Transformer idle amps are too high

[91bronc300]

I have a microwave transformer that I cut the core open on and took out the secondary windings and wound a different secondary for. I was aiming for a 120V to 240V step up transformer. With everything clamped back together and 120V fed into the original primary I am getting 244V from the secondary and the transformer hum is a little loud but not really bad. The problem is it idle at 4.94 amps. That seems way too high. Is there any trick that can be done to bring these idle amps down any?

 

[Radix2]

So this is with the output winding open - correct?

At 5 amps, with no load, the transformer would be consuming 600watts - does it heat up really fast? - probably not?

The "problem" is that the winding forms an inductor that phase shifts the current away from the voltage, so the actual power being wasted should be much less than the 600 watts you are concerned with. The phase shifted current depends on the energy required to magnetize the core and various losses as determined by the winding and core characteristics- wire size, turns, laminations, geometry, etc.

If you could plug into a actual watt meter, you would see what the effective power being consumed is.

 

[Lightning rod]

It's been over 35 yrs since I have had to think of theses things ,but the c clamps may be adding a secondary leakage flux path thru cast iron therefore increasing core losses

Remove the c clamps. Use plastic spring clamps if needed

Let us know

 

[91bronc300]

Yeah, that is with the secondary leads just hanging out in mid air. I guess you're right about if it was truly 5 amps being drawn then the transformer would have to be dissipating 600W of heat and I don't think it is. I just bought a Kill-a-watt watt meter on eBay so I'll check that out when it gets here.

I hadn't even thought about magnetic flux leaking out thru the c-clamps but that makes pretty good sense actually. Like the clamps would be conducting flux away from the windings where it doesn't return when the voltage collapses again. I'm just going to go ahead and weld the core back together tomorrow and then I'll see what the idle amps are without the clamps on. Should be interesting. Then I'll get a final answer when the watt meter gets here.

BTW I want to use this thing to run a 240V heated laboratory stir plate. It's supposed to use 1350 watts with the heater on high and all 5 stir positions on. I hope this transformer will be adequate.

 

[logixjock]

Bronc, how did you open up the core laminations? I'd like to do something similar.

 

[nehog]

These transformers are not designed to be efficient or to run continuous except under certain conditions. Microwave ovens pulse the magnetron to give a lower duty cycle.

I suspect that the core is moving well into saturation, that there are too few turns on your primary coil for the configuration.

 

[91bronc300]

Bronc, how did you open up the core laminations? I'd like to do something similar.

Just look on the side of the transformer for the seam and you will see where it's been welded.

Then use a cut off wheel to grind the welds out and just try and take as little as possible. Once the welds are gone the two halves come right apart. One shaped like an E and the other an I. I made the bobbin for the secondary from thick, un-corrugated cardboard then cut a 2x4 (don't buy at Lowe's, they're not real 2x4s) to fit into the center of the bobbin and held the 2x4 in the vise. This made it much easier to wind the new secondary. Then weld the core back together.

I didn't get much done on this today. As soon as I got outside and the core welded back together the neighbor kid came over with some steel and wanted to make some shooting targets. So I didn't get an enclosure started for the transformer like I wanted or get to test it with the stir plate. I did test the idle amps after I got it welded though and they went down from 5 to 4.25. Also, the voltage dropped from 244 before welded to 242 afterwards. However, the transformer is now whisper quiet, you can barely hear it at all. I did get a chance to test the transformer on a 230V 1A fan from an old Miller welder that was in the garage. The amps jumped up to 10 while the fan was starting and then stabilized at 5.5 once it was up to speed. The voltage dropped into the high 220s while it was starting and stabilized at 239 once it was up to speed. Under this load of about 250 watts it seems to function fine. I'm interested in seeing what readings I get from the watt meter once it gets here.

 

[91bronc300]

These transformers are not designed to be efficient or to run continuous except under certain conditions. Microwave ovens pulse the magnetron to give a lower duty cycle.

I suspect that the core is moving well into saturation, that there are too few turns on your primary coil for the configuration.

