| Inverter is often found in many places using induction motors, which is used to adjust the speed of motor by transferring its running frequency. In industrial place, it happens sometime that power is lost instantaneously (the time of losing power is quite short). After losing power instantaneously, waiting for the motor stopped is the only way to the inverter without the function of tracing rotor speed. When motor stops rotating, the inverter drives the motor to the rate speed from the beginning. For small inertia load, the time that the motor stops is not long and the motor can restart soon after coming to electricity. But to great inertial load, such as air blower load, stopping time from losing power to coming to electricity is too long to start the motor immediately.Moreover, Inverter is sensitive to the voltage change of short duration suddenly in the p lace where short duration of voltagesag and voltage interruption often happens. Then the inverter running with great inertia load is required of the capability to resisting short duration of voltage sag and voltage interruption. After losing power, rotor speed begins to drop owing to losing the source of energy. The time of losing power is not long. When power is supplied again, motor is still in the course of dropping speed. At this time restarting the motor, the output voltage frequency of inverter must be the same as the frequency corresponding to rotor speed. Otherwise the motor may be damaged in heavy loads for large difference between the output voltage frequency of the inverter and the frequency corresponding to rotor speed. So this requires the inverter has the function of following rotor speed.Themethodto solve short duration of voltage dips and voltage interruption is that particular instrument is added to the inverter to measure rotor speed. When power is lost, the inverter stops making PWM signal and inspect rotor speed in real time. After power issupplied again, the inverter output the same voltage frequency as rotor speed corresponding to. A synchronous rotator magnetic field is formed in the stator winding with three-phase voltage, which is essentially identical with rotor speed. Then the motor is driven to resume to the state before power lost by increasing frequency gradually. And the method this paper puts forward is that the inverter reduce operation frequency regularly after power lost and make the motor enter the state of generating electricity a little to maintain DC bus voltage. When power is supplied again, the inverter resumes from dropping frequency regularly to normal operation state.This paper begins with the basic motor theory and then gets the phasor plot in motor state and generator mode through an analysis of equivalent circuit of the induction motor. It also explain how the phasor of voltage and current changes in the frequency dropping course and prove that it is possible to make the motor in the generate state. Then concrete hardware and software design is carried on. Hardware design includes the circuit of measuring DC bus voltage and current, the circuit of inspecting power supply. Software design includes the definition of the variable and label and distribution of the memory unit, and corresponding regulator procedure, interrupt service procedure etc. After hard software design is finished, a trial is carried on using JD-BP32 inverter made by Shang Dong Xin Feng Guang electronic Science & Technology Development Co. Ltd. It is found that the DC voltage can be kept by making the motor in generator mode. Next test is to make DC voltage fluctuate around one voltage level, then test the case of power supplied again. Finally, the function of defending instantaneously power lost is completed. |
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