Industrial inverters use seven major misunderstandings (II)

Misunderstanding: Calculating reactive energy compensation energy gain by using apparent power

The reactive power compensation energy saving effect is calculated by the apparent power. When the fan power frequency is full load, the motor running current is 289A. When using variable frequency speed regulation, the power factor of 50Hz full load operation is about 0.99, and the current is 257A. This is because the internal filter capacitor of the inverter produces improved power factor. The energy saving calculation is as follows: ΔS=UI=×380×(289-257)=21kVA

Therefore, the energy saving effect is considered to be about 11% of the stand-alone capacity.

Actual analysis: S is the 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 a circuit with reactive reactance, the apparent power only reflects the maximum allowable output capability of the power distribution system, and does not reflect the power actually consumed by the motor. The actual power consumed by the motor can only be expressed in terms of active power. In this example, although the actual current is calculated, the apparent power is calculated instead of the active power. We know that the actual power consumed by the motor is determined by the fan and its load. The increase in power factor does not change the load of the fan, nor does it increase the efficiency of the fan. The actual power consumed by the fan is not reduced. After the power factor is increased, the motor operating state is not changed, the motor stator current is not reduced, and the active power and reactive power consumed by the motor are not changed. The reason for the increase in power factor is that the internal filter capacitor of the inverter generates reactive power to supply the motor. As the power factor increases, the actual input current of the frequency converter decreases, thereby reducing the line loss between the grid and the inverter and the copper loss of the transformer. At the same time, the load current is reduced, and the power distribution equipment such as transformers, switches, contactors, and wires that supply power to the inverter can carry more loads. It should be pointed out that if the loss of line loss and copper loss of the transformer is not considered as in this example, and the loss of the inverter is considered, the inverter not only has no energy saving but also consumes electricity when operating at 50 Hz full load. Therefore, it is not correct to calculate the energy saving effect with apparent power.

The centrifugal fan of a cement plant has a model of Y280S-4, rated power of 75kW, rated voltage of 380V and rated current of 140A. Before the frequency conversion speed regulation was reformed, the valve was fully opened. It was found through testing that the motor current was 70A, only 50% load, the power factor was 0.49, the active power was 22.6kW, and the apparent power was 4607kVA. After the frequency conversion speed regulation is adopted, when the valve is fully open and the rated speed is running, the average current of the three-phase power grid is 37A, so that the power saving (70-37) ÷ 70×100%=44.28% is considered. This calculation seems reasonable, and the energy saving effect is still calculated in terms of apparent power. After further testing, the plant found that the power factor was 0.94, the active power was 22.9 kW, and the apparent power was 24.4 kVA. It can be seen that the active power is increased, not only does not save electricity, but it costs electricity. The reason for the increase in active power is to consider the loss of the inverter without considering the line loss and the copper loss of the transformer. The key to this error is that the effect of the power factor increase on the current drop is not considered. The default power factor is unchanged, which exaggerates the energy saving effect of the inverter. Therefore, when calculating the energy saving effect, active power must be used, and apparent power cannot be used.

Misunderstanding: Contactor cannot be installed on the output side of the inverter [Inverter energy feedback unit]

Almost all inverter instruction manuals indicate that the contactor cannot be installed on the output side of the inverter. For example, the Japanese Yaskawa Inverter Manual stipulates that “Do not connect the electromagnetic switch or electromagnetic contactor to the output circuit”.

The manufacturer's regulations are to prevent the contactor from operating when the inverter has an output. When the inverter is connected to the load during operation, the overcurrent protection circuit will operate due to leakage current. Then, as long as between the output of the inverter and the action of the contactor, the necessary control interlock is added to ensure that the contactor can only operate when there is no output of the inverter, and the output side of the inverter can be installed. Device. This kind of scheme is of great significance for the case where there is only one inverter and two motors (one motor is running and one motor is used). When the running motor fails, it is convenient to switch the inverter to the standby motor. After the delay, the inverter runs, and the standby motor is automatically put into the frequency conversion operation. And it is also convenient to realize the mutual standby of the two motors.

Misunderstanding: the application of the frequency converter in the centrifugal fan can completely replace the adjustment door of the fan

The frequency converter is used to adjust the speed of the centrifugal fan to control the air volume, and has obvious power saving effect compared with the control valve to control the air volume. However, in some cases, the inverter cannot completely replace the valve of the fan, and special attention should be paid in the design. To illustrate this problem, let's start with its power saving principle. The air volume of the centrifugal fan is proportional to the primary speed of the rotational speed. The wind pressure is proportional to the square of the rotational speed, and the axial power is proportional to the cube of the rotational speed.

The wind turbine is at a constant speed, the wind pressure-air volume (H-Q) characteristic; the curve (2) is the wind resistance characteristic of the pipe network (the valve opening is fully open). When the fan is working at point A, the output air volume is Q1. At this time, the shaft power N1 is proportional to the product area of the Q1 and H1 (AH1OQ1). When the air volume is reduced from Q1 to Q2, if the regulating valve method is adopted, the pipe network resistance characteristic is changed to the curve (3). The system changes from the original operating point A to the new operating point B, and the wind pressure increases. The shaft power N2 is proportional to the area (BH2OQ2), and N1 is similar to N2. If the speed control mode is adopted and the fan speed is reduced from n1 to n2, the wind pressure-air volume (HQ) characteristic is as shown by the curve (4). When the same air volume Q2 is satisfied, the wind pressure H3 is greatly reduced, and the power N3 is reduced. (equivalent to the area CH3OQ2) With a significant reduction, the energy saving effect is very significant.

From the above analysis, it can be seen that the valve is controlled to control the air volume. As the air volume decreases, the wind pressure increases instead. The variable speed governor speed is used to control the air volume. As the air volume decreases, the wind pressure drops significantly. The wind pressure drops too much and may not meet the process requirements. That is, if the operating point is inside the curve (1), curve (2), and H-axis, simply relying on the inverter speed regulation will not meet the process requirements, and it needs to be combined with the valve adjustment to meet the process requirements. The frequency converter introduced by a factory, in the application of the centrifugal fan, because there is no design valve, simply relying on the frequency converter speed regulation to change the fan operating point, exhausted the pain. Either the speed is too high and the air volume is too large; if the speed is reduced, the wind pressure can not meet the process requirements, and the air is not blown. Therefore, when the centrifugal fan is used to speed control and save electricity by using the frequency converter, it is necessary to take into consideration the two indicators of air volume and wind pressure, otherwise it will bring undesirable consequences.