SURGING OF SYNCHRONOUS MOTORS

293

representation, and substituting in (16), gives the frequency of
oscillation:

P = 0:

e = 1600 volts; 0 = - 2°;/0 = 2.17 cycles,

or 130 periods per minute.

2180 volts
2800 volts

P = 400 kw.
e =1600 volts;

+ 3° 2.50 cycles,

or 150 periods per minute.
+ 5° 2.85 cycles,

or 169 periods per minute.

j3 = 33°;/0 = 1.90 cycles,
or 114 periods per minute.
2180 volts 21" 2.31 cycles,
or 139 periods per minute.
2800 volts 22° 2.61 cycles,
or 154 periods per minute.
As seen, the frequency of oscillation does not vary much with
the load and with the excitation. It slightly decreases with
increase of load, and' it increases with increase of excitation.
In this instance, only the momentum of the motor has been
considered, as would be the case for instance in a synchronous
converter.
In a direct-connected motor-generator set, assuming the
momentum of the direct-current-generator armature equal to
60 per cent, of the momentum of the synchronous motor, the
total momentum is MQ = 1,360,000 joules, hence, at no-load:
P = 0,
e = 1600 volts;/o = 1.72 cycles, or 103 periods per minute.
1.98 cycles, or 119 periods per minute.
1.23 cycles, or 134 periods per minute.
. 169, In the preceding discussion of the surging of synchronous
machines, the assumption has been made that the mechanical
power consumed by the load is constant, and that no damping
or anti-surging devices were used.
The mechanical power consumed by the load varies, however,
more or less with the speed, .approximately proportional to the
speed if the motor directly drives mechanical apparatus, as
pumps, etc., and at a higher power of the speed if driving direct-
current generators, or as synchronous converter, especially