Because the switching device of the frequency converter is a high-power transistor, its over-current capability is smaller than that of a thyristor, so the correctness of its capacity selection is directly related to its use safety.
Matches the power and load of the motor. The capacity of the inverter with the same capacity to drive the motor differs depending on the nature of the load carried by the motor. The same power of the motor requires different capacity of the inverter due to different load properties. The capacity of the inverter required for the square torque load (fan) is lower than that of the constant torque load. Under normal circumstances, the frequency converter has directly given the rated power suitable for driving the motor or its apparent power, in the chemical industry for the fan, pump and other square torque load, according to the motor power to select the appropriate frequency converter. In the following cases, the capacity of the inverter must also be increased:
1 The motor starts the load with large mechanical inertia for a short time;
2 requires the motor to perform acceleration and deceleration frequently;
3 In the desired acceleration/deceleration time, the maximum current of the motor is greater than the overload capacity of the inverter (when the rated current is 1.5 times in lmin). Under the above circumstances, the inverter cannot be simply selected according to the motor power.
Light-load motors are not free to use inverters of small capacity. The electric motor is large and small, and the power output is insufficient. Although the motor is allowed to be connected with a frequency converter slightly smaller than the nominal power of the motor, the larger the capacity of the motor is, the smaller the inductance is, and the pulsating current component caused by harmonics will increase. Therefore, when the load of the asynchronous motor is smaller and the inverter with smaller capacity is desired, on the one hand, the no-load current of the large-capacity motor must be taken into consideration. If the total load current after the load is greater than the rated current of the inverter, the other On the other hand, it must be noted that the above pulsating current will increase.
Inverter V/f pattern is selected correctly. For 50Hz, 380V AC asynchronous motors, in actual operation, the appropriate Vif pattern should be selected according to actual needs and the allowable working range of the motor. In the chemical industry, inverters are mainly used for the speed regulation of pumps. Therefore, when selecting Vif graphics, it is usually sufficient to meet the condition that the rated frequency is 380V and the output frequency and the maximum output frequency are both 50 Hz. The selection of the Vif graphic converter for pump speed regulation in our factory is based on the above principles.
Torque boost curve selection. Torque boost can effectively increase the output torque of the motor. If the curve is selected too small, the output torque of the motor will be insufficient and the starting will be difficult. If the selection is too large, the saturation loss of the magnetic flux of the motor will increase correspondingly, and the temperature rise of the motor will also be unfavorable for energy saving. Therefore, when selecting the torque boost curve, we should choose according to the nature of the load.
Matches the power and load of the motor. The capacity of the inverter with the same capacity to drive the motor differs depending on the nature of the load carried by the motor. The same power of the motor requires different capacity of the inverter due to different load properties. The capacity of the inverter required for the square torque load (fan) is lower than that of the constant torque load. Under normal circumstances, the frequency converter has directly given the rated power suitable for driving the motor or its apparent power, in the chemical industry for the fan, pump and other square torque load, according to the motor power to select the appropriate frequency converter. In the following cases, the capacity of the inverter must also be increased:
1 The motor starts the load with large mechanical inertia for a short time;
2 requires the motor to perform acceleration and deceleration frequently;
3 In the desired acceleration/deceleration time, the maximum current of the motor is greater than the overload capacity of the inverter (when the rated current is 1.5 times in lmin). Under the above circumstances, the inverter cannot be simply selected according to the motor power.
Light-load motors are not free to use inverters of small capacity. The electric motor is large and small, and the power output is insufficient. Although the motor is allowed to be connected with a frequency converter slightly smaller than the nominal power of the motor, the larger the capacity of the motor is, the smaller the inductance is, and the pulsating current component caused by harmonics will increase. Therefore, when the load of the asynchronous motor is smaller and the inverter with smaller capacity is desired, on the one hand, the no-load current of the large-capacity motor must be taken into consideration. If the total load current after the load is greater than the rated current of the inverter, the other On the other hand, it must be noted that the above pulsating current will increase.
Inverter V/f pattern is selected correctly. For 50Hz, 380V AC asynchronous motors, in actual operation, the appropriate Vif pattern should be selected according to actual needs and the allowable working range of the motor. In the chemical industry, inverters are mainly used for the speed regulation of pumps. Therefore, when selecting Vif graphics, it is usually sufficient to meet the condition that the rated frequency is 380V and the output frequency and the maximum output frequency are both 50 Hz. The selection of the Vif graphic converter for pump speed regulation in our factory is based on the above principles.
Torque boost curve selection. Torque boost can effectively increase the output torque of the motor. If the curve is selected too small, the output torque of the motor will be insufficient and the starting will be difficult. If the selection is too large, the saturation loss of the magnetic flux of the motor will increase correspondingly, and the temperature rise of the motor will also be unfavorable for energy saving. Therefore, when selecting the torque boost curve, we should choose according to the nature of the load.
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