Even if I don't get the science of what you're saying I think I get the gist of it. That the transformer is not even really rated for the wattage rating of the microwave it came out of. I am near positive this transformer will be fine to run the stir plate when it is only stirring but it may fall on its face if I try to use the heating element. I realize that and I will just have to see how it does and what its limits are. I may end up having to buy the transformer on Amazon that was linked to a few posts up.

 

[Milton Shaw]

Also all microwaves have cooling fans to control temp. Transformers from microwaves are not designed to be used without the cooling fan and are not designed for constant operation.

 

[91bronc300]

Also all microwaves have cooling fans to control temp. Transformers from microwaves are not designed to be used without the cooling fan and are not designed for constant operation.

I have the squirrel cage fan from the microwave and it will be part of the enclosure I make for the transformer.

 

[Tscott]

I'm going to agree with NEHOG, these transformers aren't designed to remain energized continuously, which would probably mean that not much attention was given to transformer losses in the design process. Additionally, since they are designed to cycle on and off it would stand to reason that the core and it's material would be of such a design as to promote a quick establishment of the magnetic fields and would therefore be much less efficient under prolonged loading. Also, I am guessing that a magnetron is probably more or less a fixed load and cycling it controls the duty cycle and therefore power output when used in a microwave. That being said I would imagine this transformer only operates efficiently along a certain voltage current curve. I would imagine using it as you are, you are pushing it into saturation fairly quickly which would explain the ferroresonance (the hum) you heard before it was welded back up. It will work but you will be wasting that 5A when it's on. I like the ingenuity though.

Tom

 

[91bronc300]

I got the Kill A Watt meter in the mail today and thought I'd post up some numbers for those interested.


------------------------------------Watts-------Volt Amps------Power Factor
Transformer at idle-------------------61----------789----------.07
Stir plate cooling fan-----------------75----------792-----------.09
Cooling fan + 1 stir position-----------100---------816-----------.12
Cooling fan + 2 stir positions----------125---------841-----------.14
Cooling fan + 3 stir positions----------148---------858-----------.17
Cooling fan plus heating element------1360--------1460----------.93

After running things for a little while the core was getting a little warm. So I guess the verdict is it's horribly inefficient unless the heating element is on. But would any transformer be efficient at 10% of its capacity?

Either way, it seems to work.

 

[Gerald O]

Reminds me of my very first microwave oven back in '79. It was from the cafeteria at work and they were going to throw it out. They said I could have it, so I took it and found that the transformer primary had burnt a few windings. I disassembled the transformer just like you did and just rewound it to original spec. Re-welded it and it was fixed! That thing was a beast! It was built like a tank and ran for many more years-- much to my wife's displeasure. She made me get rid of it eventually because it was too 'industrial', and had to buy pretty kitchen model.

 

[Gerald O]

By the way, idle amps are going to be high because these are a low frequency high-power transformer. The primary DC resistance is pretty low. When the transformer is driving its rated load the amps will still be high, but they will be going into the load instead of being wasted as heat. So it will be more efficient when driving a load. Figure this is designed to output around 1000 watts. At that power the primary current would be 1000VA/120V = 8.33A.

 

[wyliesdiesels]

Since were on the topic of transformers (and sorry for the high jack), i have always wondered what attribute, inside a transformer, sets or dictates the VA rating? Is it the size of the wire in the windings?

 

[Gerald O]

Its a rather complex combination of design characteristics. The wire size is only one of many factors. In simple terms, probably the greatest factor is the size of the core. That then determines how much magnetic field you can generate and how much wire you can stuff in.

 

[nehog]

Its a rather complex combination of design characteristics. The wire size is only one of many factors. In simple terms, probably the greatest factor is the size of the core. That then determines how much magnetic field you can generate and how much wire you can stuff in.

A great answer. The core is critical to the design (both size, design, and metal alloy) to total power as is the wire sizes.

 

[91bronc300]

So do you get charged for apparent power by the power company or for true power only?

 

[nehog]

So do you get charged for apparent power by the power company or for true power only?

Yes, you get charged by the power company.

(cool how I ducked the question like a politician?)

Typically you are charged for VA (Volt amps) however when non-unity power factors come into play it gets a bit complicated.

 

[wyliesdiesels]

And those factors usually only happen with 3-phase service!