DRAWING TABLES DRIVEN BY ELECTRONIC SERVOS, ETC. 
9 
The tachymeter part, set up coaxially to the biphase motor, is formed by two 
stator windings with axes at 90 0 from one another, and of a rotor, rigidly connected 
to the « squirrel cage » of the motor. One of the stator windings (excitation winding) 
Avvolgimento Avvolgimento 
di controllo di eccitazione 
Schema di un moto-tachimetro 
is fed by the network voltage (115 volts, 400 c/s), the other (<output winding) is fed 
as stated hereinafter. The rotor of the tachymeter, instead, consists of a cjdindrical 
aluminium surface {bell) wherein we induce the currents of the magnetic field of 
the stator excitation winding. These induced currents create in the rotor cavity 
an alternate magnetic field, with a stationary direction in space ; this field may 
be considered as the result of two fields of half the amplitude, rotating in a con 
trary sense and the same frequency as that of the input. 
As long as the rotor does not move, the said magnetic field does not induce 
any voltage into the output winding, as the two axes of the stator windings are at 
right angles. But if the rotor turns, (because the rotor of the biphase motor, with 
which it is rigidly connected, turns) we reduce the rotation speed of one of the 
two components in which we imagined the said magnetic field to be decomposed, 
and at the same time we increase the rotation speed of the other component : 
therefore their resultant is not an alternate field, fixed in space, but a (no longer 
alternate) field which rotates and can induce into the output winding a voltage 
proportionate to the rotation speed of the rotor. Obviously the sign of such a voltage 
depends on the rotation sense. 
If we now’ connect the input of the servo-amplifier with the output of the 
tachymeter (fig. 7), so that the latter voltage is deducted from that coming from 
the synchro-receiver we obtain the tachymetric reaction : thus, in fact, the voltage 
error which is being amplified acquires also a damping component. 
In addition to the advantage stated at the beginning, we also obtain a strong 
torque in the coincidence position which makes it insensible to any mechanical 
troubles in the controlled member. In fact, any outside disturbance, causing a 
small displacement of the synchro-receiver from the coincidence position may cause 
an appreciable y signal at the tachymeter output, the stronger the faster