Improvement of the stability of the V/f controlled induction motor drive systems

V/f control is an open loop scalar control scheme for induction motors (IM). Instability of a V/f controlled IM drive system may occur as the V/f control method has no means of controlling the transient operation of the drive system. This paper presents a new approach of V/f control, which is based on current feedback to improve the stability of the conventional V/f control. The proposed approach is based on the stator current derivatives. This new method has successfully damped the torque and speed oscillations in V/f control for low power IM drive systems. However, applying this method for high voltage IM results in more oscillations and lead to a highly unstable drive system. This paper analyzes the causes of this instability and proposes a damper to suppress the undesirable oscillations. The proposed damper is easy to implement and helps eliminate the oscillations which caused the instability of V/f for high voltage IM. The purpose of this study is to: (1) theoretically analyze the cause of the instability of the V/f based drive system; (2) propose a method to eliminate the oscillations; and (3) carry out a simulation study to verify the effectiveness of the proposed approach.