Transformers energy usage

[leehljp]

Is a transformer plugged in but not used - always burning the full amount of rated wattage?


Here is a statement that I disputed:
"The problem with a transformer is that when they aren't under load, they still have to take that energy and do something with it, hence they disperse it in heat. You have a huge 500W transformer, it's burning 500W of heat when you aren't using it."

I have 4 variacs plugged in at all times. They are all have output set to 120V and each is rated at a max of 20 to 22 amps - which if you were to convert to wattage equals 2000 to 2200 watts. (100V input) I KNOW that they do not consume that much electricity just because they are plugged in all the time.


So, how much electricity do they consume when they are plugged in but the tools/appliances that are plugged into them - are not on?

[LCHIEN]

in theory an ideal transformer will divide the voltage and multiply the current by the winding ratio. So a 1:10 transformer will output 12 volts. If the secondary (output ) load is 1 Amp, then the input will draw 1/10th of 1 Amp, or 100 milliamps.

if there is zero load - 0 amps - then the transformer will ideally draw 0 amps from the primary.

In real life there are losses, but the statement you quoted saying a 500 watt transformer will dissipate 500 watts when unloaded is totally wrong.

In theory the ideal transformers are lossless. In real life they are not 100% efficient. A trasnformer in use may be converting 500 watts, but if you feel it and its a littel warm, then you know its not 100% efficient. Offhand, efficiencies of 90-95% are not unreasonable ao 500 WATT TRANSFORMER delibvering 500 watts might dissipate 5% losses in the transformer itself which would be 25 watts. Imagine the heat given off by a 25 watt light bulb. Then imagine the heat given off by 5 100 watt light bulbs, and you know the transformer when unloaded is not disipating 500 watts.

The losses in a transformer are due to many causes... resistance of the wire its made with, and magnetic inefficiencies like eddy current losses.

An unloaded transformer will not have much loss due to resistance since little current is flowing, but it wll have magnetic leakage and allow some current o flow in the primary. depending greatly on the size of the transformer and also the quality of the magnetics used, an unloaded cheap wall wart transformer may run quite warm from drawing a few milliamps at 120 volts. a transformer leaking 10 milliamps (.010 amps) will be dissipating 1.2 watts. ( which will use 10 KWH per year costing you $1 per year to run at $0.10 per KWH).

This is of great concern to the green people who are rightly concerned that 10's of million wall warts are plugged in on standby since the power switch is usually on the other side of the tansformer. and dissipating some 10's or 100's of milliwatts continuously. Multiplied by 10 or 100 million it adds up to significcant power.

WHy? Well its cheaper and safer to design equipment with low voltage ipnut and power it will wall warts - small transformers. Only the trnasformer needs to be approved, the equipment with low voltage is easier to design and safety approve. The power switch is most logically in the unit and not at the hard to get to wall wart.

Ultimately we will have inexpensive high efficiency switching power supplies mandated that will raise the price over untis with wall warts.

[Slik Geek]

Lchien provides a good explanation. I would clarify, however, that it is the magnetizing inductance that primarily accounts for the power consumption of an unloaded transformer. When unloaded, the transformer appears as an inductor to the AC mains. This means that it still draws current, and thus the transformer dissipates heat due the the resistance of its primary winding (Irms x Irms x Rwinding). This current is called "magnetizing current".

The leakage inductance represents the portion of the magnetic flux that isn't coupled to the secondary, and hence is energy that isn't transferred to the load.

Back to the OP:
If those variacs don't have to be powered all of the time, I would put them on a switch or unplug them. You might be surprised how much current they draw when unloaded. If you have a meter, measure the current. Each of your variacs may draw about an amp of current unloaded.

[leehljp]

Thanks for the answers. Well, I did turn two off in my little shop by putting them onto switched circuits this afternoon, and will put a third on a switch but the one on our 25 year old US microwave built into the cart - it will stay on.

