AC Speed Controller

AC Speed Controller 
Rotational speed N of an induction motor can be shown by the expression (1). When the voltage applied to the motor is increased and decreased, the slip s changes, then the rotational speed N will change
N= 120·f ·(1-s)/P · · · · · · · · · · (1)

N: Rotational speed [r/min]
F: Frequency Hz
P: Number of poles of a motor
S: Slip

In the case of an induction motor as shown in Fig. 4, a stable range and an unstable range exists in the Rotational Speed - Torque curve. Since it is impossible to reliably operate in the unstable range, simple voltage control (open loop control) is limited to controlling the speed in a narrow range like, N1~N3 in Fig. 5. To make it possible to operate reliably even in the above-mentioned unstable range, it is necessary to detect the rotational speed of the motor and use a voltage control mechanism (closed-loop control) that reduces the speed error when compared to a set value.