How VFD realizes speed regulation and energy saving

VFD has the functions of voltage stabilization, speed regulation, voltage regulation, frequency regulation, etc. It applies modern science and technology. Although it is expensive and has a complicated internal structure, it has good performance and is easy to use. Therefore, variable frequency drive is not only used for motors, but also for different Power, shape, and volume are widely used in various fields. With the development of application technology and the reduction of manufacturing costs, variable frequency drive will be more widely used.

How does variable speed drive work, how to adjust speed and save energy? The following mainly starts with the working principle of variable frequency drive, and explains the speed regulation and energy saving function of variable frequency drive in simple terms.

The device that converts alternating current with constant voltage and frequency into alternating current with variable voltage and frequency is called variable speed drive, and variable speed drive is a basic AC-DC-AC single-phase inverter circuit.

We know that the rotation speed of the induction AC motor is n = 60f/p (f represents the frequency, and p represents the number of pole pairs). From this formula, it is not difficult to see that the rotation speed n depends on the number of poles and the frequency of the motor. According to the working principle of the motor, it is known that the number of poles of the motor is constant, so if you want to adjust the speed of the motor, you cannot change the number of poles of the motor. According to the calculation formula of the rotation speed of the motor, we can also control the rotation speed of the motor by adjusting the frequency before entering the motor. Therefore, the vfd drive as the motor speed control device by controlling the frequency is the best choice for the motor.

If the voltage is constant and only the frequency is lowered, the magnetic flux of the magnetic induction coil of the motor will increase when the frequency is lowered, the magnetic circuit will be seriously saturated, and even the motor will be burned. Therefore, the VFD must control the output of the VFD voltage while changing the frequency, so as to keep the magnetic flux of the motor constant and avoid magnetic saturation.

Field weakening point: When the VFD output frequency is higher than the rated frequency of the motor, the maximum output voltage of the VFD can only be equal to the rated voltage of the motor, so the output voltage of the VFD no longer continues to increase with the increase of the frequency. At this time, the frequency f continues to increase while the voltage U remains unchanged, so that U/f will decrease with the increase of the frequency, the magnetic flux in the motor magnetic induction coil will therefore decrease, and the motor magnetic flux is in a state of weak magnetic field.

We know that for a motor, its rated current and rated voltage are certain, so when the output frequency exceeds the rated frequency, the voltage will just reach its rated value, and the voltage can no longer continue to change linearly with the frequency , so the highest voltage of the motor can only be equal to the rated voltage of the motor and will not exceed this value.

When the motor is powered by a power frequency power supply, the starting current of the starter motor with direct start is 6-7 times the rated current, and the starting current of the starter motor with star/delta start and auto-coupling decompression start is also 4-5 times the rated current. The starting current is very large, which will have a great mechanical impact on the motor; when using VFD power supply, it can start or stop the motor smoothly without steps, because the output voltage and frequency of VFD are gradually added to the motor, and the starting current It is 1.2-1.5 times of the rated current, so the impact is weaker, and can effectively reduce the starting current and distribution capacity, eliminate the mechanical impact stress on the motor, and prolong the service life.

The practical application of VFD in power plants is mainly on fans and pumps, such as coal feeders, induced draft fans, condensate pumps, circulating water pumps, etc. In order to ensure the reliability and stability of these equipments during production and operation, we have left a certain margin on these mechanical equipments when selecting driving power. If the motor does not need to run at full load according to the requirements of the load, the remaining torque will become a loss, including heat energy loss and active power loss, which will cause unnecessary power consumption. waste. In the case of no application of VFD, the speed regulation of fans, pumps and other equipment is by adjusting the opening of the adjustment baffle at the inlet and outlet, the opening of the valve, the opening of the moving blade, etc., and the fan is adjusted by adjusting these openings. Or the air volume or water volume of the pump, the actual power requirements are far less than the input power used to adjust these openings, so the blocking of baffles, valves, and moving blades will cause a large amount of power consumption.
Now we use ac drive to adjust the speed of these devices. When the load decreases according to the actual requirements, the frequency of the pump or fan can be reduced through ac drive to reduce its speed to meet the actual production needs, which not only reduces the loss of electric energy, It also ensures the safety of the long-term operation of the equipment. This also applies the speed regulation and energy saving function of vfd drive.