1. Square torque load
Fan and pump loads are the most widely used equipment in industrial sites, and frequency converters are used in this type of load. It is a square torque load. Under normal circumstances, the inverter with U/f=const control mode can meet the requirements of this kind of load at all, and the following is based on the main characteristics of this type of inverter to introduce the problems that need to be paid attention to when selecting.
(1) Prevent overload
Fans and pumps are generally not easy to overload, and the capacity of the inverter can be guaranteed to be slightly greater than or equal to the capacity of the motor; At the same time, the overload capacity requirements of the selected inverter are also low, generally reaching 120%, 1min is sufficient. However, attention should be paid to the selection and presetting of the functional parameters of the inverter, because the resistance torque of the load is proportional to the square of the speed, when the operating frequency is higher than the rated frequency of the motor, the resistance torque of the load will exceed the rated torque, so that the motor is overloaded. Therefore, it is necessary to strictly control the high operating frequency of the motor and not exceed the rated frequency of the motor.
(2) The matching of the acceleration time and the deceleration time of the inverter when starting/stopping
Because the load moment of inertia of the fan and pump is relatively large, it is a very important problem to match the acceleration time and deceleration time of the inverter when it starts and stops. In the selection and application of the inverter, the acceleration time and deceleration time of the inverter should be calculated according to the load parameters to select the shortest time, so that there will be no overcurrent tripping when the inverter is started and no overvoltage tripping when the inverter is decelerating. However, sometimes in the production process, the requirements for the starting time of fans and pumps are very strict, and if the above accounting time cannot meet the demand, the inverter should be planned and selected from scratch.
(3) Prevent resonance
Because the inverter is to change the speed of the motor by changing the power frequency of the motor to achieve energy saving, it is possible to coincide with the resonance point and resonance frequency of the load shaft system at a certain motor speed, forming the intolerable oscillation of the load shaft, and sometimes the equipment is out of operation or the equipment is damaged, so when the functional parameters of the inverter are selected and preset, the resonance frequency point and width of the runout frequency should be set according to the resonance frequency of the load shaft system to prevent the resonance phenomenon of the system.
(4) Pressure holding and water hammer effect
In the actual operation process of the pump load, it is easy to produce pressure holding and water hammer effect, so when selecting the inverter, it is necessary to set it alone for this problem when the function is set up.
a. Pressure-holding When the pump load is running at low speed, the pressure rises because the outlet door is closed, and then the pump cavitation is formed. When the function of the inverter is set up, the critical point of the pump flow rate to the low speed can be limited by limiting the frequency of the inverter to the low speed, which can prevent the occurrence of such phenomena.
b. Water Hammer Effect When the pump load is suddenly powered off, it backs up due to the gravity of the liquid in the pump pipeline. If the check valve is not tight or there is no check valve, it will cause the motor to rotate, and the inverter will fail or burn out due to the motor power generation. In the planning of the inverter system, the inverter should be stopped according to the deceleration curve, and then the main circuit should be disconnected after the motor is completely suspended, or the function of "deceleration abort after power failure" may be set to prevent the occurrence of this phenomenon.
2. Constant torque load
A belt conveyor is a typical example of a constant torque load. The fundamental feature of constant torque load is that under the condition of a constant load, the load resistance torque depends on the friction resistance between the belt and the drum and the radius of the drum. This type of load torque has nothing to do with the speed of the rotation, so the resistance torque of the load remains unchanged during the speed adjustment process. When selecting the frequency conversion speed regulation system for constant torque load, in addition to the conventional requirements, the control method of the frequency converter should be selected.
a. The speed regulation scale of the load. In the case of a small scale of speed regulation, choose an inverter with a relatively simple V/F control method. When the speed regulation scale is large, a responsive vector control method should be considered.
b. Constant torque load is only under the condition that the load resistance torque is unchanged, but the rotation distance still changes with the load when the load changes. When the scale of torque change is not large, the inverter with a relatively simple V/F control method can be selected, but for the load with a large torque change scale, the non-responsive vector control method should be considered.
c. If the load does not have high requirements for mechanical characteristics, you can consider choosing a simpler V/F control method of the inverter, and in the case of higher requirements, you must choose a responsive vector control method.
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