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AC Speed ControllerRotational 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 slipschanges, then the rotational speed N will change N= 120f(1-s)/P (1)
N: Rotational speed [r/min]F: Frequency HzP: Number of poles of a motorS: 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, N1N3 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.
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