A New Discrete Frequency Control Method for an Improved Soft-Start of Induction Motors Part 1: CLSSR-DFC Development

Abstract

There are many starting methods that an induction motor (IM) can undergo. The direct-on-line method has the advantage of producing maximum torque. However, it also produces significant torque rippling and high inrush currents, leading to high mechanical and electrical stress. Traditional fixed frequency, soft-start methods were proposed to overcome these issues by ramping the voltage/current feed to the IM, which dramatically reduces torque ripple and inrush currents. Unfortunately, this method introduces another problem - a significant reduction in overall torque - which drastically increases the likelihood of stalling under heavy loads. As a result, a variable-frequency soft-start method known as Discrete Frequency Control (DFC) was proposed. This method utilises the same hardware as traditional fixed-frequency methods. However, unlike traditional soft-start methods, DFC methods are capable of producing high overall torque. This is achieved by starting the motor under a series of discrete subharmonic frequencies, where lower subharmonics produce higher torque but lower speeds. Unfortunately, current DFC research has failed to present any strategy that optimally selects switching times between the discrete harmonics. As a result, torque production significantly drops and even oscillates about zero during a frequency transition, resulting in a low average torque being produced. Additionally, current research fails to present a method for limiting RMS currents. To overcome these problems, this paper presents the development of a new DFC method - Current Limiting, Sensorless Speed Referenced DFC (CLSSR-DFC). This new CLSSR-DFC method can control RMS currents to a preset limit (current limiting) by utilising the same voltage/current ramping function as traditional fixed-frequency methods - which has the additional benefit of reducing torque ripple and inrush currents. Additionally, the CLSSR-DFC method optimally times the transitioning of frequencies by using the rotor's speed as a reference - which is estimated using an Extended Kalman Filter (EKF) sensorless technique. For simulation development and results refer to part 2 - A New Discrete Frequency Control Method for an Improved Soft-Start of Induction Motors Part 2: Simulation Studies - under the same authors. Where in part 2 simulations comparing the CLSSR-DFC method against a traditional fixed-frequency, current limiting - soft start (CL-SS) method under a constant load were carried out. Results show the CLSSR-DFC method's superior torque production. This method produced a successful start at 100% of the IM's rated load; in comparison, the traditional CL-SS method failed to start under 75% of the rated load. Results also show the current limiting strategy of the CLSSR-DFC method limiting the RMS current near to the preset limit. Additionally, the strategy for optimally transitioning frequencies prevented any significant torque oscillation occurring about zero, unlike previous research. Potential problems associated with high torque ripple and high peak raw currents of this CLSSR-DFC method have also been highlighted.