Re: vfd load reactor suggestions?
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Hi Vedran
Don¡¯t know what the B-H curve for Microwave Oven Transformer looks like . It would make sense to operate near saturation as that would give a constant voltage feed for the magnetron
Maybe one of our members with a spare transformer , a variable supply and a couple of multimeters could measure it for us
Don¡¯t think it would impact this application as the value is not critical
Regards,
Carvel ?
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From: [email protected] [mailto:[email protected]] On Behalf Of Vedran via groups.io
Sent: Sunday, March 23, 2025 10:47 PM
To: [email protected]
Subject: Re: [emcoV10lathe] vfd load reactor suggestions? ? I'd chisel or hacksaw-off the secondary... 2000V is not something one just shakes off and hobbyists sometimes tend to keep kludges outside of appropriate enclosures. No need to press out what is left of copper inside the core after you cut the exposed parts off. Broken microwaves are regularly found at e-waste and metal scrapping places. Transformers are usually undamaged.? When taking one apart there are a few things to know 1. They contain a HV capacitor... usually it has a high value resistor in series so it drains in minutes to hours after being unplugged, but always assume the thing is alive and deadly. These also go up to 2000V. You can use a high value resistor across its terminals to drain the charge but you need to connect it with a known-good insulated pliers or similar tool. If you think the thing is discharged, confirm by sticking something metal across its contacts... needle nose pliers work pretty good. This tool also needs to be well insulated. And if it turns out the thing still has charge, you'll get a loud crack, big spark and a black mark on your tool to remind you of the event. 2. Magnetron contains two big and somewhat powerful ring magnets. These can be extracted, but they are secured with white or pink ceramic heat resistant material. Supposedly white is aluminium based and not particularly dangerous, but pink one is beryllium based and is a big health hazard if you inhale it (in dust form). I'm not sure how much of a risk this is, like if you'd have to sand the thing and sniff the dust or would just breaking the thing apart and being in the same room be enough. I also have no idea if color is actually tightly correlated to the material type, ie. if the white ones are always safe. In any case, do your own diligence if you plan to mess with one. As I understand, MOT cores are made to run on the edge of saturation - they even have magnetic shunts to dump a bit of flux. Does this matters in our case? You want magnetic coupling in transformers and in common/differential mode chokes, but not in this case. ? ? The goal is to have the magnetic fields linked.
Hi Bill
An interesting idea ?
You would just need to find 3 transformers with intact primaries with the same inductance
Coincidentally this might even be in the correct ball park ?
I have a figure in my head for the VFD inductor of ?10-30 ¦ÌH/kW? . . . maybe someone else can confirm? . . . and a quick search throws up values of 20-30 ¦ÌH for medium size microwaves ? !
Just need to have someone with a multimeter that can measure inductance and start scrounging !
If you do go this route and leave the secondary winding ( unused for this ) in place just make sure the output terminals are well screened as you could have very high voltages appearing across them when there is current flowing through the Primary !
Regards,
Carvel ? From: [email protected] [mailto:[email protected]] On Behalf Of Bill Nite via
Sent: Sunday, March 23, 2025 7:05 PM
To: [email protected]
Subject: Re: [emcoV10lathe] vfd load reactor suggestions? ? If you¡¯re looking for cheap, I have a DIY choke idea for you. The concept design came from a project I was researching to convert a straight AC welder to DC output, so the purpose of the choke was to help smooth the output of a bridge rectifier. I don¡¯t know how if its capabilities falls within the range of what¡¯s needed to protect your motor from the high voltage in your situation, but maybe Carvel or others can chime in to add their expertise to either say it won¡¯t work, or possibly suggest tweaks to make it work. The design centers around the transformer from a scrap microwave oven. You cut the weld on each side of the transformer that holds the two halves of the laminations together. This allows you to remove the two coils and replace them with a single coil that can handle the current it needs to pass. In the case of the welder this choke is in series with the stinger of the welder, so it has to carry in the range of 100 to 200 amps, big wire, so you can only get about 9 wraps of wire, but this was supposedly sufficient. In your case you can use much smaller wire which allows more wraps and as a result better smoothing of the signal. Just thinking off top of my head here, but the primary coil originally in the transformer might have enough ampacity for your application. Maybe you won¡¯t even have to cut it apart and install a new coil, just wire through the primary side of the transformer. I¡¯m sure that the number of coils affects the inductance, and that in turn likely affects the frequency that it filters out, but how to figure that out I have no clue.?
Ok people who know the electrical theory please comment. It¡¯ll be cheap but it might not work. Oh and I guess you¡¯re going to need three of these, one for each leg coming off the VFD? Remember though, they're cheap. They¡¯re on the side of the road all the time. I happen to come across something saying that treadmills have chokes on their motors, so that¡¯s another possible source that you can scrap from things people throw away. Search microwave oven transformer on YouTube and you can find all sorts of fun things they¡¯re good for.? Bill Nite (knows enough electrical to be dangerous)
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