AC asynchronous motors are widely used in all walks of life, and frequency converters and power frequency power supplies are often required to switch in the selection of frequency conversion speed control system. The primary types of switching are: fault switching and multi-machine system switching. In the process of operation of many production machinery, the motor is not allowed to stop running, such as the exhaust fan of the textile and chemical plant, the blower of the boiler and the induced draft fan, etc., in the process of the inverter put into operation, once the inverter trips due to failure, it is necessary for the motor to be able to quickly switch to the power frequency power supply; For example, in the multi-pump water supply system, a frequency converter is often used to control the plan of multiple pumps, which is generally called 1 tow N, and the system also requires the switching of the frequency converter to the power frequency power supply.
When switching, because the motor is out of the power supply and the rotor rotates at high speed, coupled with the existence of the DC magnetic field in the rotor, the motor is in the state of synchronous generator at this moment, if it is directly switched to the power frequency power supply, it will present a large impulse current, which will have a negative impact on the power grid, the inverter and the motor, and the frequent switching will present the phenomenon of the inverter blowing up and incineration of the motor. The use of synchronous switching skills can prevent the huge inrush current caused by the phase of the power supply and the output power phase of the inverter when the inverter is switched, and then improve the reliability of the switching to a large extent, effectively protect the motor and the inverter, and prevent the disturbance of the power grid.
2. The occurrence and impact of the inrush current 2.1 The occurrence of the inrush current occurs when the frequency and phase of the output power supply of the inverter are not in line with the frequency and phase of the power frequency power supply, and the asynchronous motor is switched from the power supply of the inverter to the power supply of the power frequency, and the impulse current will occur due to the excessive back-EMF of the stator winding and the rotor slip in this process, and the impulse current can reach about 30 times of the rated current.
Inrush current caused by electromotive force in the stator windings. When the motor disconnects the power supply, the high-speed rotating rotor cuts the magnetic field lines in the DC magnetic field generated by the rotor winding, and the stator winding is an open circuit, at this moment, the asynchronous motor is in the synchronous generator state, with the decrease of the rotor speed, the amplitude and frequency of the three-phase voltage in the stator winding also gradually decrease, then the phase between the power frequency voltage and the voltage on the stator winding must be out of sync, and will follow the addition of the power-off time, and the phase difference will continue to change. When the two are in the same place (phase difference is 0), the two voltages cancel each other out, and no large inrush current will occur; If the two are in inverted phase (phase difference of 180), the two voltages will be superimposed, and a large impulse current will occur, and the value will reach about 3 times the current when the motor is directly started.
When the motor is disconnected from the power supply, because the stator is open, the magnetic field energy stored in the stator winding has no release loop, and a large back-EMF will occur in the stator winding, and a large inrush current will occur if it is switched to the power frequency power supply at this moment.
Inrush current caused by excessive slip. After the motor is disconnected from the power supply, because most of the motors have load switching, the speed of the motor will drop rapidly, the slip difference between the actual speed of the rotor and the synchronous speed is larger, and because of the existence of remanence in the stator winding of the motor and the DC magnetic field of the gradual attenuation of the rotor current, the induced electromotive force and current that occur when the rotor winding cuts the magnetic field line are larger (MCX: DI/DI, ICCDW/DI), and then the impulse current occurs.
2.2 The influence of impulse current because the back electromotive force in the stator winding and the electromotive force in the state of the motor in the synchronous generator state and the inrush current in the self-induced electromotive force in the rotor of the self-formed circuit must have an impact on the motor, inverter and power grid.
