FREQUENCY CONVERTER 187

2. The compounding curve; that is, the change of primary
impressed voltage required to maintain constant secondary
terminal voltage.

In this case the impressed frequency and the speed are con-
stant, and consequently the secondary frequency is also constant.
Generally the frequency converter is used to change from a low
frequency, as 25 cycles, to a higher frequency, as 60 or 62.5
cycles, and is then driven backward, that is, against its torque,
by mechanical power. Mostly a synchronous motor is em-
ployed, connected to the primary mains, which by overexcitation
compensates also for the lagging current of the frequency
converter,

Let:

Y = g — jb = primary exciting admittance per circuit of
the frequency converter.

Zi = TI + jxi = internal self-inductive impedance per sec-
ondary circuit, at the secondary frequency.

ZQ = r0 + jxQ = internal self-inductive impedance per primary
circuit at the primary frequency.

a = ratio of secondary to primary turns per circuit.

b = ratio of number of secondary to number of primary
circuits.

c = ratio of secondary to primary frequencies.

Let:

e = generated e.m.f. per secondary circuit at secondary
frequency.

Z = r + jx = external impedance per secondary circuit at
secondary frequency, that is load on secondary system, where
x = 0 for non-inductive load.

To calculate the characteristics of the frequency converter,
we then have:
the total secondary impedance :

the secondary current:

where:

r + rx , x + ,

ai — ~—-----.^^——----- anci tt2 =

(r + n)2 + (x + x,y *nu "2 " (r + n)2 + (x