volts watts and amps - their relationships

[Pat]

Is there a simple explanation on the internet someplace of the relationships among watts, volts, and amps? 

An example.  You have two 30amp outlets.  One is wired with 115 volts, and one is wired with 120 volts.  If idential 1000-watt appliances are plugged into both, would one appliance use more power than the other?  Is the 120 volt wired outlet more efficient?  Assuming 120 volts is preferable to 115 volts, why? 

Thanks.

--pat

 

[Ned]

Outlets aren't "wired" for 120V or 115V, they will show the voltage that is being supplied from the power source.  That is nominally 117V but can vary by +-10% and still be within the acceptable range.  The relationship between volts, amperes, and watts, is Watts=VoltsxAmps, generally, but for most purposes this is close enough without getting into AC theory.  A 1000W appliance will draw more current (amps) when supplied with 115V than when supplied with 120V, but it will still draw 1000W.

 

[Karl]

Pat,

Using a water analogy, think of an electrical system as plumbing in your house or MH. The size of the wire is equivalent to the size (inside diameter) of a pipe. Therefore, a small wire (pipe) cannot flow as much electricity (water) as a larger one. This flow rate is called Amperes, or simply amps. The water pressure determines how strong the flow is, and this is measured in Volts. Sometimes you can open a faucet and the water spurts out with great force. That's because the pressure (volts) is high. Other times, you get barely a dribble; pressure (volts) is low. The other factor is power, and is measured in Watts. Power (watts) is a measure of how much total electricity is flowing at any given instant and, as Ned said, is simply P=E x I where P=Watts, E=volts, and I=amps.

Appliances are designed to work within a specific range of volts - usually from 110 to 125 volts, or in the case of an electric range, from 220 to 240 volts. Let's use a toaster for an example. If it's rated to use 1000 watts of power at 120 volts, you can plug these figures into our equation and see that it will draw 8 1/3 amps. Now let's say the voltage at your site is only 105 volts. Your power usage will drop to 875 watts. Great, you say; I'm using less electricity!! Well, yes you are, but you're also not getting as much 'work' done. Because your voltage is so low, the heating element in the toaster willl barely glow, and it may take twice as long to make a slice of toast! That's why microwave ovens cook slower, air conditioners don't cool as well, lights appear dim, and (some) t.v. pictures shrink. Low voltage.

 

[Lou Schneider]

In a nutshell -

Watts (Power) = Voltage (Pressure) times Current (volume).

As Karl said, if your voltage drops, heat producing devices will generate less heat.  Usually this is the only drawback.

The exception is the motor in your air conditioner.  This is a synchronous motor, that tries to run at a constant speed.  Since Power = Voltage x Current, if the voltage drops the motor draws more current to develop enough power to run at the proper speed.  The higher current creates extra heat inside the motor, and if the voltage is low enough the motor can overheat and burn out.

109 volts is the low limit for your air conditioner.

 

[Ned]

Karl and Lou have done a better job than I did of explaining this.? I was thinking mostly of motor driven devices as Lou mentioned, but for resistive loads, like heaters and such, the current will indeed be lower at lower voltages.

However, none of us have addressed your first question

Is there a simple explanation on the internet someplace of the relationships among watts, volts, and amps?

http://science.howstuffworks.com/question501.htm is about as basic an explanation as I could find on short notice.

 

[John From Detroit]

I think your question was worded wrong.. .The relation between Volts, Amps and Watts is Volts * Amps = Watts

However you went on to ask about two otherwise identical 30 amp outlets, one marked 115 volt and one 120 and how appliances would work when plugged in.

That one is a bit tricker to answer

Espically since 115 and 120 are, for practical purposes, identical voltages (Less than 10 percent difference)

Appliances can be devided into three categories.. Inductive (Motors) Resistive (Lights, heaters, toasters, ovens, the like) and electronic (TV, Microwave, Radio, Battery charger)

Inductive devices may work better with a slightly lower running voltage, but start up better with a higher one, The explanation gets a bit involved but if you have a "Green Plug" or other POWER FACTOR CONTROLLER, it's job is to reduce the voltage when the motor is running for maximum efficency.

