Wallside power transformer.
36.5 ohms (DC) primary.
Output listed as 1.3 amps.
Input (of course) 115VAC.
I want to figure out what it costs me while the device is not being used. Transformer is powered up 24/7. PLEASE do not say little. I have heard this all my life. I have about 12 of these devices in my house. This largest one (above) is quite warm when the device is turned off. It is also big.
So, how can I calculate the KWH used while the device is off?
These are called “phantom loads”
Unfortunately the DC resistance doesn’t tell us anything. You’d have to measure it with a full-blown wattmeter.
If you have access to one, a clamp style AC ammeter usually has a shunt included allowing you to plug your load into the shunt, and the shunt into the wall. Choose one of the ports for an x1, x10, or x100 reading, clamp the ammeter around one of the shunts, do the match for your amperage draw. Multiply the amperage determined by 120 for wattage. Assuming you ended up with 4.6A x 120V = 552W, or .552kWh.
This won’t give you the power. The current into a transformer with an open circuit secondary has a large reactive component. Unless you can also measure the voltage to current phase angle you can not compute the power. A wattmeter is your best bet.
You can make an estimate by computing the output volt-amperes. Multiply the maximum output current by the output voltage. Say 24 v. times 1.3 amp… That equals 31 volt-amps. If that is all real power and the transformer is 85% efficient then the power lost is 4.7 watts. This is about the maximum that you would expect to be wasting for each transformer. For 12 of them your maximum power loss would be in the neighborhood of 56 w. In a month your maximum watt-hours wasted would be 562430 or about 40 kilowatt hours.
Remember, that is the maximum. Chances are a lot of the volt-amps at idle are reactive and you don’t pay for them. I would make a WAG that the power loss that you actually pay for is less than 20 kwh/month for this sample problem.
[QUOTE]
*Originally posted by David Simmons *
**… Say 24 v. times 1.3 amp… …
That’s the full load usage, correct? Is there no way to figure no load ?
You’d measure no-load current (which for a plain transformer is called excitation or magnetization current) the same way, only with (duh) no load attached, and you’d measure input voltage and curent, instead of the output. If you then multiply by volts in, you’ll wind up with no-load VA. Let’s say our transformer is pluged into 117 VAC, and with no load attached, the primary draws .08 A. That works out to a no-load loss of 9.36 VA.
I should have noted that. This assumes that all transformers are loaded fully at all times, which isn’t the case. The is an extreme worst case and if you can stand about $3/month full-load loss then the no-load loss should be a piece of cake.
There is no way you can compute no-load loss without knowing the input current, voltage and the phase angle between them.
You say the transformer is “quite warm” even when unloaded. I would be more concerned with that than with the idle power loss. Can you squeeze the transformer with your hand at idle and still hold onto it? And make the same test with the load. If you can’t it is too hot and there is something wrong. Maybe a shorted turn. Most such devices have a maximum allowable 30-40 C. temperature rise. For most houses this would be 60 C or about 140 F which is right at the maximum most people can take with good thermal contact such as squeezing a metal object.
See http://electronics.howstuffworks.com/question291.htm.
FYI, this question gets asked about twice per month here. The SDMB search feature isn’t perfect, but you can try it before asking a simple question. Oftentimes you’ll get a hit.
Q.E.D.
I,m sorry, I should have said that I do not have an AC current meter.
David Simmons
Thanks for the warning. It is hidden behind the desk, but I will grab it later and see how long I can hold it.
And remember if it’s that hot on the outside, it’s much hotter deep in the windings, since the insulation helps keep the heat in, causing a temperature drop. In fact, you can use this effect to roughly calculate the watts loss. Meaure the DCR of one of the windings cold, then let it warm up, plugged in but unconnected to a load. Measure the DCR again, and using the thermal coefficient of copper, you can calculate the temperature rise. My books are still packed away, so I can’t find the relevant formulae, but it is then possible to work backwards and calculate the loss roughly from the winding temperature rise. No, it’s not very accurate, but it’s kind of a neat way to get a ballpark figure.
Last month the neighbors house almost burned down. The cause was a wall-wart transformer from a cheap answering machine. Apparently this isn’t a rare event. I’d never heard of this before. I didn’t find out whether there was a short in the answering machine, or whether the transformer just had a spontaneous melt-down.
That’s a bit scary. Most of those things carry a UL listing, and one of the requirements for that is that the transformer won’t catch fire under a dead short secondary condition. At EWC, we had to test our new designs for UL listing requirements with a short-circuit temperature rise test. Man that was fun.
Another way to estimate the unloaded wattage would be through crude “calorimitry.”
Find a plastic box of the same size and shape as the transformer. Place a small 5-watt bulb within it, let it run for an hour or two, then feel both devices in order to compare the temperature of the transformer with the temperature of the bulb enclosure. If the transformer is hotter, then it’s drawing more than five watts. (Perhaps include some fiberglas in the bulb enclosure to distribute the heat more evenly, and to keep the bulb from touching anything flammable.)
Now I wonder if the burning transformer was simply plugged into a wall outlet? Many people plug them into extension cords (and perhaps lay them on the shag carpet, and perhaps throw a pile of blankets on top for good measure.)
Also, I wonder if cheap manufacturers ever use the wrong lamination material (too thick, or using steel rather than silicon-iron.) If steel is cheaper than the proper material, and the things are being made in some tiny overseas country, all bets are off. Ship the proper transformers first, then ship the 5x cheaper fire-hazard transformers later on when nobody is checking.
I have an old VHS tape-rewinder with a bum transformer internally, with buzzing laminations. I only plug it in when actually using it. I have visions of the long-term vibrations causing the coil insulation to abrade away, and shorted turns to appear. It’s not hot though, just noisy. (I wonder if most wattmeters would pick up the nonlinear stuff, the Barkhausen core losses, etc.)
Not where I used to work. But then our primary customers were military, government and aerospace sorts who expected our stuff to be MIL-T-27 compliant and all soldering to be done to QQ-S-571. Everything got tested to AQL I or II levels, and consistent bad batches being shipped got us an angry source inspector up our ass. Of course, we wern’t your average wall-wart manufacturer.
It is my opinion that our electrical equipment codes are inadequate. It is possible to burn a building down with all wiring within code and all appliances UL approved. The wiring circuit, for example, will have a 15 amp breaker for #14 wire. You plug an appliance using #22 wire leads into that circuit and it can start a fire without ever blowing the breaker. I think the UL should require each appliance to be fused consistent with its wiring.