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February 22, 2010 10:36 PM Subscribe
How can I identify the approximate properties of an electrical transformer?
My adventurous friend has acquired some large transformers., and as a cautious engineer wants to be careful to preserve life and limb. How does my friend begin to identify the transformer's characteristics, without identifying markings? It's a single phase device. It would be good to know the transformer ratio, which we think can be found by inputting a very low AC voltage in one coil and checking the output volts from the other coil. Is there a way to estimate a maximum voltage and amperage?
My adventurous friend has acquired some large transformers., and as a cautious engineer wants to be careful to preserve life and limb. How does my friend begin to identify the transformer's characteristics, without identifying markings? It's a single phase device. It would be good to know the transformer ratio, which we think can be found by inputting a very low AC voltage in one coil and checking the output volts from the other coil. Is there a way to estimate a maximum voltage and amperage?
The voltage rating is a function of the insulation inside the transformer, and that's very difficult to determine without taking it apart.
It isn't really worth it to try to figure out ratings like this on old transformers. Send them to the recycling bin and get new ones with known characteristics.
posted by Chocolate Pickle at 11:02 PM on February 22, 2010
It isn't really worth it to try to figure out ratings like this on old transformers. Send them to the recycling bin and get new ones with known characteristics.
posted by Chocolate Pickle at 11:02 PM on February 22, 2010
The wire gauge can give you a ballpark estimate of maximum current, though you'll have to take it apart. Maximum voltage depends on the thickness and material used in the insulation, and will be difficult to determine even if you do take the device apart.
Needless to say, don't use these for any application where it's important not to set things on fire or avoid the death of the user.
posted by Behemoth at 11:10 PM on February 22, 2010
Needless to say, don't use these for any application where it's important not to set things on fire or avoid the death of the user.
posted by Behemoth at 11:10 PM on February 22, 2010
A good experimental project for curious students, as long as you're careful. Measure the dc resistance of both windings. If one is pretty low (few ohms) figure it's the secondary. If one is pretty high (hundreds of ohms) figure it's the primary. Then yeah, you can put a low voltage (few volts) into the primary and see what shows up on the secondary, to determine the voltage ratio. But what voltage is the primary designed for? It's tempting to guess 120 volts, but (looking at the photo) it could be a special-purpose transformer with some strange primary voltage. As for the max current in the secondary, operate the transformer with a load on the secondary and monitor the temperature of the transformer's windings as you slowly and carefully increase the secondary current. When the transformer heats up, decrease the current. (Then there's the question of what frequency the transformer is designed to work at. Since you're in the US, 60 Hz is probable but not guaranteed.) (Disclaimer: I have done things like this in the past and am still alive, but I am in no way responsible for anything that happens to you. Be careful. Don't touch bare wires. Don't let bare wires touch. Don't apply large voltages "just to see what will happen." Don't do anything rash. )
posted by exphysicist345 at 2:47 PM on February 23, 2010
posted by exphysicist345 at 2:47 PM on February 23, 2010
If you really want to characterize the transformer you need to find out where it saturates. Transformers work by passing energy through the magnetic field via flux, but the physical composition of the core (drive frequency, material, and cross sectional area) will put a limit on the highest achievable magnetic flux which puts an effective limit on how much power can be fed through the transformer.
If you've ever wondered why the transformer in a standard 60Hz wall-wart is huge and heavy whereas the transformer in a switchmode power supply (like in a computer or perhaps a cell phone charger or electric razor) is tiny and light, this is why. Specifically, it's because the switchmode supply uses a much higher frequency (on the order of tens or hundreds of kHz) which means that the peak magnetic flux does not have to be as large because there are more cycles. (As an analogy, a smaller bucket, filled and emptied 100,000 times per second moves the same amount of water as a huge bucket filled and emptied 60 times per second.) This means you can use a smaller (= less cross sectional area, less iron, less weight and volume) transformer and still push the same amount of power through it.
posted by Rhomboid at 2:46 AM on February 24, 2010
If you've ever wondered why the transformer in a standard 60Hz wall-wart is huge and heavy whereas the transformer in a switchmode power supply (like in a computer or perhaps a cell phone charger or electric razor) is tiny and light, this is why. Specifically, it's because the switchmode supply uses a much higher frequency (on the order of tens or hundreds of kHz) which means that the peak magnetic flux does not have to be as large because there are more cycles. (As an analogy, a smaller bucket, filled and emptied 100,000 times per second moves the same amount of water as a huge bucket filled and emptied 60 times per second.) This means you can use a smaller (= less cross sectional area, less iron, less weight and volume) transformer and still push the same amount of power through it.
posted by Rhomboid at 2:46 AM on February 24, 2010
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As far as I know the only empirical test of max amperage is having two of the thing, and turning up the amperage on one of them until it melts or catches on fire or disintegrates in a puff of smoke, and you know your max amperage is lower than the amperage that destroyed the other one.
posted by idiopath at 10:49 PM on February 22, 2010