Dae-Young Yang

A soft starter is commonly utilized in various industrial induction motor systems to reduce the inrush current during motor startup. The conventional voltage ramp-up method, used in the typical soft starter, gradually increases the voltage over the startup period. However, this method can result in a significant voltage increase as the induction motor approaches its rated speed, resulting in a rapid torque increase compared to the beginning part of the startup phase. This sudden torque increase can potentially cause physical damage to the induction motor system due to a transient current overshoot, which in turn can result in resonance. Additionally, the changes in the parameters of the induction motors can lead to the variations in torque during startup, especially because of the air gap permeance. Therefore, a firing angle control method for the soft starter that maintains a constant torque, facilitating a gradual increase in speed, is desirable. This paper proposes a novel firing angle control method, to achieve a constant torque and a gradual speed increase, using only current feedback. This paper is the first time that a change in air gap permeance is considered and compensated for in controlling the firing angle of the soft starter. The air gap permeance variation is modeled as an effective inertia in the transient response of the induction motor. Experimental results demonstrate that the proposed method effectively reduces rapid torque increases and speed variations compared to the conventional voltage ramp-up method, through its application to a 30kW industrial induction motor system.

This paper presents a novel resonance suppression method for mover-position-dependent resonance. In industrial equipment and robots, increasing controller bandwidth for higher performance can induce inherent resonances. Various filter-based methods have been developed and utilized to compensate for the resonances. However, in applications, such as the belt-drive loads, where the resonance frequency can change by two to three times depending on the position of the mover, suppressing the resonance using fixed-parameter filters is not effective. This paper proposes a new resonance suppression method combining a decoupled persistent oscillation compensator (DPOC) and a disturbance observer (DOB). In the proposed method, the DPOC compensates for the mover-position-dependent resonance by attenuating the magnitude of the measured velocity signal within a specific frequency range, whereas the DOB compensates for external disturbances. Therefore, the control system with the proposed method can enhance control performance by automatically adapting to both resonance characteristics dependent on the mover position and the external disturbances. The implementation of the proposed method in the discrete-time domain is also presented, and its stability and robustness are analyzed. Experiments are performed on an industrial belt-drive servo system to demonstrate the effectiveness of the proposed method. Using the proposed method leads to an 84% reduction in overshoot and a 20% reduction in tack time compared with the results of using a well-tuned set of fixed-parameter filters, demonstrating its superiority in resonance suppression.