SPEED CONTROL

13

motor thus has rather series characteristic than shunt character-
istic, except that its speed is limited by synchronism.
Series resistance in the armature thus is not suitable to produce
steady running at low speeds.
To a considerable extent, this disadvantage of inconstancy of
speed can be overcome:
(a) By the use of capacity or effective capacity in the motor
secondary, which contracts the range of torque into that of
approximate resonance of the capacity with the motor inductance,
and thereby gives fairly constant speed, independent of the load,
at various speed values determined by the value of the capacity.
(6) By the use of a resistance of very high negative temperar
ture coefficient in the armature, so that with increase of load and
current the resistance decreases by its increase of temperature,
and thus keeps approximately constant speed over a wide range
of load.
Neither of these methods, however, avoids the loss of efficiency
incident to the decrease of speed.
9. There is no method of speed variation of the induction
motor analogous to field control of the shunt motor, or change
of the armature supply voltage by a multivolt supply system.
The field excitation of the induction motor is by what may be
called armature reaction. That is, the same voltage, impressed
upon the motor primary, gives the energy current and the field
exciting current, and the field excitation thus can not be varied
without varying the energy supply voltage, and inversely.
Furthermore, the no-load speed of the induction motor does not
depend on- voltage or field strength, but is determined by
synchronism.
The speed of the induction motor can, however, be changed:
(a) By changing the impressed frequency, or the effective
frequency.
(6) By changing the number of poles of the motor.
Neither of these two methods has any analogy in the direct-
current shunt motor: the direct-current shunt motor has no fre-
quency relation to speed,, and its speed is not determined by the
number of poles, nor is it feasible, with the usual construction
of direct-current motors, to easily change the number of poles.
In the induction motor, a change of impressed frequency corre-
spondingly changes the synchronous speed. The effect of a
change of frequency is brought about by concatenation of the