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600 HANDBOOK OF PROJECTION FOR
because 1,000 watts -5 50 volts = 20 amperes, and with 100 per cent, efficiency the primary wattage divided by the secondary voltage must and would be equal to the secondary amperage. This always holds true, subject only to modification by the losses inherent in the transformer itself.
A step up transformer is one in which the voltage induced in the secondary is higher than the voltage impressed upon the primary, and the amperage correspondingly lower, thus reversing the action of the step down transformer. As in the case of the step down transformer, however, the wattage of the secondary will be equal to the wattage of the primary, less the losses inherent in the device itself.
The ratio of transformation depends upon the relative number of turns of wire in the primary and in the secondary. If the number of turns in the primary exceed the number of turns in the secondary, or in other words if there are a greater number of turns of wire in the primary than in the secondary coil, the action will be that of a step down transformer. In the case of a projection transformer, the voltage of the secondary must be just sufficient to force the desired number of amperes against the resistance of the secondary circuit and the arc.
This is easily calculated. For example : A transformer with 100 turns in its primary and 10 turns in its secondary coil, will have a transformation ratio of 10 to 1, and if the primary voltage be 100, the secondary voltage, at no load, will be 10. If 10 amperes flow in the primary of a transformer having a transformation ratio of 10, then 10 x 10 amperes will flow in the secondary. The whole matter is summarized in the following:
Primary voltage : Secondary voltage = primary turns : secondary turns.
Primary amperes: Secondary amperes = secondary turns: primary turns, from which it is evident that, except for losses in the device, the wattage of primary and secondary will always be equal.
In examining the step-down transformer it will be found that the wires of the secondary coil are larger than the wire of the primary coil. This is because of the fact that the secondary amperage will be higher than the primary amperage, hence a wire of larger capacity is required.
The coils are completely insulated electrically from each other and form the core, but magnetic lines of force pass through insulation just as though it were not there.