AC asynchronous motors are widely used in various industries. In variable frequency speed control systems, switching between the frequency converter and the mains frequency power supply is frequently required. The main types of switching are fault switching and multi-machine system switching. In many production machinery operations, the motor cannot be stopped, such as exhaust fans in textile and chemical plants, and blowers and induced draft fans in boilers. When a frequency converter fails and trips, the motor must be able to quickly switch to the mains frequency power supply. Similarly, in multi-pump water supply systems, a scheme where one frequency converter controls multiple pumps (often called 1-to-N) is commonly used. This system also requires switching from the frequency converter to the mains frequency power supply.
During switching, the motor is disconnected from the power supply while its rotor rotates at high speed. Combined with the presence of a DC magnetic field in the rotor, the motor operates in a synchronous generator state. Directly switching to the mains frequency power supply would result in a large inrush current, adversely affecting the power grid, inverter, and motor. Frequent switching could lead to inverter failure and motor burnout. Synchronous switching technology avoids the massive inrush current caused by the mismatch between the mains frequency power supply phase and the inverter's output power phase during inverter switching. This significantly improves switching reliability, effectively protects the motor and inverter, and prevents interference with the power grid.
2. Generation and Impact of Inrush Current 2.1 Generation of Inrush Current When the frequency and phase of the inverter output power supply are inconsistent with the frequency and phase of the power supply, the asynchronous motor is switched from inverter power supply to power supply. During this process, an inrush current will be generated due to the back EMF of the stator winding and the excessive rotor slip. This inrush current can reach about 30 times the rated current.
The inrush current is caused by the electromotive force of the stator winding. When the motor is disconnected from the power supply, the high-speed rotating rotor cuts the magnetic lines of force in the DC magnetic field generated by the rotor windings. Since the stator windings are open-circuited, the asynchronous motor is in synchronous generator mode. As the rotor speed decreases, the amplitude and frequency of the three-phase voltage generated in the stator windings gradually decrease. At this time, the phase difference between the power frequency voltage and the voltage generated on the stator windings is inevitably asynchronous, and this phase difference will continuously change as the power outage time increases. If the two phases are in the same phase (0° phase difference) during switching, they cancel each other out, and no large inrush current is generated. However, if they are out of phase (180° phase difference), the two voltages will superimpose, resulting in a large inrush current, approximately three times the current when the motor is directly started.
When the motor is disconnected from the power supply, the magnetic field energy stored in the stator winding has no release path due to the stator being open-circuited. This results in a large back electromotive force in the stator winding. If the motor is switched to the mains frequency power supply at this time, a large inrush current will be generated.
Inrush current caused by excessive slip. After the motor is disconnected from the power supply, the motor speed will drop rapidly due to the load switching of most motors. The slip between the actual rotor speed and the synchronous speed will be large. Due to the presence of residual magnetism in the stator winding and the gradually decaying DC magnetic field generated by the rotor current, the induced electromotive force and current generated by the rotor winding cutting the magnetic lines of force are large (mcx: di/di, iccdw/di), thus generating inrush current.
2.2 Impact of Inrush Current The inrush current generated by the back electromotive force in the stator winding, the electromotive force generated by the motor in synchronous generator mode, and the self-induced electromotive force in the self-circuited rotor will inevitably affect the motor, frequency converter, and power grid.
