Frequency converter performance calculations

In its general construction the induction-type frequency converter is identical with a standard slip ring motor. Connecting its primary to a line of frequency f1 sets up an air gap flux revolving at synchronous speed ns in respect to the primary winding, inducing in the secondary a voltage of a frequency f2 in direct proportion to the difference between shaft speed n and ns. Introducing the slip ±S = ±f2/f1 and the number of poles P, two convenient formulas for the selection of converters are obtained: (1) and (2)