While what you say is quite true for pulse transformers, where Volt-Second capacity and core saturation are directly related, it's for me a bit of a stretch to apply the same concept to an a.c. power transformer where entirely different issues are at play.
In designing a power transformer, core characteristic are more often determined by an analysis of the combined magneto-motive force of all its windings vis-a-vis the BH curves of the core material.
Harry C.
So, why does a microwave oven power transformer go into saturation with no load? I tried one awhile back and it drew over 3 amps from the line with no load. I assume it was running into saturation since it was getting hot.
-Bill
Sean,
The bottom line is the internal temperature should not exceed that specified for the class of insulation. If the unit isn't marked then assume commercial grade class A which is 105C. I have used the resistance change method recommended by UL and find it to be simple and accurate. Also useful for motors and solenoids.
Roughly determine the load from the core size/weight. Measure the primary and secondary cold resistances. The more accurate the better.
Let loaded unit cook for 15 minutes while monitoring for burning odor or smoke which indicates an overload. Disconnect power and load, and measure resistances again.
Use the following formula to determine the hot temp of each winding.
R2/R1 = (x + T2) / (x + T1) solve for T2.
R1 = cold resistance R2 = hot resistance T1 = cold temp (ambient, usually 22C) T2 = hot temp in deg C x = 234.5
If T2 is less than 105C then increase cooking time and measure again. The winding closest to the core should be hotter.
Grumpy
I don't know what's going on, it seems a little lightly built for the job. The core cross section area is 1.25 by 1.75 inch or .00141 sq meter. The laminations are all welded together on a base plate so the eddy currents are quite high and probably account for most of the heat. But I don't think it should draw
3 amps without only eddy currents as a load. The primary only has 160 turns of looks like 12 gauge wire which is a little over one turn per volt. Most power transformers are at least 2 turns per volt.There is a transformer formula relating voltage, frequency, turns, area, and flux density: Erms =4.44*FNAB
and using B=1.1 for transformer steel, I get:
Erms = 4.44 * 60 * 160 * .00141 * 1.1 = 66 volts rms.
Looks like a 66 volt transformer running at 120.
-Bill
Bill: If may paraphrase? "resistance change from cold to hot". Sounds like a smart idea; using the resistivity/temperature coefficient of copper before and immediately after power is disconnected. Later I must look that up and estimate what sort of numbers we would be working with for typical gauges of wire. In my mind it would be the 'ratio' of the resistance change that would indicate the temperature difference; am I right there? (I'm not sure if I've worded that correctly or very well?) Winding resistance of transformer at say, room temperature to start; then after testing at load? I guess it would be most accurate, with most tools (meters) available to us to measure the winding with the highest resistance? Cheers. PS. I've got the remains of a Wheatstone Bridge somewhere! Wonder if one would need to measure that accurately?
The problem with using the AC is that the heating effect will not be just related to current in the wire. Better to do it with a DC current source and just measure the volt drop as you increase the current. The readings from that will give you length of wire and therefore a fairly accurate number of turns, assuming your into length of spirals (don't fancy working that out on a square former). Current would need to be kept small in order to be sure that the temerature wasn't greater where the wires were in the middle of the windings. Using DC would also remove any skin effect too. Getting a bit hypothetical here :)
Good point Mjolinar; but what I was assuming was that one would measure a typical winding, cold. i.e. at room temperature. Then run the transformer under its AC load for a period of time. Disconnect, and immediately measure the selected winding again, using a DC ohmmeter or Wheatstone bridge of some sort. The change of temperature, due to all losses, including core flux and resistive losses, within the transformer which has been turned into heat, could then be determined by looking at the ratio of the two DC resistances? Before and after. Regardless of the gauge of wire or its resistance would not the ratio of those two resistances be an indication of how much hotter the wire and therefore the interior of the transformer had become? Must admit hadn't even thought about skin effect; wonder if that is a concern at frequencies such as 50 or 60 cycles? (Oops sorry!
50 or 60 Hertz. Mustn't forget about him and his Hertzian Waves.) Terry. PS. We had some customers telephone lines at one time that we knew were at or even beyond their design limit. They were several miles out on very fine gauge cable. But they worked OK. Then we had some unusually hot sunny weather and some of these customers then complained that they couldn't get dial tone; the loop resistance had increased due to temperature. Each night the black pole suspended telephone cables cooled down and service returned. I was surprised that the temperature had so much effect; it was the only time I ever experienced that.
I agree that the temperature is what you have to worry about. It doesn't matter what caused it as long as you can reduce the load and keep the temperature under control. Measuring the resistance change will tell you the temperature rise.
Bill K7NOM
--------------- Sure. Yes, I'm sorry, I was tired and forgot which group I was on.
However, people should not really buy a meter without getting educated in Ohms Law and basic circuits at their library. Meters should carry a warning to that effect and extensive pages first on what NOT to do, and why!!:
You see:
-- Each year at Xmas and also Father's Day, particularly, thousands of meters are destroyed almost immediately upon being opened, by folks trying to measure the "resistance" of an active powered circuit, or by trying to measure the "current" capacity of a power source that way, which you cannot do. And many families of wives and children who were talked into buying him this present, unknowing, are disappointed that it never really works and there is great gnashing of teeth that convinces one and all into the very direst technophobia, that spread familialy thereafter, and without any good cause, lest someone finally become educated enough to explain it and break the "family curse". I knew one family that went through three such meters before discovering this, thinking each time that "they weren't really very well made, now, are they?", or that they were congentially too stupid to ever do electronics. Their family of several teen boys had assiduously avoided science in school because of such experiences. And all it would have taken is a couple federally mandated pages in a manual, and whole lives/self-esteem/careers might be saved.
-Steve
Steve; I agree with you and that many unnecessarily go through life with a fixed idea that anything electrical or technical is beyond them! But, IMHO it is also a part of a snobbishness by some people, who, from some imagined lofty educational height have an attitude of; "I'm too much of an intellectual (with a couple of degrees perhaps!) and too highly educated to learn or understand something like that". These are the people who are equally dangerous because they will put 35 amp fuses in a blown 15 amp circuit and when the lights come on think they've fixed it! This business, in our societies, of looking down on persons who understand and can 'actually do things' (and get their hands dirty) is really annoying. Sure dreamers can dream, intellectuals can learn academic facts; but when it comes down to the electricity being off (Hey there's a current topic!) or fixing the TV, the computer or the car, it's at the point that a person who technically actually knows how it works is needed! Some people worsen the situation by using nicknames or abbreviations, the word "Sparky" for an electrician for example. Possibly that's Ok within a peer group of electrically minded equals, or in the navy perhaps, where job functions and skills are recognized; but in society generally it is in my opinion demeaning. When we post in this group asking for advice/guideance we are sort of opening up to others acknowledging we don't know and asking for help on a particular item. That was a gracious apology btw. Terry. PS. Our toaster went on the fritz yesterday! Haven't a clue yet what's wrong! First task is to drill out the screws holding the end with the controls into the main frame of the unit, which have rusted and won't budge! I may be back asking for advice; stand by!
Best advice...Unplug it first.
Find where the fuse is, saw the head off a bolt correctly chosen for it's similar diameter to the fuse and insert in place of fuse. If resistance of said bolt is too high then use a copper nail instead of said bolt.
Hope this helps :)
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