Microwave transformer reversed? - Page 1
Forum section: electronics / beginners

author: chiken
author_rank: Contributor
timestamp: March 27, 2019, 10:52:10 am
content: Hello, does anyone know what could be microwave's transformer secondary coil inductance, to measure impedance?So if transformer is connected reversely (secondary coil to mains 240V (resistance in ohms is 200, dont have meter for inductance nor impedance) ), what could be output at "primary", i connected it and voltage is 24V, but im interested how many amps (or better VA)

author: Ian.M
author_rank: Super Contributor
timestamp: March 27, 2019, 12:27:07 pm
content: The answer to the VA question is not a lot! Its got a 10:1 ratio and you shouldn't exceed the original secondary current when using it as the primary. Assuming its out of a nominally 800W oven at 80% efficiency, the secondary must have been about 415mA RMS @2.4KV, so its only good for at most 100VA which is 4.15A @24V intermittent duty cycle (e.g. 1/2 hour on 1 hour off). , with fan cooling! For other oven power ratings, scale proportionally.

author: chiken
author_rank: Contributor
timestamp: March 27, 2019, 01:43:58 pm
content: If thats true, that is EXACTLY what i need, max 3AMPS @ 24V,but, why would it heat so much with misserable amperage as that?

author: Cliff Matthews
author_rank: Unknown
timestamp: March 27, 2019, 02:02:01 pm
content: I've never hooked one up in reverse, but the shunts should be removed before connecting it to a 6-ohm load. As I understand it, shunts pre-load (regulate?) so the HV is stable. Judging on core weight, one might expect to get 200VA* without high temps.*edit - Well, I was wrong. I just tested a 700 watt MOT with a "114 ohm primary". I had to turn my Variac to 140v to get 2.7 amps through a 100 watt 1 ohm resistor (it was 7v unloaded..) even shorted, I could only measure 6 amps.Summary: These HV windings have potential and nothing more..

author: chiken
author_rank: Contributor
timestamp: March 27, 2019, 04:34:51 pm
content: ehmmm,So you connected 120V to 100-Ohm "primary" @ 60Hzmaybe with 240V on 200-Ohm @ 50Hz would provide a bit more current, since lower frequency, lower impedance..I guess ill need to wait a bit more and try it directlyBut if i get a bit more than 3A then i dont even need fuses

author: Jwillis
author_rank: Super Contributor
timestamp: March 27, 2019, 04:36:46 pm
content: The shunts are there to limit current to the magnetron by adding leakage induction. Under load the transformers field decreases keeping the core from over saturating.Under no load conditions the core will heat very fast so thermal switches are used to shut down the transformer in case the magnetron fails.MOTs are designed to use as little copper and iron possible so if your re purposing them you need to calculate the potential VA according to the core area. Simply removing the shunts won't work because the air gap still produces leakage inductance.By rewinding the secondary you can eliminate the air gap and get your desired voltage and the transformer will heat less.

author: Cliff Matthews
author_rank: Unknown
timestamp: March 27, 2019, 04:53:40 pm
content: Quote from: chiken on March 27, 2019, 04:34:51 pmYou won't even get 24v unloaded.. put any load on it and you'll get basically nothing useful, that's what I was saying

author: chiken
author_rank: Contributor
timestamp: March 27, 2019, 06:31:52 pm
content: Resistance of real primary is cca 3 ohms,and my point was that i did actually connect secondary to mains and got 24volts on "secondary"but dont have ac ampermeter nor could find 1ohm shunt so couldnt measure amperage

author: Cliff Matthews
author_rank: Unknown
timestamp: March 27, 2019, 07:49:50 pm
content: Lowly hair dryer's do fine as LV loads. Due to a high primary impedance, you'll find the 24v drops considerably under load.

author: chiken
author_rank: Contributor
timestamp: March 27, 2019, 08:30:41 pm
content: Yep, :/thats why i asked what could i expect with this setupAs i said i need 3 amps at 24voltsbut im afraid that it will drop voltage to point where it is useless at that loadBut it looks like that noone really has done that to confirm that

author: Cliff Matthews
author_rank: Unknown
timestamp: March 27, 2019, 09:37:38 pm
content: ..and 3a @ 24v is plentiful, you'd have to live on the moon not to find itYou're not looking hard enough. Place an AD in the local newspaper "cash and pick-up for broken stereo's" - have fun tearing stuff apart, that's how we learn.

author: Zero999
author_rank: Super Contributor
timestamp: March 28, 2019, 10:06:15 am
content: I wouldn't recommend this.In all of the microwave transformers I've seen, one side of the secondary is connected to the transformer's core. To use it in reverse, it would involve connecting the neutral of the mains to the core, which is dangerous because the neutral could be live under certain fault conditions.

author: Cliff Matthews
author_rank: Unknown
timestamp: March 28, 2019, 11:54:58 am
content: Yep, this is why when testing the premise, it was disconnected..