Question about microwave oven transformer losses - Page 1
Forum section: electronics / projects

author: Ben321
author_rank: Frequent Contributor
timestamp: January 12, 2021, 09:50:18 am
content: I have read some stuff that these transformers tend to run hot due to core losses, thanks to them being made cheaply with a poor design. However, it's my understanding that all transformers use laminations to avoid core losses (aka eddy currents). So if there's not current being induced in the core, then they should NOT be getting hot. However, a couple places I've read about online say these transformers do run hot. Is this because unlike most transformers, MOTs actually use an unlaminated, solid iron core? Or is it because they don't use a thick enough layer of insulation between the laminations, so some eddy currents still flow in the core? Or is there some other process that's taking place that I'm not aware of that causes the these transformers to run hot?

author: Zero999
author_rank: Super Contributor
timestamp: January 12, 2021, 10:38:26 am
content: Microwave transformers do have a laminated core. They tend to run hot because they use the bare minimum about of steel and copper for a the power rating. The core runs just short of saturation and the copper losses are relatively high.Some microwaves use a high frequency inverter to drive the transformer, so a much smaller, cheaper one can be used, at the expense of the more complex drive electronics. The advantage is the duty cycle of the AC waveform, at the transformer's primary can be controlled, to vary the power in a linear fastion, rather than just switching it on & off at a low frequency, as is the case with the low frequency transformer.

author: Ian.M
author_rank: Super Contributor
timestamp: January 12, 2021, 11:12:16 am
content: It also doesn't help having welds securing the I laminations to the E ones, rather than having interleaved E and I laminations secured by insulated clamps or resin bonding. The welds are supposed to be in a low flux area of the core but inevitably will result in some local eddy currents.Generally a 20% reduction in input voltage can be expected to keep a line frequency microwave transformer out of saturation. Unfortunately there is no easy way to get that reduction without either another buck transformer (large and fugly) or adding 25% extra turns to the primary (as an extra winding), which involves knowing how to wind a mains transformer primary safely and still achieve class II insulation between the added winding and whatever secondaries you have, without the benefit of a split bobbin.

author: Gyro
author_rank: Super Contributor
timestamp: January 12, 2021, 12:15:36 pm
content: Quote from: Zero999 on January 12, 2021, 10:38:26 amThe 'Copper' is often Copper clad Aluminium too, further increasing winding losses.

author: CaptDon
author_rank: Super Contributor
timestamp: January 12, 2021, 03:17:13 pm
content: I run some MOT's in a high voltage power supply for a lowband ham amplifier. They do run very near fully saturated incurring huge losses. I had to knock out the magnetic shunts (they give the transformer a ballasting action which helps control the magnetron peak current and current shape during conduction) and fortunately the transformer produced way more voltage than I needed so I unwound part of the secondary to get closer to my desired voltage and then added some heavy gauge turns into the empty space and put them correctly in phase and in series with the primary to make the transformer act like it had a 140vac primary so that when run from the normal 120vac it would be well out of the saturation region. The transformer was also wound in a way that allowed me to 'unground' the secondary and drive a full wave bridge without fear of arc-over. It was an older higher quality transformer and nearly twice the size of most you see today. Due to the high resistance of the secondary winding MOT's offer very poor high voltage stability with the voltage dropping significantly under load.

author: richard.cs
author_rank: Super Contributor
timestamp: January 12, 2021, 03:59:16 pm
content: Quote from: Ben321 on January 12, 2021, 09:50:18 amCheaply made, yes. Poor design, I am not sure I would agree. They are designed for a specific purpose, in which they only ever operate at full load (which reduces core flux compared to the no-load condition and means the high magnetising current is unimportant), where short-term operation is the normal use case, fan cooling is present and ambient temperature is modest. They are excellent for cheaply supplying power to a magnetron in a cost-constrained consumer product. That just means they are optimised for a bit of a corner-case compared with general purpose transformers, they're not poor at what they do, and they rarely fail.

author: Gyro
author_rank: Super Contributor
timestamp: January 12, 2021, 07:32:16 pm
content: Agreed, they have been carefully value engineered over the years to be as perfect a functional and cost match to that very specific application as possible. It's when you try to move them into other applications that things start to go downhill.