1) Impact on the frequency converter. When the frequency converter is operating under normal load, the current flowing through the power devices in the frequency converter passes through the motor windings. Its energy and voltage are mainly consumed in the motor windings and do not have an adverse effect. However, once the frequency converter suddenly disconnects the load, the current through the power devices loses its circuit, generating a very large d/df ratio, causing a sharp rise in the voltage at the power device terminals. This causes the power devices to withstand excessive current surges, which can damage them. 2. This surge current can also damage the freewheeling diodes, filter capacitors, and insulation of the frequency converter, which will inevitably shorten the frequency converter's service life. 3. Impact on the motor. The degree of impact varies depending on the load on the motor shaft and the motor's capacity. If the motor is equipped with a blower or exhaust fan, the back pressure generated by the air during switching is small. After a 13-second delay, it avoids the influence of back EMF and switches to the mains frequency power supply, thus avoiding the impact of a large current. The motor can fully withstand this inrush current. However, if the motor is equipped with a pump-type load, the "water hammer" effect will occur. Combined with the back EMF and high water pressure during switching, the motor will experience a current surge greater than 20 times its rated current and a huge torque surge, causing damage to the motor. If the motor is an older model, due to its lower efficiency and power factor, and larger copper and iron losses, most of the inrush current generated during switching is consumed by the motor's losses, and the motor can withstand this inrush current. If the motor is a newer model, due to its higher efficiency and power factor, lower power consumption, smaller size, and lighter weight, most of the impact generated during switching becomes a torque surge, thus causing greater damage to the motor.
Impact on the power grid. If the switching timing is good, the inrush current will not have a significant impact on the power grid; however, if the switching time is not chosen properly, it may cause the circuit breaker to trip, or even cause interference and fluctuations in the power grid, which will have an adverse effect on the safety of the power supply system and product quality. The impact will be even greater if the motor has a large capacity.
3. Analysis of the Principle of Synchronous Switching The principle of switching from frequency converter to power frequency can be analyzed using the phasor of any phase winding of a three-phase asynchronous motor, as shown. When the asynchronous motor is operating normally, the main magnetic flux is less than m, rotating at synchronous speed. The induced electromotive force generated in the stator winding is given by and the electromotive force balance equation of the stator winding is given by . 7. It consists of 3 basic circuits, the basic components of which are shown in the figure.
A phase detector is a phase comparison device used to detect the phase difference G between the input signal phase ft(t)(t) and the feedback signal phase 0(G)(t). The output error signal Ud is a linear function of the phase difference signal 0(t), so the phase detector is a proportional element.
The loop filter uses a passive proportional-integral (PI) filter. When the loop is locked, the output frequency (the inverter's output frequency) is the same as the input frequency (the power grid frequency), with only a steady-state phase difference between them. When the open-loop gain is sufficiently large, this phase difference is very small. When the input signal undergoes phase or frequency changes, the loop output signal, i.e., the inverter's output frequency and phase, will track the input signal changes through the loop's own adjustment. Even if the input signal fluctuates under the following conditions, a large phase difference will not occur.
The input signal experiences a frequency step. When the input signal undergoes a frequency step, its Laplace transform, according to the Laplace transform final value theorem, results in a smaller steady-state phase error when a larger value is selected.
When the input signal experiences a phase step, its Laplace transform changes according to the final value theorem. At this point, the synchronizer can achieve smooth system switching.
Controller 1 inverter "captures" the in-phase point within 100ms and switches to the mains frequency power supply at 5.5s.
As mentioned above, it is difficult for the output frequency and phase of the frequency converter to be perfectly synchronized with the power frequency voltage frequency and phase at the moment of switching. A synchronizer using the "differential frequency and same phase" switching method successfully solved the switching problem between frequency converter and power frequency, and simulations were performed on numerous systems. The stator current waveform of the motor during synchronous switching is shown in the figure. After the motor outputs the upper limit frequency of the frequency converter and confirms the time, the frequency converter is disconnected at 5.4 seconds. This shows that the synchronous switching method using a synchronizer results in a small current surge, approximately twice the rated current. The motor can re-enter a new stable state 0.2 seconds after switching to the power frequency.
6. Conclusion If the timing of the switching from frequency converter to mains frequency is not properly selected during the control system transition, the inrush current generated during the switching process can severely impact the power grid, the frequency converter, and the motor. This paper proposes using a synchronizer to achieve the switching from frequency converter to mains frequency, discusses the switching principle and implementation method, and simulates the switching process using simulation software. This synchronizer has been put into use in a company's paint line blower system, boiler exhaust system, and multi-pump constant pressure water supply system, operating stably and reliably. Practice shows that using a synchronizer to achieve the switching from frequency converter to mains frequency results in an inrush current of no more than twice the normal value, effectively avoiding excessively large switching currents.