Resistive devices (light bulb, heater, etc) will draw slightly more power on the higher voltage, this may mean a slightly higher current or not, depending on the exact temperture coefecent of the heat element, They will work faster at the higher voltage  however the difference between 120 and 115 is such that what takes an hour at 115 may take 59 miutes 45 seconds at 120, that is, you won't notice the difference

Electronics will compensate for the voltage difference and work the same not matter what

 

[Pat]

This stuff is priceless.  I have it copied and saved.  Thanks a zillion.  The water analogy makes it understandable. 

The a/c was my main concern.  I'll watch available voltage before running it. 

Some of the RV parks are not too well wired; although, the one I'm in seems surprisingly consistent for its age.

--pat

 

[Karl]

Time out, John...

A microwave oven is almost purely inductive, except for the electronic timer/heat controls. It's got a huge high voltage transformer as its' main component, which accounts for most of the weightand power consumption. Most other small electronics are also affected by voltage fluctuations unless they have a buck/boost regulator that maintains the proper voltage. Clocks, for the most part, use either a synchronous motor or are clocked to line frequency, so they are exceptions. 

if you have a "Green Plug" or other POWER FACTOR CONTROLLER, it's job is to reduce the voltage when the motor is running for maximum efficency.

The so-called "Green Plug" does not reduce running voltage; it limits the inrush current to a motor for approx. 85 msec's. It does very little if anything to improve the power factor. There is a lot of controversy about green plugs actually causing damage to refrigerator and other appliance motors by something called 'half cycling'; a failure mode where the appliance motor is getting only one-half of the sine wave voltage, i.e. a plus (or minus) half cycle of the ac current. The jury is still out, but all test results I've seen seem to indicate that their use is being penny wise and pound foolish. You may save a dollar or two a month, but if you have to replace the refrigerator compressor after a few years, it doesn't sound like a very good trade-off.   

If anyone is interested, I would be happy to explain power factor in detail, but for us household and RV types, it matters little, and the cost to correct it is prohibitive and it has virtually no effect on our daily lives. If you're a large industrial user, that's a different story. 

 

[blueblood]

? Assuming 120 volts is preferable to 115 volts, why??

Thanks.

--pat

ANSI sets standards for US and ANSI C84.1 (and CAN3-C325) set 120 volts as nominal and the allowable range from 114 to 126. The use of 110,115 and 117 have been eliminated.

Leo

 

[John From Detroit]

Karl, I don't want to argue with you, but most electronics have voltage regulators in them.  I mean, very little in the electronic world runs on 120 vac, most of it runs on something closer to 5 9 or 12 VDC (and yes, I know that some stuff runs on other voltages as well)  To get from 120 vac to say, 12VDC for your portable television you use a power supple conisiting of a transformer, rectifier, filter and VOLTAGE REGULATOR if it's a good design.  And when I build them they are always done that way,  Good regulation is very necessary to getting a proper picture.

Yes, I build them

The load in the microwave is not the transformer, it's the magnatron tube  The transformer can be ignered so long as you keep close to 60 hz (50 in some countries) as it is near resonance and thus resistive, not reactive

However most of the microwaves I've worked on did not have voltage regulation to the magnatron  so different supply voltages will affect them

Finally, a Green Plug works by reducing the voltage on the motor till the power factor approaches one, I've used them, I've measured what they are doing. This matches the power to the load and causes the motor to run cooler, the savings in heat is multiplied during the summer when you use AC.  During motor startup the device should be dormat, that is passing full power to the load.

I have the instructions for making one lying about here somewhere, Never made one, but I have taken one apart and as I said, measured it's operation.  Made a big difference on a De-humidifier