Figuring watt draw??

Anonymous-0 · Apr 19, 2011

Hey Folks, If I use anything that is 110-130 volt here, I have to use a transformer. Some stuff has watts on the label so I know how big of transformer I need but some just have amps. Does volt x amps = watts (120 volt x 20 amps = 2400 watts or 240 volt x 10 amp = 2400 watts)? or is it more complicated than that?

Thanks, Dave

IaGary · Apr 19, 2011

Yes volts times amps equals watts.

I"m confused though. Isn"t your line voltage a
constant over there in Germany. And you want 120
volts for the US appliance right? So the same
transformer should work for all.

Because a transformer changes the voltage only.

Unless you need a larger transformer to maintain
the load.

I may have just answered my own confusion.

Gary

Jim350 · Apr 19, 2011

Amps = Watts divided by Volts.
Volts = Watts divided by Amps.
Watts = Volts times Amps.

Anonymous-0 · Apr 19, 2011

Yea, there are 75 watt transformers and 3000+ ones. I have a 300 watt one that I use so I can buy cordless tools in the States (cheaper) and run the charger here and a couple other things.Was thinking of something in the line of the HF 120 volt inverter welder but the size transformer needed would make it cheaper to just buy the 240 volt one or buy one over here. Most things I just buy here but I couldn't beat the price of a makita cordless drill, impact driver, and light set (less than half the price having my brother send it to me, just use my transformer for the charger. Fell in to a deal on some 110 volt CF lightbulbs (2 bucks for 10) so thats what I have for lights in the 2 stalls that are outside of town (no electricity) and have one of the 5 in 1 power supplies from HF that I carry with me to plug in to and have to use my transformer to charge that.

Dave

MarkB_MI · Apr 19, 2011

To be precise, transformers are rated in volt-amps, not watts. For resistive loads such as lights, volt-amps and watts are the same. For reactive loads like motors they are not, but in either case volts x amps = volt-amps.

John T · Apr 19, 2011

Dave, to answer your questions:

1) Does volt x amps = watts?
YES if you have a Unity One Power factor
NO if its not a unity One Power Factor

2) Or is it more complicated than that?
YES its more complicated then that

A pure resistive load has a unity one Power Factor, but a high inductive load such as an electric motor or a transformer does not. However, its possible to use a capacitor to offset the inductance which can improve the Power Factor. With an inductive load current lags the voltage but with a capacitive load voltage lags the current (Im sure that helps lol)

Volts x Amps = a Volt Amp, but is only a Watt when you have a Unity One Power Factor CLEAR AS MUD IM SURE LOL

John T

Kirk Grau · Apr 19, 2011

You should see the eyes glaze over when I have this conversation with non-technical clients. Sometimes try the "real/imaginary power" route, that can send them over the edge. Usually just end up stating that it is not really true, but think of power factor as analogous to efficiency.

Kirk

John T · Apr 19, 2011

Been there done that and got the T Shirt also lol. And when I tell them PF is the cosine of the angle between voltage and current and if its unity (voltage and current together like if pure resistive laod) the cosine of zero is one, then they really glaze over lol Its all about real versus apparent power and the utility is reallyyyyyyyyyyyyy concerend about PF

Fun Chat (If youre a sparky)

John T

KEB1 · Apr 19, 2011

Dave,

Without getting into a long involved discussion of power factor, apparent power, real power, reactive power, etc., etc., there's a simple approach.

What mostly drives wire and core size in a tranformer is the current through the windings, not power. This is why most transformers are actually rated in volt-amperes rather than watts.

Therefore, if you know the current (amperes) draw of the loads you want to use, any transformer that has the right output voltage and an output current rating equal to or greater than the current draw of the biggest load (or combination of loads) will work. In this case, worrying about watts just confuses the issue.

If all you have are watt requirements for the loads and a watt rating for the transformer, then buy a transformer that's 50% or so bigger than the watt requirements and you'll be covered for any additional current requirements associated with reactive loads.

There are a number of other possible permutations, but this should cover the vast majority of situations you may encounter.

Keith

karl f · Apr 19, 2011

and if you're planning on using a Hammond organ ('74
and prior with synchronous ac motor) it's best to
either get one for your local market, or use a sine
wave generator at 60 hz driven through a good
amplifier that can deliver the wattage at 120v...

karl f

Geo-TH,In · Apr 19, 2011

Dave,
Most things in the US not only need 120 vac they need 60 Hz. Not sure what the frequency you have in Germany. If you put 50 hz in a transformer, you are going to get 50 hz out. The speed of a motor is also determined by frequency. Inverter technology has advanced to a point here that we can control the speed of AC motors by changing the frequency.

I think some gas generators also incorporate inverter tech to regulate the output and frequency. I'm guessing generators may be too expensive for you to use because of the price of your gas. What if you got a generator made for US that uses inverter tech, replace the engine with an electric motor that works on your power and make 120-240 vac 60 hz from the generator?

Just a crazy red neck idea. In theory, 1 horsepower = 746 watts. If you would find a 1000 watt generator, I would overpower it a little and use 2 hp.

George

guido · Apr 19, 2011

Hello dave2,
Yes,you multiply volts x amps to get watts.
If you have any device, such as a 120V turntable, than the speed will be 1/6 slower.
If I remember right your A/C frequency is 50Hz?
For lights it should not matter.

Guido.

Figuring watt draw??

Anonymous-0

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IaGary

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Jim350

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Anonymous-0

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MarkB_MI

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John T

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Kirk Grau

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John T

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KEB1

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karl f

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Geo-TH,In

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guido

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