From http://international-electrical-supplies.com/transformers16.html  
 

How To Choose A Transformer!
What Is A Transformer?

In its most basic form, a transformer is a laminated steel core wrapped with insulated copper wire with at least two coils or windings. A transformer, again in its most primitive state, has no moving parts and is normally a trouble-free device. Transformers are designed to step up (increase) or step down (decrease) AC voltages through the principle of mutual inductance which causes a voltage to be induced from one coil into another coil because of the relative motion of the collapsing and expanding magnetic field resulting from the constantly alternating current (60 times per second). The primary coil is the input coil or power source and the secondary coil is the output coil or load.

The core is not a solid component but rather is many layers of thin steel called laminations. This core is the "link" that magnetically "connects" the primary winding to the secondary winding. When AC power is applied to the Primary coil a magnetic field is produced within the core, also known as Flux, that induces voltage in the Secondary coil or output.

The ratio of the number of wire turns in the primary windings versus the secondary windings determines whether a transformer is a step up or step down. For example, 120 turns on the primary versus 30 turns on the secondary is a 4:1 ratio and if the input voltage was 480 volts the secondary or output would be 120 volts. This would, obviously, be a step down transformer.

The type of transformer we have been describing so far is an isolation transformer. That is, the windings or coils are physically separated from each other. Another, very common, type of transformer is the autotransformer which consists of a single "tapped" coil in which the turns between the tap and one end of the primary winding comprise one coil of the transformer and the entire primary winding comprises the other coil. An autotransformer in the lower winding ratios is very lightweight, physically smaller, and therefore more economical than its isolation transformer counterpart. The transformers we sell here and elsewhere for international voltage converter purposes are all autotransformers.

  
 
Transformer Calculations
Wattage refers to the amount of power (electricity) consumed by your appliances and equipment. Sounds simple, doesn't it? Actually it is very simple and it is important to know a little about it because you need to know wattage in order to choose a Converter or a Transformer. The wattage of most appliances and equipment is found on decals or labels of some sort on the appliance or equipment or in the owner's manual. In addition, the decals or labels also usually give the voltage and/or amperage of the appliance.

If the wattage isn't given you can still figure it out if you know the voltage and amperage. If you multiply the voltage times the amperage, the result equals the wattage of the appliance. For example, the decal on the appliance you want to take overseas doesn't list the wattage but gives the voltage as 120 volts and the amperage is 1.1. To find the wattage of that particular appliance, multiply the voltage 120 times the amperage 1.1, 120X1.1=132 watts.

TRANSFORMER SELECTION

The manufacturers of transformers recommend that the connected wattage not exceed 90% of the Transformer's rated wattage. For example, don't connect more than 450 watts to a 500 watt transformer.
Now that you know how to figure the wattage (either look on the label or do the simple math) you are ready to select a converter and/or a transformer. To choose either one you first decide which appliances you want to use, second get the wattage of each item, and third, decide how many appliances you will use at one time. See which of the three situations below applies to you, then look at the solutions and examples:

•1). You will bring and use only 1 appliance.

•2). You will bring more than one appliance but use only one at a time.

•3). You will bring and use more than one appliance.

•SOLUTION 1). Just find the wattage of that one appliance and choose the converter or transformer that suits your needs.

•SOLUTION 2).If you know for a fact that you will never use more than one appliance or piece of equipment or combination thereof at one time then you find the highest wattage of your appliances and choose the converter or transformer that has a higher wattage than that one appliance.

•SOLUTION 3). Add all of the appliance wattage you will use at one time and once again choose the Converter or Transformer with a wattage rating higher than the total of all the appliances you intend to use at one time:

•EXAMPLE 1. If your ungrounded appliance has a wattage of 550 watts you can choose between the ungrounded step down Hi-Lo converter (use it with the wattage switch set before plugging into the wall at the 25-1875 watts setting) or the VC-1000 step up / step down transformer. If your appliance is grounded or you want to use it for long periods of time, you will have to choose one of the Transformer model because the converters aren't grounded and can't be used for longer than 45 minutes to one hour. The advantages of converters are: inexpensive, lightweight, and compact.

•EXAMPLE 2.In this example you could choose the ungrounded Hi-Lo converter that has two settings 0-25 watts and 25-1875 watts. WARNING! Always switch to correct wattage range before plugging into an adapter that should already be in the wall outlet, if an adapter is used. You could also choose the step up / step down transformer VC-2000. The Hi-Lo converter is not intended for continuous use and must be unplugged from adapter which also must be removed from wall outlet as soon as you are finished with appliance or equipment. The advantages of converters are they are: inexpensive, lightweight, and compact.

•EXAMPLE 3A. You are bringing 3 ungrounded appliances that you have to use at the same time with the following watts: 1400, 350, and 150. The total of all the watts is 1900 watts which is too high for any of the converters but you could choose the step up / step down transformer model VC-3000 . Remember you don't want to load the transformer more than approximately 90% of its rated wattage.

•EXAMPLE 3B. You want to bring a 300 watt grounded appliance, a 500 watt ungrounded, and a 800 watt grounded appliance and you have to use them all at the same time for long periods of time. You would add all of the watts, 1600 watts total, and choose the VC-2000 model transformer. You couldn't choose a straight converter, even though it has a wattage rating above the 1875 watts of all these appliances for two very important reasons, 1). you have some grounded appliances and 2). you intend to use them for long periods of time. Either of the previous reasons preclude the use of a Converter, they can't be used for long periods of time and they aren't grounded.