Experimental Study and Comparative Analysis of Transients of Induction Motor with Soft Starter Startup

Abstract

Abstract — This paper investigates the influence of the parameters of the machine and of the soft starter on the dynamics of the induction machine start. In order to evaluate the effects of this variation we have used a design of experiments (DOE). The situations may reproduce actual situations occurred in practice, for example the variation of initial voltage U i , modification of the start time and load value. In the present paper we have investigated the relation between the inrush current, voltage dip at the startup of one industrial soft starter. Using an already predefined fire angle characteristic the influence of the initial voltage was also evaluated. Index Terms—AC Motor drives, Converters, Power semiconductor switches, Speed control. I. INTRODUCTION Electrical drives based on induction motors are the most widely used electromechanical systems in modern industry. Due to their reliability, ruggedness, simple mechanical structure, easy maintenance and relatively low cost, induction motors are attractive for use in a new generation of electrical transportation systems, such as cars, buses and trains [1], [2]. From the variety of electric en ergy consumers in industry one of the largest is without any doubt the induction machine operating as motor. Besides the classical destination of the induction machine as motor this machine is more and more used in the latest period as generator in the conversion chain of wind or micro-hydro-energy into electricity [3], [4], [5]. Variable voltage operation of a squirrel cage induction machine at part load is receivi ng considerable attention as an energy conservation measure. In particular, the power factor controller has created an interest in energy-saving schemes for all types of electric motors [6]. The switching of three-phase induction machine under different operation conditions is one of the processes frequently performed through speed control, soft starting , energy-saving and in renewable energy applications. It is well-known that the transient behavior associated with frequent switching of induction machines is characterized by high current peaks and pulsating torques. Such performance is most undesirable for both electrical supply and mechanical gearing systems [7]. Direct online induction machine starts have many disadvantages. Torque pulsations are often large and modify from positive to negative values. These torque transients in a motor shaft are transmitted to the load, resulting in mechanical wear in the motor bearings and load couplings. Therefore, properly controlling the starting currents and torques of induction machines is of great importance in many instances. Additionally, the resulting starting currents are high, especially during the first few cycles of a starting transient. This high currents are endured by the motor and power system, causing the heating of the machines windings [8]. In recent years, a variety of power electronics equipment with voltage fed pulse width modulation inverters (VSI - PMW) used widely in industrial applications and power network systems have caused significant inherent problems, such as generation of reactive current and power, as well as higher harmonic distortion in the power sources. The selection of the best PWM technique for most applications is uncertain, which can lead to less than optimum results [9]. Soft starters using silicon-controlled rectifiers (SCRs) are now used extensively in the industry. This starting method essentially allows the control of the voltages applied to an induction motor and hence, control of its torque and the acceleration of a machine during its starting transient [8], [10]. Appearance of soft starters produced a qualitative raise in starting, stopping or braking matter of induction motors with squirrel cage. These equipments are useless at starting-up of induction motors with phase wound rotor. There are numerous producing companies that offer soft starters at low voltages (400 and 690 V) or medium voltages (3.3, 4.2, or 6 kV) [11]. Can be started-up even electric drive systems that have a load torque and an inertia moment of equivalent high values. The basic way of finding out the inertia moment is through analytic calculation, based on the rotor drawing and materials. Modern design rules take full benefit of the powerful capabilities of computer-aided design (CAD) tools, and once the rotor is entirely defined geometrically and material wise, automatic calculation of the inertia moment for the rotating parts may be easily achieved. Based on the accu rate measurements brought by the modern digital-data-acquisition tools, the moment of inertia can be obtained in different manner [12]. The soft starters of switch all three phases are controlled can use the starting-up or shutting-down by means of voltage, current or torque control [13]. At voltage control, is achieved a soft start-up, but it’s not generated any current or torque reaction. The typical quantities for starting-up with voltage raise are the initial voltage U