Are you 7 "missing" in the 7 major mistakes of the inverter application?

With the advancement and diversification of user needs, the functions of inverter products are constantly improving and increasing, the degree of integration and systemization are also getting higher and higher, and special energy-saving inverter products in some fields have emerged. Looking at China's inverter market, it has maintained a growth rate of 12% to 15% in recent years. Industry insiders predict that the growth rate of inverter installation capacity (power) in the Chinese market is actually around 20%, and the potential market space is about 120 billion to 180 billion yuan. However, the application of the frequency converter is very easy to cause people to misunderstand.

Misunderstanding 1. Power consumption can be saved by using the inverter

Some literature claims that the frequency converter is a power-saving control product, giving the impression that as long as the inverter can save power, in fact, the reason why the inverter can save electricity is because it can adjust the motor. speed. If the inverter is a power-saving control product, then all the speed control equipment can also be said to be a power-saving control product. The inverter is only slightly higher in efficiency and power factor than other speed control devices.

Whether the inverter can achieve power saving is determined by the speed regulation characteristics of its load. For loads such as centrifugal fans and centrifugal pumps, the torque is proportional to the square of the speed, and the power is proportional to the cube of the speed. As long as the original valve is used to control the flow, and it is not working at full load, it can be saved to speed control. When the speed drops to 80% of the original, the power is only 51.2% of the original. It can be seen that the application of the frequency converter in this type of load has the most obvious power saving effect. For loads such as Roots blowers, the torque is independent of the magnitude of the speed, ie the constant torque load. If the air volume is adjusted by using the venting valve to remove the excess air volume, and the speed control operation is changed, the power saving can also be realized. When the speed drops to 80% of the original, the power is 80% of the original. It is much less efficient than the application in centrifugal fans and centrifugal pumps. For constant power loads, the power is independent of the magnitude of the speed. Constant power load of cement plant, such as batching belt scale, under the condition of set flow rate, when the material layer is thick, the belt speed is slowed down; when the material layer is thin, the belt speed is accelerated. The application of the inverter in this type of load cannot save power.

Compared with the DC speed control system, the DC motor is more efficient than the AC motor and has a high power factor. The digital DC governor is as efficient as the inverter. Even the digital DC governor is slightly more efficient than the inverter. Therefore, it is claimed that the use of AC asynchronous motors and frequency converters is more energy efficient than the use of DC motors and DC converters. Theory and practice have proved that this is not true.

Misunderstanding 2, the capacity selection of the inverter is based on the rated power of the motor.

Compared with the electric motor, the price of the frequency converter is relatively expensive, so it is very meaningful to reasonably reduce the capacity of the frequency converter under the premise of ensuring safe and reliable operation.

The power of the inverter refers to the power of the 4-pole AC asynchronous motor to which it is applied.

Due to the same capacity motor, the number of poles is different and the rated current of the motor is different. As the number of poles of the motor increases, the rated current of the motor increases. The capacity selection of the inverter cannot be based on the rated motor power. At the same time, for the transformation project that did not use the inverter, the capacity selection of the inverter can not be based on the rated current of the motor. This is because the capacity selection of the motor should take into account factors such as maximum load, richness factor, motor specifications, etc., and often have a large margin, and industrial motors often operate at 50% to 60% of rated load. If the capacity of the inverter is selected based on the rated current of the motor, the margin is too large, resulting in economic waste, and the reliability is not improved.

For squirrel-cage motors, the capacity of the inverter should be selected based on the principle that the rated current of the inverter is greater than or equal to 1.1 times the maximum normal operating current of the motor, which can save money. For heavy-duty starting, high temperature environment, wound motor, synchronous motor, etc., the capacity of the inverter should be appropriately increased.

For the design of the inverter from the beginning, the choice of the inverter capacity is based on the rated current of the motor. This is because the inverter capacity cannot be selected in actual operation at this time. Of course, in order to reduce investment, in some occasions, the capacity of the inverter can be determined first. After the equipment is actually running for a period of time, it is selected according to the actual current.

In a Φ2?4m&TImes;13m cement mill secondary grinding system of a cement company in Inner Mongolia, there is a domestically produced N-1500 O-Sepa high-efficiency classifier with a motor type Y2-315M-4 and a motor power of 132kW. However, the FRN160-P9S-4E inverter is used. This inverter is suitable for 4-pole, 160kW electric motor. After being put into operation, the maximum working frequency is 48Hz, the current is only 180A, less than 70% of the rated current of the motor, and the motor itself has a considerable margin. The inverter selection specification is one level larger than the drag motor, which causes unnecessary waste and the reliability will not be improved.

The No. 3 limestone crusher of Anhui Chaohu Cement Plant adopts 1500 & TImes; 12000 plate feeder, and the traction motor uses Y225M-4 AC motor. The rated power of the motor is 45kW and the rated current is 84.6A. Before the frequency conversion speed regulation transformation, it was found through the test that when the plate feeder was running normally, the average current of the three phases was only 30A, only 35.5% of the rated current of the motor. In order to save investment, the ACS601-0060-3 type inverter is used. The rated output current of the inverter is 76A. It is suitable for 4-pole, 37kW electric motor, and has achieved good results.

The two examples are reversed and explained. For the transformation project that did not use the inverter, the capacity of the inverter can be greatly reduced according to the actual working conditions.

Misunderstanding 3, using reactive power to calculate reactive power compensation energy saving income

The reactive power compensation energy saving effect is calculated by the apparent power. For example, in the literature [1], when the original system fan power frequency is fully loaded, the motor running current is 289A. When using variable frequency speed regulation, the power factor of 50Hz full load operation is about 0.99, the current is 257A, which is due to the internal filter capacitor of the inverter. Improve the power factor. The energy saving calculation is as follows: ΔS=UI=&TImes;380&TImes;(289-257)=21kVA

Therefore, the paper believes that its energy-saving effect is 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 4, the contactor on the output side of the inverter cannot be installed.

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.

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