2-Phase A-C Servo Motor Operation for Varying Phase Angle of the Control Winding Applied Voltage

The results of the empirical and analytical methods are identical. Since the motor design data are not always easy to obtain, the motor time constant can be computed directly from the torque speed curve. It is interesting to note that the motor velocity constant is twice the carrier or excitation frequency. This fact is readily proved experimentally providing another check on the validity of the motor equation derivation. The system is thus basically nonlinear as shown by the sine function in the torque equation. To use this motor with phase control in a servo system requires that the error (difference between input and output of closed loop system) be transferred into a directly proportional amplified phase shift. The signal then applied to the motor must be of the form sin K
₁
(θ
_input
-θ
_output
) giving: T
_s
d
²
θ
_o
/dt
²
+dθ
_o
/dt=K
_s
sin K
₁
(θ
_input
-θ
_output
) where T
_s
is the over-all system time constant and K
_s
is system velocity constant. In a practical system employing phase shift control some means will have to be employed to cause the motor to exert maximum torque (90-degree phase shift) even when the quantity K
₁
(θ
_i
-θ
_o
) is greater than 90 degrees. This is necessary in order to prevent false zeroes since the motor torque decrease when the phase shift across its terminals is greater than 90 degrees. This is probably one of the principal disadvantages of a system employing phase shift control.