.
There is no ballast resistor in series with the on-off switch for the following captures.
The line is connected to the variac at a point most of the way up the winding. When the wiper of the variac is set to that same place, the insertion impedance of the variac is at a minimum, and the electrolytic charging surge would be expected to be a maximum there.
When the turn-on surge of the toroidal transformer (AN-10435) by itself is measured, what determines the peak current is the resistance of the house wiring and the resistance of the primary of the transformer; note that the resistance of the secondary is not involved. But for the case of the charging surge of the filter capacitors, the resistance of the variac and the secondary of the transformer come into play, and we shouldn't expect such a large surge.
Setting the variac wiper to the same point where the line is connected and applying line voltage the the input of the variac repeatedly, I captured the maximum peak current into the bridge rectifier. This is the same as the current out of the secondary of the AN-10435 transformer. The first image here is with a single 34,000 uF capacitor on the output of the bridge. The green trace is the current into the bridge rectifier, and the purple trace if the voltage across the filter capacitor:
The peak surge current is nearly 200 amps and you can see the voltage across the filter cap increase rapidly during that first current pulse. Subsequent current pulses gradually decrease and the voltage also gradually approaches its final value. Since this current is the current out of the secondary of the transformer, we could expect that the primary current surge would be less by a factor equal to the turns ratio of the transformer, which is 120/35 = 3.43. This would give a primary peak surge current of 58 amps. Of course, this is separate from the transformer turn-on surge due to saturation of the core.
Next I connected another 34,000 uF capacitor in parallel with the existing one. Again line voltage was applied repeatedly to the input of the variac until I captured the maximum charging surge. Here's the scope capture:
Notice that the peak current of the first pulse is the same as with only one capacitor. This shows that it's the resistance in the variac and the primary and secondary of the AN-10435 that limits the current, not the capacitance (total microfarads) of the filter. However, one difference is that the capacitor voltage (purple trace) increases more slowly.
Now for the dirty little secret. The "heavy duty" bridge rectifier I used for these measurements was this: https://www.digikey.com/product-detail/en/on-semiconductor/GBPC3502/GBPC3502-ND/1057390
Having a look at the spec sheet: http://www.onsemi.com/pub/Collateral/GBPC3510-D.pdf we see in figure 2 that the non-repetitive surge rating for the 35 amp bridge is 400 amps. Since HP wants the power supply to supply 30 amps, for some extra margin of safety I used two such bridges in parallel. Since the peak surge current I measured is 200 amps, and since I have two bridges in parallel (I'll discuss sharing in a moment), the actual peak surge seen by each bridge is only 100 amps, well below the allowable 400 amps.
Adequate sharing of current in the two bridges is accomplished by using longer than needed (12 inches) hookup wire to each bridge from the secondary of the AN-10435 transformer, and making the lengths of the "longer than needed (12 inches)" identical. Measuring the actual currents into each bridge showed that sharing is so good one can hardly tell any difference.
And finally, all this so far has been without any ballast resistor in series with the line at turn-on. With a 10 ohm ballast resistor the initial charging current surge from the secondary of the transformer into the bridges is greatly reduced. Here's a capture of the turn-on current onto the filter caps with a 10 ohm ballast resistor. Notice that the scale for the purple current trace is 10 amps/division rather than the earlier 100 amps/division:
Using a ballast resistor and a switch with make-before-break contacts tames the surge due to saturation of the variac and transformer, and also the filter cap charging surge.
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∨The setup (kluge?) so far:
By the way, the little clamp-on meter seen in the photo would be a good thing for a builder to have. It's a UT210E and can be had on eBay for less than $50. It's a voltmeter and a clamp-on ammeter. The clamp-on ammeter feature not only measures AC current as clamp-ons have done for years, but it can also measure DC current. This is a feature that is fairly new in a low-cost clamp-on.
/////In an earlier post: https://forum.allaboutcircuits.com/threads/rewinding-2dary-of-toridal-pwr-xfmr.149589/#post-1283322
I discussed the effect in capacitor input power supplies like this where the secondary RMS current is larger than the DC output current due to the very peaky current pulses drawn from the transformer.
It's difficult to model and calculate the ratio of secondary RMS current to DC output current without knowing the many parameters involved, such as DC resistance of windings, ESR of the filter caps, apparent impedance of the line at the service entrance, resistance of the house wiring, etc. So we resort to measurement.
For the following captures, the variac was set to output a voltage equal to the grid voltage of 120 VAC which is applied to the transformer primary. The filter capacitance was 68,000 uF (two 34,000 uF in parallel) and for a load I used a 4 ohm, 120 watt rated power resistor (the green thing in the picture above) and severely overloaded it to 400 watts for 5 seconds at a time. The variac was turned up until the DC output voltage was 40 volts and the current in the resistor was 10 amps.
The measured ratio of secondary RMS current to DC output current is 18.6/10 = 1.86. This is not as high as 2 or more as I speculated it might be for a large toroidal transformer, but a value of 1.86 means we have to significantly derate the transformer for continuous duty.
A linear extrapolation tells us that if we want 30 amps DC out of this supply, the secondary RMS current would be 55.8 amp, almost double the rated 30.8 amp output for the AN-10435 transformer. Antek says that their transformers can be overloaded 20% without any problems which helps a little.
Another matter of concern is the ripple current in the filter caps. With 30 amps DC out, the total filter cap ripple current would be 48.6 amps, a value not to be ignored.
This capture shows the current in the secondary of the transformer (green trace) and the ripple voltage riding on the 40 volt DC output. The ripple voltage was about .85 volts peak-to-peak. The secondary RMS current was 18.6 amps:
This next image shows the ripple current in the parallel combination of the two 34,000 uF filter capacitors:
And, finally, the current in the primary of the transformer:
Re: Reluctance-gap spacer fabrication:
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