72 ELECTRICAL APPARATUS
. Thus the machine couple has synchronous-motor character-
istics, and runs at a speed corresponding to synchronous speed
of a motor having the sum of the induction-motor and syn-
chronous-motor poles as number of poles.
If n = 1, that is, the synchronous motor has the same number
of poles as the induction motor,
5 = 0.5,
1 ~- s = 0.5,
that is, the concatenated couple operates at half synchronous
speed, and shares approximately equally in the power output.
If the induction motor has 76 poles, the synchronous motor
four poles, n = 19, and:
s = 0.05,
1 - s = 0.95,
that is, the couple runs 'at 95 per cent, of the synchronous speed
of a 76-polar machine, thus at synchronous speed of an 80-polar
machine, and thus can be substituted .for an 80-polar induction
motor. In this case, the synchronous motor gives about 5
per cent., the induction motor 95 per cent, of the output; the
synchronous motor thus is a small machine, which could be con-
sidered .as a synchronous exciter of the induction machine.
48. Let:
$o = e'o + jV'o = voltage impressed upon in-
duction motor.
#1 = e'i + je"i = voltage induced in induc-
tion motor, by mutual
magnetic flux, reduced to
full frequency.
$2 = e'2 + je"% = nominal induced voltage
of synchronous motor, re-
duced to full frequency.
/0 = i'Q — jy0 = primary current in induc-
tion motor.
/i = i'\ — ji"\ — secondary current of in-
duction motor and cur-
rent in synchronous motor.
Denoting by Z* the impedance, Z, at frequency, s, it is:
Total impedance of secondary circuit, at frequency, s:
. Z8 = Zi8 + Z2*
i + a?2), (3)