I am surprised at the term wall-wart. What? - no one builds them into the carts, boxes, etc with plenty of space and air vents? Missionaries here must be far more resourceful with esthetics than other transformer users. They are not so common today but this is the way it was done 40 and 50 years ago until recently.

[LCHIEN]

Thanks for the answers. Well, I did turn two off in my little shop by putting them onto switched circuits this afternoon, and will put a third on a switch but the one on our 25 year old US microwave built into the cart - it will stay on.

I am surprised at the term wall-wart. What? - no one builds them into the carts, boxes, etc with plenty of space and air vents? Missionaries here must be far more resourceful with esthetics than other transformer users. They are not so common today but this is the way it was done 40 and 50 years ago until recently.

More on Wall Warts:
http://en.wikipedia.org/wiki/Wall_wart

this device is great for monitoring power usage from small appliances up to HF DCs and are available at HF and on eBay for aroun $20. Readout the line voltage, current, power, power factor and can integrate power usage over time (to get watt hours) to monitor devices that switch on an off like a refrigerator.
http://www.p3international.com/produ.../P4400-CE.html

[LCHIEN]

thanks for the help slik geek. magnetics was not really my strong point.

I just measured my Variac autotransformer, rated for 120V and 16 Amps max.
Off, of course it drew 0.00 amps. On, but with the output unloaded it drew 0.03 amps at any setting of the dial. So that's almost certainly primary leakage current.

[leehljp]

More on Wall Warts:
http://en.wikipedia.org/wiki/Wall_wart

Whoa! The ones that I have - don't look anything like that. Now I understand why you refer to them as "Wall Warts". Mine are more like this, only older and no onboard switch.

this device is great for monitoring power usage from small appliances up to HF DCs and are available at HF and on eBay for aroun $20. Readout the line voltage, current, power, power factor and can integrate power usage over time (to get watt hours) to monitor devices that switch on an off like a refrigerator.
http://www.p3international.com/produ.../P4400-CE.html

I gotta get me one of those!

[LCHIEN]

Whoa! The ones that I have - don't look anything like that. Now I understand why you refer to them as "Wall Warts". Mine are more like this, only older and no onboard switch.



I gotta get me one of those!

I own two of them (P3 Killa-a-watt), I use one at work, it's easier to use than some of the instrumentation I have access to.

As for the wall warts, they are used here for AC adapters for battery-powered equipment, and for small appliances using around 10 watts or less of power and typcially do 6 to 18 volts or AC or DC rectified.
Variacs are very uncommon in the US for residential use - most everything is 120V only and even 220V appliances from abroad would use a 2:1 step up transformer instead of a much more expensive and easy to abuse variac. By abuse, I mean some kid or adult playing with the dial could destroy connected equipment very easily. Variacs are basically confined to the electronics sevice and testing labs, for running lights or heaters at partial power, or for testing operation of equipment at line voltage extremes. Mine at home at and work looks exactly like your linked picture. Variac was a registered trademark of General Radio and the proper generic name is an autotransformer or variable transformer and even autoformer, however the name variac has lapsed into a generic term.

Variacs are characterized by using the same winding tapped for input and output, and one serious concern for use in consumer gear is that a single point failure of insulation in the winding can expose the output to full voltage. Most variacs are used in industrial equipment as voltage controllers and test gear. You might consider trading out your variacs for isolated winding step up or step down transformers for the specific voltages you need, for more safety. I understand that 100V is an odd voltage specific to some parts of Japan, however, there should be enough local market that step up transformers to 120 or to 220 should be available.

[leehljp]

Thanks Loring, great advice from a great guy! Now, for me . . Got to get moving into the electronics age . . .

[KenBurris]

Now, for me . . Got to get moving into the electronics age . . .

By the time you get there, you'll be back here !

[leehljp]

By the time you get there, you'll be back here !

Well, I was just being kind to myself! Somethings I like, somethings I don't.

One thing I do like - the toilet LIDS in some toilets raise up when you walk into the bathroom. There is a setting for the seat to raise and lower automatically too! LOML will go for that. These have been around a few years here and I understand that they are being built in the US now.