Adding Primary turns to a Microwave Oven Transformer,  22Volt secondary - Page 1
Forum section: electronics / projects

author: Qmavam
author_rank: Regular Contributor
timestamp: June 29, 2020, 09:22:06 pm
content: Hi all,I removed the HV winding from a MOT, and thought I could just wind a new secondary. I find the Primary magnetizing current is very high. Another group said I needed to add primary turns.I'm trying to figure out where to stop adding turns. I have a graph showing the original, with 37 turns added, 48 turns added and 60 turns added.Can anyone look at the graph and say, that's good enough, put your secondary on.The 22V secondary will have an 8 amp load.Thanks, Mikek

author: duak
author_rank: Super Contributor
timestamp: June 29, 2020, 10:06:39 pm
content: The desired magnetizing current is up to you. You can reduce it but then the maximum power the transformer can deliver is also reduced. I think the function is roughly reciprocal ie. Imag = f(1/N) - try plotting magnetizing current as a function of the number of turns for your expected primary voltage.I remember reading an article somewhere that described the rationale behind MOT design. I thought it was this one:https://www.qsl.net/kh6grt/page4/xfmr/xfmr.htmbut it's not. It's worth a read if you haven't seen it.I understand that for the most part, MOTs run at full secondary current except during the time that the magnetron's heater is warming up and the anode current is reduced. Because the secondary load current cancels out the magnetic flux developed by the primary current, increasing the secondary current moves the transformer away from saturation. The designer can then use a smaller core that can saturate at a lower primary voltage than for equipment that does not operate at full current.Bottom line, if the transformer will not operate at high load current almost all the time, the losses due to the magnetizing current can be significant.

author: Qmavam
author_rank: Regular Contributor
timestamp: June 30, 2020, 12:29:41 am
content: My use is to power a Battery operated leaf blower. (The battery and charger are gone)Yes, I'll use a bridge rectifier.It will draw 8 amps when running and 0 amps when the leaf blower is switched of.I can't make sure power to the primary will always be switched off, so I should lower themagnetizing current. I should not have much problem getting a 180 watts from a transformerthat did output 900 watts.Any other advice?Mikek

author: floobydust
author_rank: Super Contributor
timestamp: June 30, 2020, 02:52:49 am
content: I see why you are lowering the flux density in the transformer, but it has consequences. You will need more turns on both windings, so copper losses go up.If a typical MOT is 0.8V/turn your (sec) 28 turns may go up to 33. Measure the window size and see if you can still fit the ~20% more (secondary) turns plus the extra primary turns. I'd guess you have a 150T primary to start with.If you are adding primary turns in the secondary window, there is a safety issue in keeping decent high-voltage insulation and spacings between the primary to secondary.Magnetization current is a non-linear waveform and you need to use a decent true RMS ammeter for accurate data or better yet a true power meter to compare those losses in the transformer.

author: duak
author_rank: Super Contributor
timestamp: June 30, 2020, 03:13:50 am
content: I happened to have a 120 V, 50 VA transformer on the bench and measure about 120 mA AC with the secondary unloaded. Extrapolating to 200 W, a reasonable magnetizing current might be 480 mA. 60 extra primary turns should easily meet this and also allow for the output current. As Floobydust points out, is there room?How hot does the transformer get? The last microwave I looked at air cooled the transformer as well as the magnetron.BTW, I found out from that article that some MOTs have a magnetic shunt. I didn't know that. I knew that some battery chargers have them to act as a voltage regulator and current limiter. Does your MOT have a shunt?

author: Qmavam
author_rank: Regular Contributor
timestamp: June 30, 2020, 12:05:10 pm
content: Quote from: floobydust on June 30, 2020, 02:52:49 amAdding turns on the Primary will reduce the turns needed on the secondary.Yes that 0.8y/turn is a accurate. I have lots of room for all the extra turns and more. They are a little hard to get an accurate count, but I measured the voltage ratio between the 60 added turns and the original winding with 100V applied. 60 turns had 30v across it, the original 70v, so... I think the original winding has 140 turns.QuoteI have lots of room for seperation and I have hybrid insulating sheets made for electric motor rewinding.QuoteHmm... I'm using Dave's BM235 meter, I don't know how to relate the unknown primary current waveform to the crest factor numbers listed in the manual, Maximum Crest Factor <2:1 at full scale & <4:1 at half scale. I'm using a 1 ohm resistor in series to measure current.https://www.eevblog.com/files/BM235-Manual-EEVblog.pdf

author: Qmavam
author_rank: Regular Contributor
timestamp: June 30, 2020, 12:14:51 pm
content: Quote from: duak on June 30, 2020, 03:13:50 amIF extrapolating is an accurate method, I'm looking good, with the added 60 turns I have 470 ma of magnetizing current!Loads of room.Quote