Tips: How to solve the inverter capacity problem

Since the switching device of the inverter is a high-power transistor, its overcurrent capacity is smaller than that of the thyristor, so the correctness of its capacity selection is directly related to its safe use.

Match the power and load of the motor. The ability of the inverter with the same capacity to drive the motor varies depending on the nature of the load carried by the motor. The capacity of the inverter required for the motor with the same power varies due to the different nature of the load. The capacity of the inverter required for the square torque load (fan) is lower than that for the constant torque load. Usually, the inverter has directly given the rated power or its apparent power suitable for driving the motor. In the chemical industry, for square torque loads such as fans and water pumps, the corresponding inverter can be selected according to the motor power. In the following cases, the capacity of the inverter must be increased:

① The motor starts the load with large mechanical inertia in a short time;

②The motor is required to accelerate and decelerate frequently;

③ Within the desired acceleration and deceleration time, the maximum current of the motor is greater than the overload capacity of the inverter (when it reaches 1.5 times the rated current within 1 minute). In the above case, the inverter cannot be selected simply according to the motor power.

Lightly loaded motors cannot be equipped with small capacity inverters at will. The motor is like a big horse pulling a small cart, and the power output is insufficient. Although the motor can be equipped with an inverter slightly smaller than the motor's nominal power, the larger the motor capacity, the smaller its inductance, and the larger the pulsating current component caused by harmonics. Therefore, when the load of the asynchronous motor is small and you want to use a smaller capacity inverter, on the one hand, you must consider that the no-load current of the large-capacity motor is large, and once loaded, whether the total load current exceeds the rated current of the inverter, and on the other hand, you must pay attention to the fact that the above-mentioned pulsating current will increase.

Correct selection of the V/f graph of the frequency converter. For 50Hz, 380V AC asynchronous motors, the appropriate Vif graph should be selected according to actual needs and the allowed working range of the motor in actual operation. In the chemical industry, frequency converters are mainly used for speed regulation of pumps. Therefore, when selecting the Vif graph, the output frequency and maximum output frequency should usually meet the condition that the rated voltage is 380V and the output frequency and maximum output frequency are both 50Hz. The selection of the Vif graph of the frequency converter for pump speed regulation in our factory is based on the above principles.

Selection of torque boost curve. Torque boost can effectively increase the output torque of the motor. If the curve is selected too small, the motor output torque will be insufficient and starting will be difficult; if it is selected too large, the motor flux saturation loss will increase accordingly, and the motor temperature rise will increase, which is not conducive to energy saving. Therefore, when selecting the torque boost curve, we should choose it according to the nature of the load.