Actually, the microwave oven transformers are true engineering marvels
in which the $2 technology was delivered for $1 safely and effectively.
Most of these transformers are ferroresonant types which rely on
saturation to regulate the output voltage. Ferroresonant transformers
will run quite hot as the core losses are 2 to 4x per pound those of
high quality linear transformers but they do the one thing that no
linear transformer can do, deliver a constant voltage with no active
devices.
Tomm
Will Matney wrote:
>Marv,
>
>I can see what they're doing, probably running the transformer and tube maybe
>at 25% duty cycle by pulsing it. The thing is, even doing this, the
>transformer is going into saturation most likely. Hipersil or M-6 states that
>it can be ran up to 17 kilogauss (max), but they recommend 15 kilogauss. The
>reason being is that after 15 kg, the magnetizing current starts to raise
>rapidly, and that means the permeability will go to crap. That goes into what
>they call incremental permeability when the straight line starts to bend over
>into a knee and at some point in higher flux density, it will top out and
>begin a sharp fall. At the top of course is the highest permeability that can
>be reached but is well out of the linear region of either it or flux density.
>When they do the test to obtain a BH curve using a hysteresis loop tracer (The
>BH curve is inside 1/2 the hysteresis loop and divides it in half), and a
>scope, the curve pattern will go up linearly, then bend to a knee, keep goi
n
> g to a point until it takes a sharp nose dive back to the start then form the
> negative portion of the waveform. That sharp point is saturation. Believe it
> or not, it is around 20 kilogauss at this tip on a lot of material, but the
> permeability has fallen off so low that the primary looks almost like a short
> (but not quite enough current to be one) to the line because of the very high
> magnetizing current. They rate material at a point on down the curve, but
> above the start of the upper knee for maximum flux density. Also, design says
> your better to run a transformer at a flux density just below the knee. That
> is the linear region, and is just below where maximum permeability is, and
> that's below the curve where incremental permeability is. To find maximum
> permeability on a BH curve, you look and see there's two knees. There's a
> small lower knee where the curve starts, then almost a straight leaning line,
> and at last the line forms into a curve again being the upper knee. If
y
> ou draw a straight line from the point where the BH curve starts (where the
> lower knee begins at 0), and go towards the upper knee with it until it
> becomes tangent with the curve, the tangent point is maximum permeability.
> What they've done is throw the book out the window over cost in my opinion.
> If they would have designed it to operate normally, a mans power bill would
> be less too!
>
>Best,
>
>Will
>
>
>*********** REPLY SEPARATOR ***********
>
>On 3/14/06 at 8:35 PM wc6w@juno.com wrote:
>
>
>
>>Hi Will,
>> Not certain but, I believe 1KW. It had a 1uF cap which is typical for
>>that rating.
>>
>> I didn't see a model number anywhere on the frame which was returned to
>>the "alley" after the autopsy; Then, claimed by the Morlocks.
>>
>>73 & Good afternoon,
>> Marv WC6W
>>
>>
>>-- "Will Matney" <craxd1@verizon.net> wrote:
>>Marv,
>>
>>20 kilogauss is even over the maximum flux density of Hipersil or M-6
>>which is 17 and recommends 15. M-2, the best is still 17 kg. I'd say that
>>transformer is running into stauration under full load, and runs hot as a
>>firecracker. In microwave ovens, even on high power, the tube is on for
>>maybe a minute, than off so long, and then on again. The lower powers,
>>this delay is even slower. I've noticed this while mine has been heating a
>>meal, listening to it kick on and off over the fan noise. Since they're
>>pulsing the power, they're giving the transformer time to cool down a
>>little with help from the fan. A 1-3/8 x 2-3/8 core is only 3-1/4 square
>>inches and is way to small for 1 KVA, maybe 500 watts at the most. What
>>size of mocrowave was this in watts?
>>
>>Best,
>>
>>Will
>>
>>*********** REPLY SEPARATOR ***********
>>
>>On 3/13/06 at 6:32 PM wc6w@juno.com wrote:
>>
>>
>>
>>>Hi Will,
>>> No holes in the lams except the one for the secondary termination rivet.
>>>
>>> The numbers are worse than you imagined.
>>>
>>> The core cross section is 3.5cm x 6 cm.
>>>
>>> The primary is wound with (approx.) 103 turns #14; .33 ohms DCR, 34 mHy
>>>inductance. Even with the mag. shunts removed, it takes quite a bit of
>>>idle current.
>>>
>>> The secondary is wound with #27; 95 ohms DCR.
>>>
>>> The above numbers figure out to about 40 Watts coppper loss, and over
>>>20K gauss in the core which is likely another 40 Watts loss at 1KW. Eeek!
>>>
>>> Inspires a new meaning for ICAS: Inappropriate Consumer Appliance
>>>Service.
>>>
>>> I think if I was going to rewind one for something I'd add at least 20
>>>turns to the primary and only run the thing at 750 watts or so to stay
>>>within "amateur" ICAS.
