High Frequency Apparatus Page 3

THE ALTERNATING CURRENT and the other in a negative direction. In one second of time, it will have made 60 complete cycles; it is therefore called a 60-cycle current. Change of Frequency.-Now let us assume that the alternator be supplied with eight field or stator poles instead of four. As a reversal of current occurs when the rotor windings pass from the influence of one pair of stator poles to the next pair, it is obvious that to double the number of stator poles is to double the frequency, if the speed at which the armature is driven remains the same in each case. On the other hand, precisely the same result is obtained if the number of poles remains fixed and the speed of the rotor is doubled. Therefore, the matter sums itself up into a simple formula which will be useful to the worker if he will but understand it and not fear it as some intangible form of "mathematics." The formula is: Frequency == R.P.M. x Number of Poles 2 x 60 Therefore, if we know the number of poles of a given machine and the speed at which it is driven, we may multiply the number of revolutions per minute by the number of poles and divide this product by 120 to find the frequency of that particular machine. On the other hand, suppose we know the number of poles and we are required to produce a certain frequency from a given machine; we must determine the number of revolutions at which the rotor must be driven. This formula is: R.P.M.= 2 x 60 x Frequency Number of Poles