time:2023-02-07 15:38:32Views:
The frequency conversion speed controller is a device that converts the power frequency power (50Hz or 60Hz) into AC power of various frequencies to realize the variable speed operation of the motor. The control circuit completes the control of the main circuit, and the rectifier circuit converts the AC into DC. The DC intermediate circuit smoothes and filters the output of the rectifier circuit, and the inverter circuit inverts the DC power into AC power.
We know that the speed expression of the AC motor is: n=60 f(1-s)/p
Where n is the speed of the asynchronous motor; f is the frequency of the asynchronous motor; s is the slip rate of the motor; p is the number of pole pairs of the motor.
It can be seen from formula (1) that the speed n is proportional to the frequency f, and the speed of the motor can be changed as long as the frequency f is changed. When the frequency f changes within the range of 0 to 50 Hz, the motor speed adjustment range is very wide. The frequency converter realizes the speed adjustment by changing the frequency of the motor power supply, and is an ideal high-efficiency, high-performance means of speed regulation.
When the VFD output frequency of the variable frequency speed controller (also called frequency converter, the English abbreviation is VFD (Variable-frequency Drive)) is 0, the motor cannot be started because the motor does not have enough starting torque. Only when the output frequency of the VFD reaches a certain value, the motor starts to accelerate, and the output frequency of the VFD is the starting frequency fs. At this time, the starting current is larger and the starting torque is larger. The setting of the starting frequency is to ensure that the motor has sufficient starting torque when starting, and prevent the motor from failing to start or causing an over-current trip during the starting process. In general, the starting frequency should be set according to the characteristics of the load driven by the VFD. On the one hand, it is necessary to avoid the low-frequency under-excitation area to ensure that the motor has sufficient starting torque; on the other hand, the starting frequency cannot be set too high, otherwise it may cause a large current impact when the motor starts, or even an over-current trip. The starting frequency depends on the specific load situation.
1. How to determine the starting frequency?
Generally, most motors start to accelerate from 0Hz, but in some special cases, it is necessary to directly accelerate from a certain frequency. At this time, the frequency output by the VFD at the moment of starting is the starting frequency. Generally, you need to set the launch frequency in the following situations.
1)A system with multiple water pumps supplying water at the same time
For a system that uses multiple water pumps to supply water at the same time, since there is usually a certain water pressure in the water supply pipeline, if it is started at 0Hz, the water pump is difficult to start. In this case, it is necessary to start directly from a certain frequency in order to start the rotation smoothly.
2)Load
If the VFD is driven by a load, the static friction is large, and it is difficult to start from 0Hz. When this happens, it is necessary to set the starting frequency to ensure that a certain starting impact is generated at the moment of starting, so that the system starts smoothly.
3)Conical motor
In the process of starting, there will be friction between the stator and the rotor, so it is necessary to set the starting frequency to ensure that sufficient magnetic flux is quickly established at the moment of starting, so that a certain air gap is maintained between the stator and the rotor, so that the motor able to start successfully.
2. VFD starting frequency setting method
Constant torque load. Generally, the synchronous speed of the motor should not exceed the rated slip when starting. Right now:
Among them, P is the number of poles of the motor; △n——rated slip, △n = n1-ne; N1—synchronous speed; ne—rated speed; (1) square torque load. Since the resistance torque of the square wave torque load is small at low speed, the starting frequency can be increased appropriately.
In practical applications, the starting frequency should be set reasonably to solve the problems of motor starting difficulties and overcurrent tripping. For a motor with a large starting torque, first consider setting an appropriate starting frequency parameter, and then set a reasonable torque boost curve according to the actual load situation. If the VFD current is too high when starting, or even an overcurrent trip occurs, it can be solved by prolonging the acceleration time. But generally speaking, as long as there is no excessive flow, the speed-up time should be as short as possible to improve efficiency.
