Microwave oven transformers

See:

"As it is intended to drive a capacitive load, the leakage inductance of the tranformer is deliberately increased by adding a small magnetic shunt between the primary and secondary coils. The inductance is roughly equal and opposite to the doubler capacitance, and so reduces the output impedance of the doubler. This specified leakage inductance classifies the transformer as non-ideal."

BE CAREFULL!!!

--
Virg Wall, P.E.

"terryS"

** As another poster says, it increases the leakage inductance. ** There is no DC current and only fairly smooth AC. ** Transformer lams are normally stacked as close and tight as possible, plus interleaved to avoid any air gaps.

Only inductors and single ended (valve) output transformers have deliberate air gaps in the cores.

** Oven transformers operate under full load whenever the AC power is applied and have a fan to keep them cool.

If operated with no load, the core will be driven into saturation.

..... Phil

te

Thanks much for the answers. Yes the 'load' is some sort of single diode voltage doubling circuit using, in this case, a 2500 volt 1 mfd capacitor by look of it. So the magnetron presumably gets half cycle pulses of Hi Voltage DC? When the 'other' high voltage half cycle is in series with the discharging capacitor? Yup will be very careful, have worked on high voltage transmitters etc. and well aware of high voltage and up to kilowatt of RF radiation. Yes there is/was or is usually a fan from which some of the air is aimed at the transformer. magnetic blocks between sections. Yes was thinking of single ended non- push/pull audio transformers carrying for example some 20- 50 milliamps of DC plate current to a single pentode. But come to think of it typical laminations are interleaved etc. to obtain a good 'magnetic' circuit, right? But are not laminations frequently insulated 'electrically' from each other to reduce 'hysteresis' (induced currents?) through the metal core? Or am I confusing things here? The term 'non-ideal' rings a faint bell from some almost 60 years ago, so I'll think about that 'leakage inductance' term. I only spent a few days in the transformer department! They were just starting to use 'C' core transformers mainly for 50 cycle AC power IIRC.

"terryS" "Phil Allison"

Thanks much for the answers. Yes the 'load' is some sort of single diode voltage doubling circuit using, in this case, a 2500 volt 1 mfd capacitor by look of it.

** The other "diode" is the magnetron itself.

So the magnetron presumably gets half cycle pulses of Hi Voltage DC? When the 'other' high voltage half cycle is in series with the discharging capacitor?

** Yep.

But are not laminations frequently insulated 'electrically' from each other to reduce 'hysteresis' (induced currents?) through the metal core?

** There is a coating on the lams that provides adequate insulation for that purpose.

The term 'non-ideal' rings a faint bell from some almost 60 years ago,

** No transformer is " ideal " - the closest among AC supply types is the toroidal.

so I'll think about that 'leakage inductance' term.

** Silly name for a simple thing - the simple fact that the coupling from primary to secondary diminishes at high frequencies just as if there was an inductor in series with the secondary.

With small E-core step-down types, that inductor might be 0.5mH.

With the large step-up job used in ovens the number is many Henries.

..... Phil

Audio Xformers have the E's and I's going in the same direction, with some paper to establish the gap width between the two.

Power transformers are interleaved 1X1 or 2X2 to reduce eddy current losses. Lamination thickness also counts - high frequency power and audio transformers either use very thin laminations or special "boutique" iron to limit losses in both hysteresis and eddy currents.

The type of iron, and to a lessor extent how it is rolled, determines the hysteresis. The iron should be "relaxed" to work in a transformer. You don't want it staying magnetized after the current is at zero . . . Rolling the iron sheets aligns the domains, annealing afterwards relaxes them.

Grain oriented, silicon steel

"default"

** ONLY true for single ended, tube output transformers !!!

They are just like inductors and have to carry unbalanced DC current.

All other audio transformers are made with interleaved lams closed up as tight as possible.

.... Phil

Only ones that need to pass net DC in the primary for single-ended applications, most of which involve(d) vacuum tubes.

That does not affect eddy current loss, but it does reduce magnetizing current and copper loss in the primary.

Thickness only affects eddy current losses.

That is largely true.

However, audio transformers needing to work at full power at bass frequencies do not need to be less lossy than power transformers. One thing they do often need is windings interleaved in multiple layers to reduce leakage inductance, which affects performance at high frequencies.

The need for such interleaved widnings is reduced if turns count is skimped in applications where low frequency performance is compromised, or if the application has less requirement for good high frequency performance (amplifiers intended mainly for voice applications, AM-only radios such as "5 tube" radios, and most tube TV receivers).

There are audio transformers smaller than the smallest common sizes of power transformers. They are mostly not fully functional at lower audio frequencies near and below power line frequencies.

- Don Klipstein ( snipped-for-privacy@misty.com)