>>>
>>>73 & Good morning,
>>> Marv WC6W
>>>
>>>-- "Will Matney" <craxd1@verizon.net> wrote:
>>>Marv,
>>>
>>>Well, they're getting cheaper, no screws now! Does the lams still have the
>>>screw holes or are they solid? A 1/8" fillet weld wouldnt be too hard to
>>>cut out. The problem with welds here is causing eddy currents that are
>>>higher than normal thus running the losses up. At one tune per volt and
>>>about 12 kilogauss, it would use about a 5 square inch core and be rated
>>>at about 1KVA or just under. I'd say though they're running it higher than
>>>12 kilogauss, maybe 15-16. The easiest way to tell the turns per volt is
>>>if there is any room between the coil and the cores outside legs, snake in
>>>some wire, and do a 10 turn winding on the outside of the coil. Any size
>>>small wire will do. Then, power up the primary and read the voltage of the
>>>10 turn coil. Take that reading and use it to figure the turns per volt
>>>with the line voltage. I use 10 turns so it's easier to figure it math
>>>wise. At least on this one, the secondary connection to the core was
>>>easily cut, some aren't. The shunt can be
>>>driven out with a hammer and a block of wood or a punch. The shunt was
>>>used as a current limiter type of arrangement if I recall, or something to
>>>that effect. On a power transformer for an amp, it would not be needed or
>>>wanted. It's a shame how they've cheapend everything to cut manufacturing
>>>costs using welding on a core.
>>>
>>>Best,
>>>
>>>Will
>>>
>>>
>>>*********** REPLY SEPARATOR ***********
>>>
>>>On 3/13/06 at 12:50 AM wc6w@juno.com wrote:
>>>
>>>
>>>
>>>>Hi Will,
>>>> This one has four 1/8" wide welds down the opposing sides. And yes,
>>>>
>>>>
>>it
>>
>>
>>>>has the shunts but, they are encased in transformer paper so, I'd guess
>>>>that they could be easily driven out without disturbing anything else.
>>>>
>>>>
>>No
>>
>>
>>>>screws what so ever. The lams are even welded to the mounting frame in
>>>>four spots. The "cold" end of the secondary was soldered to a lug which
>>>>was riveted to the lams -- I necessarily disconnected that wire for the
>>>>hi-pot check.
>>>>
>>>> It looks like it's wound at about 1 turn per volt. As the core
>>>>wouldn't practically dissassmble, the only easy rewind would be for a
>>>>filament transformer. The secondary could be sliced out by a careful guy
>>>>and rewound with a bundle of large wires in parallel making a KW filament
>>>>transformer... for say a 4CX5000/10000? Or twenty 813's? :-)
>>>>
>>>>73 & Good afternoon,
>>>> Marv WC6W
>>>>
>>>>**********************************
>>>>
>>>>
>>>>-- "Will Matney" <craxd1@verizon.net> wrote:
>>>>Marv,
>>>>
>>>>Did you check to see if one of the windings was connected to the core,
>>>>
>>>>
>>and
>>
>>
>>>>if the core had a shunt in it? That is what most have that I've seen.
>>>>Another thing that really ticks me about their manufacture, and a few
>>>>other transformers too, is they weld the lams together. They put a weld
>>>>right down one side of the lams, front to back, one bead about 1/4" wide.
>>>>That makes them a bit*h to use for a rewind. A person with a mill, or be
>>>>good with a disc grinder can remove the weld. Also, the shunt needs to be
>>>>driven out with a hammer. This is really good though as it gives you more
>>>>vertical window area for a new coil. What gets me though is why weld them
>>>>and still use screws to hold the lams together?
>>>>
>>>>Best,
>>>>
>>>>Will
>>>>
>>>>*********** REPLY SEPARATOR ***********
>>>>
>>>>On 3/12/06 at 11:57 PM wc6w@juno.com wrote:
>>>>
>>>>
>>>>
>>>>>QST
>>>>> Yesterday, I acquired a 2004 vintage Sharp microwave oven, from the
>>>>>"alley exchange", which was light (weightwise) enough that I thought it
>>>>>might contain a switching power supply.
>>>>>
>>>>> Upon examining the unit, I found that the incredible lightness was due
>>>>>to its construction with frame metal of soda can thickness, and that it
>>>>>contained a conventional transformer manufactured by one Digital Power
>>>>>Communications Co, Ltd.
>>>>>
>>>>> There was an article in QEX about 10 years ago that discussed the use
>>>>>
>>>>>
>>>of
>>>
>>>
>>>>>these microwave oven transformers in plate power supplies. It advised
>>>>>against using them in a conventional full wave bridge but, rather used
>>>>>them in an unorthodox half wave connection.
>>>>>
>>>>> Examining the transformer at hand, it appeared that the insulation was
>>>>>uniform over the full length of the secondary.
>>>>>
>>>>> I disconnected the "low side" secondary lead from the frame and
>>>>>hi-potted the secondary (also the primary just for science...) up to
>>>>>
>>>>>
>>4KV,
>>
>>
>>>>>referenced to the frame, with zero leakage.
>>>>>
>>>>> This suggests that one of these transformers might be employed in a
>>>>>"normal" connection for a low voltage (2500V) KW input amplifier. Or a
>>>>>pair of identical units in parallel for a 2KW input amp.
>>>>>
>>>>> Caution, this suggestion MAY NOT hold true for other makers of this
>>>>>style of transformer.
>>>>>
>>>>>73 & Good afternoon,
>>>>> Marv WC6W
>>>>>
>>>>>
>>>>>P.S. -- The magnets from the magnetron assembly make great refrigerator
>>>>>magnets!
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>*
>>>>>
>>>>>
>>>>>________________________________________________________________________
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>>>>>
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>>>>>
>>>>>
>>>>
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>>
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>
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