| Most loads of petrochemical and steel enterprises are induction motors. And manyof those devices used during the production process of those enterprises are flammableand combustible. That demands the produce process continuous. However, unexpectedcases such as faults are inevitable, which means the switch of power supply forimportant loads are necessary. If the time to switch the power supply is not appropriate,there would be a huge current impact because of the residual voltage on the bus. Thiswould cause the operation of the current protection and result in a failure of switching,or damage the equipment, even cause safety accidents. Hence, research in the powersupply switching for induction motors to find a fast and smooth switch strategy issignificant. Nowadays, automatic switch are widely used. Because of the lack ofdetection of the frequency and phase and delay, the failure rate of switching is high. Soit is gradually replaced by fast switch. Although the result of fast switch is pretty good,it can only be used in the situation when the backup power supply and the work supplyare synchronous.Considering the demand of the important loads and the defects of existing switchmodes, this paper researched the switch modes of the power supply for inductionmotors. The analytic works in concrete are mainly as follows:Firstly, the current situation of the power supply switching in the enterprise wasintroduced. Then it derived the analysis formula of the residual voltage, whichexplained the change rules of it. After all, the relevant factor of the residual is indicated.As the effect of switching mostly related with the difference of phase, this paperproposed an innovative switching scheme to supply the load with the inverter after ithad been switched smoothly when the working power supply was interrupted.Then, the inverter power supply for the power supply switching for the inductionmotor was designed. The control system both of the rectifier and the inverter wasdesigned after modeling based on the dq-axis synchronous rotating frame. The powerfactor correction was realized by the rectifier, and the fast dynamic response wasachieved by the inverter, meeting the requirements of the load.Based on the analysis of the preamble, the switching scheme for the inductionmotor was explained in detail. A new method to modulate the phase difference betweenthe back power supply and the output voltage of the inverter was proposed. It succeeded in changing the phase of the output voltage of the inverter to be as the samewith the back power supply voltage. Then, the back power supply was switched tosupport the induction motor before making the inverter power supply unconnected.Finally, a simulation using Matlab was completed. The load was a model of a4kW-induction motor which drags the shut-off valve of coal gas. The result of theswitching strategy was compared with that of the existing automatic switching mode.The result of the comparison shows that: the proposed strategy could effectivelyrestrain the impact current which exists in the direct switching mode when the backuppower supply is not synchronous with the work supply. So the effect of the proposedstrategy is more effective than the existing switching modes. |
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