1) Impact on the inverter. When the inverter is working under normal load, the current flowing through the power equipment in the inverter flows through the windings of the motor, and its energy and voltage are mainly consumed on the windings of the motor, which will not adversely affect it. Once the inverter suddenly throws off the load, the current in the power equipment loses the circuit, and a great d/df occurs, which constitutes a sharp rise of 70 high in the terminal voltage of the power equipment, so that the power equipment accepts excessive current impact, which will pose a hazard to it 2. The impulse current will also pose a hazard to the insulation of the freewheeling diode, filter capacitor and inverter in the inverter, which is bound to greatly shorten the service life of the inverter 3. The load on the shaft of the motor is different, and the degree of influence is also different; Depending on the capacity of the motor, the degree of influence is also different. If the motor with the supply and exhaust fans of the motor, due to the small backpressure of the air during switching, and the power frequency power supply is switched to avoid the influence of the back-EMF after a delay of 13s, it will not be affected by the impact of large current, and the impulse current motor is completely acceptable; If the motor is loaded with pumps, it will exhibit the "water hammer" effect, coupled with the back EMF and high water pressure during switching, which will make the motor present a current impact greater than 20 times the rated current and a huge torque impact, causing damage to the motor. If the motor is an old model, due to the low efficiency and power factor of the motor, the copper loss and iron loss are large, and most of the inrush current generated by switching is consumed in the loss of the motor, and the motor can accept the impulse current; If the motor is a new model, due to the high efficiency and power factor of the motor, small power consumption, small size and light weight, most of the impact that occurs during switching becomes torque impact, so it is more harmful to the motor.
Impact on the power grid. If the switching opportunity is good, the inrush current will not have much impact on the power grid; If the switching time is not selected properly, the air switch will trip, or the power grid will be disturbed and cause the power grid to fluctuate, which will adversely affect the safety and product quality of the power supply system, and the impact will be greater if the large-capacity motor is greater.
3. The principle of synchronous switching can be analyzed by the phasor of the one-phase winding of the three-phase asynchronous motor, as shown in. When the asynchronous motor is working normally, the main magnetic flux is less m to synchronize the speed. Rotation, the induced electric potential that occurs in the stator winding is the potential equilibrium equation of the stator winding is 7 from 3 basic circuits, the basic composition of which is shown below.
The phase detector is a phase comparison device used to detect the phase difference between the phase ft(1) of the input signal and the phase of the feedback signal 0G). The output error signal Ud is a linear function of the phase difference signal 0(1), so the phase detector is a proportional link.
The loop filter is a passive proportional integration filter. When the loop is in a definite state, the output frequency (the output frequency of the inverter) is the same as the input frequency (the frequency of the power grid supply), and there is only a steady-state difference between the two, and when the open-loop gain is large enough, this difference is very small. When the phase or frequency of the input signal changes, after the adjustment of the loop itself, the loop output signal, that is, the output frequency and phase of the inverter, will track the change of the input signal. If the input signal fluctuates in the following conditions, there will be no large phase difference.
Input signal frequency step. When the frequency step of the input signal occurs, the Ralph conversion is based on the Ralfalis conversion terminal value theorem: when the selection is larger, the steady-state phase error is smaller.
Input signal phase step. When the phase step of the input signal occurs, the Rashorn switch at this moment is based on the Rallar conversion final value theorem: the synchronizer can complete the smooth switching of the system from time.
The inverter of controller 1 "captures" the in-phase point within 100ms, and switches to the power frequency power supply at 5.5 s.
As mentioned above, at the moment of switching, it is difficult to display the output frequency and phase of the inverter and the power frequency, voltage, frequency and phase together. The synchronizer using the "differential frequency in-phase" switching method successfully deals with the switching problem of frequency conversion and power frequency, and simulates in a large number of systems, and the stator current waveform of the motor during synchronous switching is shown as shown in. After the motor outputs the upper frequency of the inverter and acknowledges the time, it cuts off the inverter at 5.4s, from which it can be seen that there is a small current impact when the synchronous switching method is controlled by the synchronizer, which is about 2 times the rated current. Switch to power frequency power 0. After 2s, the motor can re-enter a new stable state.
6Conclusion: If the switching opportunity is not selected properly when the frequency conversion to power frequency switching is carried out in the control system of the inverter, the impulse current in the switching process will have a severe impact on the power grid, inverter, motor and other equipment. In this paper, the synchronization device is proposed to complete the switching from frequency conversion to power frequency, the switching principle and completion method are discussed, and the switching process is simulated by simulation software. The synchronizer has been put into use in a company's topcoat line blast system, boiler exhaust system, multi-pump constant pressure water supply system and other products, and the operation is stable and reliable. The practice shows that the switching from frequency conversion to power frequency is completed with a synchronizer, and the impulse current of the switching is not more than 2 times, which effectively prevents the switching process from being too large.
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