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 f
1
sets up an air gap flux revolving at synchronous speed n
s
in respect to the primary winding, inducing in the secondary a voltage of a frequency f
2
in direct proportion to the difference between shaft speed n and n
s
. Introducing the slip ±S = ±f
2
/f
1
and the number of poles P, two convenient formulas for the selection of converters are obtained:
$n=[1 \mp S]n_{s} \eqno{\hbox{(1)}} $
(1) and
$n=1 \mp f_2 = P_n/120 \eqno{\hbox{(2)}}$
(2)