Application Error of Inverter in Practical
3. Use apparent power to calculate reactive power compensation for energy saving benefits
The apparent power is used to calculate the energy saving effect of reactive power compensation.For example, in reference , when the fan frequency of the original system is working at full load, the motor's operating current is 289A. When frequency conversion is adopted, the power factor of 50Hz at full load is about 0.99 and the current is 257A. This is due to the improvement of the power factor generated by the filter capacitance inside the converter.The energy saving calculation is as follows: ΔS = UI = * 380 * (289-257) = 21 kva
Therefore, this paper considers that its energy-saving effect is about 11% of the capacity of a single machine.
Practical analysis: S stands for apparent power, that is, the product of voltage and current. When the voltage is the same, the apparent power saving percentage is the same as the current saving percentage.In reactance circuits, the apparent power only reflects the maximum permissible output capacity of the distribution system, not the actual power consumed by the motor.The actual power consumed by the motor can only be expressed in terms of active power.In this case, the actual current is used, but the apparent power is calculated, not the active power.We know that the actual power consumed by the motor is determined by the fan and its load.The increase of the power factor did not change the load of the fan, nor did it improve the efficiency of the fan, nor did the actual power consumption of the fan decrease.After the increase of the power factor, the running state of the motor does not change, the stator current of the motor does not decrease, and the active and reactive power consumed by the motor does not change.The reason for the improvement of the power factor is that the reactive power generated by the filter capacitor in the converter supplies the motor consumption.With the increase of the power factor, the actual input current of the converter decreases, thus reducing the line loss between the power grid and the converter and the copper consumption of the transformer.At the same time, the load current is reduced, the transformer, switch, contactor, wire and other distribution equipment that supply frequency converter can carry more load.It should be pointed out that if, as in this case, the loss of the converter is taken into account instead of the saving of the line loss and the copper consumption of the transformer, the converter will not save energy but also consume power when running at full load at 50Hz.Therefore, it is wrong to calculate the energy saving effect with the apparent power.
4. Contactor shall not be installed on the output side of the frequency converter
Almost all the specifications of the variable frequency governor point out that the output side of the variable frequency governor cannot be equipped with contactors.For example, the specification of Yaskawa frequency converter stipulates that "do not connect electromagnetic switch and electromagnetic contacter in output loop".
The manufacturer's regulation is to prevent contactor action when the variable frequency governor has output.When the converter is connected with the load during operation, the overcurrent protection loop will operate due to leakage current.So, as long as in the variable frequency governor output and contactor between the action, to the necessary control interlock, to ensure that only in the variable frequency governor no output, the contactor can act, variable frequency governor output side can be installed contactor.This scheme is of great significance in the case of only 1 inverter governor and 2 motors (1 motor running and 1 motor standby).When the motor is in trouble, the frequency converter can be easily switched to the standby motor, and the frequency converter can be run after a delay to realize the automatic operation of the standby motor.In addition, the two motors can be conveniently used for each other.