Suppliers of frequency converter supporting equipment remind you that a frequency converter can drive several or even dozens of motors simultaneously, and the speed of all motors is controlled by the output frequency of the same frequency converter. In theory, the speed of all motors is the same, and it can ensure simultaneous speed increase and decrease.
However, due to differences in motor manufacturing or the size of the load carried by the motor, the actual operating speed of each motor varies, and there is no mechanism in the system to correct this difference, nor can a mechanism be installed to correct the difference. Therefore, in some situations where there is no connection between devices, this control method will definitely accumulate errors.
Consider the frequency converter as a power supply. In some rigidly connected systems, motors that run slightly faster may have heavier loads; And motors that run slightly slower will have lighter loads. But because it is driven by the same frequency converter, the slip rate of the load increases and the slip rate of the light load decreases. This will provide a certain degree of automatic correction ability, ultimately keeping each motor running synchronously. However, the load distribution is uneven, and the motor power should be amplified by one level when selecting the motor.
So, when using a frequency converter to drive multiple motors, pay attention to the following issues:
1. The power of the motor should not differ too much, generally not by more than two power levels.
2. It is best for the motor to be manufactured by the same manufacturer. If it is a motor of the same power, it is best to use the same batch to ensure consistent motor characteristics and maximize the consistency of the motor's slip rate (the difference between the stator rotating magnetic field speed and the rotor speed) to ensure good synchronization performance.
3. Fully consider the length of the motor cable. The longer the cable, the larger the capacitance between the cables or between the cables and the ground. The output voltage of the frequency converter contains rich high-order harmonics, which will form high-frequency capacitance grounding current and affect the operation of the frequency converter. The length of the cable is calculated based on the total length of all cables connected to the frequency converter. Ensure that the total length of the cable is within the allowable range of the frequency converter. When necessary, an output reactor or output filter should be installed at the output end of the frequency converter.
4. The frequency converter can only operate in V/F control mode (relative to vector control mode), and the appropriate V/F curve should be selected. The rated working current of the frequency converter should be greater than 1.2 times the sum of the rated currents of all motors.
To protect the motor, a thermal relay should be installed in front of each motor, and it is not recommended to install an air switch. In this way, the main circuit can be continuously opened when the motor is overloaded, avoiding the impact on the frequency converter itself when the main circuit is interrupted during the operation of the frequency converter.
For applications that require rapid braking, in order to prevent overvoltage during stopping, a braking unit and braking resistor should be added. Some low-power frequency converters already have a built-in braking unit, so only the braking resistor needs to be connected.