14 ELECTRICAL APPARATUS
motor with a second motor, or by internal concatenation of the
motor: hereby the effective frequency, which determines the
no-load or synchronous speed, becomes the difference between
•orimary and secondary frequency.
Concatenation of induction motors is more fully discussed in
Chapter III.
As the no-load or synchronous speed of the induction motor
depends on the number of poles, a change of the number of poles
changes the motor speed. Thus, if in a 60-cycle induction motor,
the number of poles is changed from four to six and to 6ight, the
speed is changed from 1800 to 1200 and to 900 revolutions per
minute.
This method of speed variation of the induction motor, by
changing the number of poles, is the most convenient, and such
"multispeed motors77 are extensively used industrially.
A. Pyro-electric Speed Control
10. Speed control by resistance in the armature or secondary
has the disadvantage that the speed is not constant, but at
a change of load and thus of current, the voltage consumed
by the armature resistance, and therefore the speed changes.
To give constancy of speed over a range of load would require
a resistance, which consumes the same or approximately the
same voltage at all values of current. A resistance of very
high negative temperature coefficient does this: with increase of
current and thus increase of temperature, the resistance decreases,
and if the decrease of resistance is as large as the increase of
current, the voltage consumed by the resistance, and therefore
the motor speed, remains constant.
Some pyro-electric conductors (see Chapter I, of "Theory
and Calculation of Electric Circuits") have negative tempera-
ture coefficients sufficiently high, for this purpose. Fig. 6 shows
the current-resistance characteristic of a pyro-electric conductor,
consisting of cast silicon (the same of which the characteristic
is given as rod II in Fig. 6 of " Theory and Calculation of Electric
Circuits")• Inserting this resistance, half of it and one and one-
half of it into the secondary of the induction motor of constants:
eo = 110; Fo = 0.01 - Q.lj;ZQ = 0.1 + 0.3 j; Zl = 0.1 + 0.3;?"
gives the speed-torque curves shown in Fig. 7.
The calculation of these curves is as follows: The speed-
torque curve of the motor with short-circuited secondary, r = 0,