78 ELECTRICAL APPARATUS

P = /(0.836HY- 10) +j (0.048 *'„ + 270),
(508 = 0.241 z'0) - j (32 + 0.326 i'0/ '
* (0.836 1'0 - 10) (508 - 0.241 i'0) - (0.048 f0 + 270)
(32+ 0.326 i'o).
As seen from the curve, e^ of the nominal induced voltage, the
synchronous motor has to be overexcited at all loads. However,
62 first decreases, reaches a minimum and then increases again,
thus is fairly constant over a wide range of load, so that with
this type of motor, constant excitation should give good results.
Fig. 30 then shows the load curves of the concatenated couple
for constant excitation, on overexcitation of the synchronous
motor of 70 per cent., or
e2 = 850 volts.
(It must be kept in mind, that e2 is the voltage reduced to full
frequency and turn ratio 1 : 1 in the induction machine : At the
slip, $ = 0.05, the actual voltage of the synchronous motor would
be sea = 42.5 volts, even if the number of secondary turns of the
induction motor equals that of the primary turns, and if, as
usual, the induction motor is wound for less turns in the secondary
than in the primary, the actual voltage at the synchronous motor
terminals is still lower.)
As seen from Fig. 30:
the power-factor is practically unity over the entire range of
load, from less than one-tenth load up to the maximum output
point, and the current input into the motor thus is practically
proportional to the load.
The load curves of this concatenated couple thus are superior
to those, which can be produced in a synchronous motor at con-
stant excitation.
For comparison, the curve of apparent efficiency, from Fig. 30,
is plotted as CS in Fig. 28. It merges indistinguishably into the
unity power-factor curve, So, except at its maximum output
point.
Induction Motor Concatenated with Commutating
Machine
52. While the alternating-current commutating machine, espe-
cially of the polyphase type, is rather poor at higher frequencies,
it becomes better at lower frequencies, and at the extremely low
frequency of the induction-motor secondary, it is practically as