Three-phase AC motor application research
4 degaussing and re-switching schemes and limitations
The scheme is implemented on the traditional direct switching scheme without adding hardware, and only by the inverter software to increase the degaussing function. Before the inverter switches the power frequency, the inverter demagnetizes the motor in advance to reduce the back electromotive force of the motor to zero. The advantage of this scheme is that the maximum current switched to the power frequency is lower than the original scheme, but the disadvantage is that it is only suitable for asynchronous motors. For a synchronous machine, the magnetic field is constant and the back electromotive force cannot be reduced by degaussing, so the solution is not suitable.
5 realization principle of the same phase switching
As shown in Figure 3, the driver has a built-in phase switching control function that requires the driver to install a voltage detection card to detect the real-time phase of the three-phase grid voltage. The parameter P48.02 of the drive is set to 1 to activate the in-phase switching. At this time, the driver enters the phase synchronization phase. After receiving the user's running command, the motor starts running. If the actual frequency of the motor is close to the grid frequency, the driver starts to adjust the motor phase until the phase error is 0. At this time, the first bit of the status word P48.00.00 becomes 1, indicating the in-phase switching request. Sign. The relay R2 of the control board is recommended for in-phase switching control, and its signal source P14.32 needs to be modified to be P48.00.00. When the user's stop command is received, the drive immediately resets the in-phase switching request flag P48.00.00.
For heavy-duty applications, while the drive is shutting down and the power frequency contactor has not been closed (measured approximately 10 to 30 ms), the motor may fall due to load and cause phase error. It is perfectly solved by adding the appropriate lead compensation (parameter P48.03, compensation of about 5° to 15°) by trimming the motor phase.
6 experimental analysis
As shown in Figure 4, the experiment uses a domestic common synchronous motor, the rated data is 90kw/380V/160A/1500rpm/50Hz, the experimental measurement signals are ch2 (red): motor line current, ch3 (blue): the line voltage of the motor , ch4 (green): Switch the contactor auxiliary contact signal.
In the first stage, the motor is in variable frequency control, the current effective value is about 15A, and the line voltage is PWM wave. The second stage is the free running of the motor, the current is zero, and the voltage is the back electromotive force of the motor, which lasts for about 15 ms, which is caused by the mechanical action delay of the contactor. In the third stage, the motor is in the power frequency state, the power frequency contactor is closed, and the maximum current effective value of the motor is about 25A, which is about 15% of the rated value, and the motor has no impact. From the second stage to the third stage, the back electromotive force of the motor completely coincides with the grid voltage waveform, that is, the result of the same phase.
to sum up
Switching frequency conversion to power frequency is of great significance in energy-saving applications. The solution to achieve the most perfect frequency conversion to power frequency is the same phase switching. The solution is characterized by smooth and no impact, reducing the requirements for power distribution capacity and low voltage electrical capacity. Improve the life of motors and machinery.
The Kumak ES850 series inverter has the advantages of simple low cost and smooth switching in the application of frequency conversion in phase switching to power frequency. It is suitable for long-term work in power frequency and energy saving applications, such as oil fields and textiles.
