Research On Soft Starting Based On Asynchronous Motor

The AC asynchronous motor is simple in structure, highly reliable in work, strongly adaptable in environment, good in drive, so it is widely used in many fields. The starting current is 4-7 times the nominal current under the nominal voltage without load, and even 8-12 times for some motors made in China. The extremely big starting current makes the big wallop in stator winding and squirrel cage, which damages the winding isolator and the squirrel cage, in addition produces the large thermal which damages the winding isolator and reduces the motor life. Especially when the high voltage motor starts the big starting current makes the grid voltage drop sharply and makes another appliance out of work normally in the same grid, and even makes the grid unsteady and then makes the bigger accident at the same time; So there is a big current in the starting process, which is harmful for the motor itself, the power grid and load. In addition the big torque pulsation and a small torque confine its usage.To reduce the starting current it is traditional to employ the drop voltage starting including:stator in series with resistor or the induction, frequency sensitivity variable resistantce, in series with water resistance/liquid risistance, Y/A transformation, auto-transformer, extended triangle, magnetic-control soft starting and so on, which have such some shortcoming as large volume, big weight and wide area, low technology without harmonic. Because the torque is proportional to the square of the voltage, when the voltage drops a little the torque reduces very much and then becomes smaller.Until 1970s when the electrical and electric device develops rapidly, the thristor is employed in drop voltage starting, namely the electric soft starting, which overcomes the traditional soft starting above shortcoming and brings much harmonic, expensive price, high consistency requirement for the thristor and difficulty in repairing while damage. Nowdays a few big companies oversea master the kernel technology to make the appliance.The variable frequency technology appears until 1980s, which can regulate the motor speeed and perfectly solves its starting question. Because its expensive price it is not economic only for the motor starting without regulating speed to use and beyond affording for the common industrial company.To raise the starting torque and reduce its current, the paper mainly researches the discrete AC/AC variable frequency method systematically, which acquires three phase AC regulating voltage soft starting hardware structure, change the triggering meathod to study the motor starting character.The studies carried out in the thesis and the corresponding results are elaborated as follows.1) Consult the material widely, generalize the soft starting technology developing history, actuality and significance, elaborate the traditional soft starting sort, character and the electric soft starting appliance character, control manner, usage condition. Analyse the single-phase and three-phase regulating voltage principle, the common control tactic and set up the corresponding model and make their simulation for the voltage ramp starting and limited current starting.2) Elaborate the discrete variable frequency basic principle. Firstly the unbalanced discrete voltage is acquired after discrete variable frequency, which the fundmental wave is derived from, which is decomposed the positive sequence voltage, negative sequence voltage and zero sequence voltage by three phase symmetry component method. Finally the balanced voltage is supplied to the motor respectively, which sums up to get the character under the unbalance voltage.3) Aiming at geting the max torque and analysing the umbalance voltage combination one by one such as 2 discrete frequency,3 discrete frequency,4 discrete frequency,7 discrete frequency to find out the desired voltage combination which produces the max torque. Additionally to reduce the big starting current analyse the above voltage combination, set up a model and simulate to find out that exits the voltage combination which produces not only a smallest current but also the max torque, which is desired when the motor starts.4) The thristor acts as the main on-off device with the discrete variable frequency method, thus the harmonic is big, which harmonic and base wave are analyzed with FFT. The corresponding model is set up with MATLAB to analyze them. The above two results are uniform by comparing.5) To get the max torque to study the motor steady model, analyse the positive sequence voltage, the negative sequence voltage under any discrete variable frequency, generalize the starting torque formula to find out when the induction is bigger in the stator and rotator the starting torque is bigger, and at the same time the discrete variable frequency degrade is decided theoretically.6) The model is set up to analyse the torque pulsation reason. In order to get the steadily ascending torque it is acquired to let the U/f constant, U represents the basic wave voltage,frepresents the basic wave frequency. The basic wave is separate from the voltage wave after the discrete variable frequency. The simulation and theory analysis are carried out to get the desired voltage to decide the appropriate triggering method.7) Design the hardware circuit and the corresponding software about the discrete variable frequency. After the experiment of the different disctete frequency degrade respectively the frequency is raised to have effect on the resistance and the motor to find out that the motor can start with a small current and a high torque.