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Misunderstandings in the Application of Inverter

Sep 03, 2020

Misunderstandings in the Application of Inverter

Misunderstanding 1. Use inverter to save electricity


   1. Introduction to the working principle of the equipment.


   Some documents claim that the frequency converter is a power-saving control product, which gives people the feeling that as long as the frequency converter is used, it can save electricity.


  In fact, the reason why the inverter can save electricity is because it can adjust the speed of the motor. If the frequency converter is a power-saving control product, then all speed-regulating equipment can also be said to be a power-saving control product. The frequency converter is just slightly higher in efficiency and power factor than other speed regulating equipment.


   Whether the variable frequency speed controller can achieve power saving is determined by the speed regulation characteristics of its load. For loads such as centrifugal fans and centrifugal water 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 valve is used to control the flow, and it is not working at full load, it can save electricity if it is changed 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 variable frequency speed regulator in this type of load has the most obvious power saving effect. For loads such as Roots blowers, the torque has nothing to do with the speed, that is, constant torque load. If the original method of using the air release valve to release the excess air volume is used to adjust the air volume, and it is changed to speed control operation, it can also save electricity. When the speed drops to 80% of the original, the power is 80% of the original. Compared with centrifugal fans and centrifugal water pumps, the power saving effect is much smaller. For constant power loads, the power has nothing to do with the speed. For constant power loads in cement plants, such as batching belt scales, under a certain set flow condition, when the material layer is thick, the belt speed slows down; when the material layer is thin, the belt speed increases. The application of frequency converter in this kind of load cannot save electricity.


   Compared with the DC speed control system, the DC motor has higher efficiency and higher power factor than the AC motor. The efficiency of the digital DC speed controller and the variable frequency speed controller is about the same, and even the digital DC speed controller is slightly more efficient than the variable frequency speed controller. Therefore, it is claimed that the use of AC asynchronous motors and frequency converters is more energy-saving than the use of DC motors and DC governors. The theory and practice have proved that this is incorrect.


  Misunderstanding 2. The capacity of the inverter is selected based on the rated power of the motor


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


  The power of the variable frequency speed controller refers to the power of the 4-pole AC asynchronous motor to which it is applicable.


   Because of the same capacity motors, the number of poles is different, and the rated current of the motor is different. As the number of motor poles increases, the motor rated current increases. The capacity selection of the frequency converter cannot be based on the rated power of the motor. At the same time, for the original transformation projects that did not use the frequency converter, the capacity selection of the frequency converter cannot be based on the rated current of the motor. This is because the selection of the capacity of the motor should consider factors such as maximum load, richness factor, motor specifications, etc., which are often richer, and industrial motors often run under 50% to 60% rated load. If the capacity of the frequency conversion governor is selected based on the rated current of the motor, the margin left is too large, causing economic waste, and the reliability has not been improved.


   For squirrel-cage motors, the capacity selection of the frequency converter should be based on the principle that the rated current of the frequency converter is greater than or equal to 1.1 times the maximum normal working current of the motor, which can save money to the greatest extent. For heavy-load starting, high temperature environment, wound motor, synchronous motor and other conditions, the capacity of the frequency converter should be appropriately increased.


For the design that adopted the frequency converter from the beginning, it is understandable that the capacity of the frequency converter is selected based on the rated current of the motor. This is because the inverter capacity cannot be selected based on actual operating conditions. Of course, in order to reduce investment, in some occasions, the capacity of the inverter can also be determined first, and after the actual operation of the equipment for a period of time, the selection can be made according to the actual current.

Misunderstanding 3: Use apparent power to calculate reactive power compensation energy-saving benefits


   Use apparent power to calculate the energy saving effect of reactive power compensation. For example, in document [1], when the fan of the original system works at full power frequency, the motor running current is 289A. When using variable frequency speed regulation, the power factor at 50Hz full load operation is about 0.99, and the current is 257A. This is due to the internal filter capacitor of the inverter. The role of improving power factor. Energy saving calculation is as follows: ΔS=UI=×380×(289-257)=21kVA


   Therefore, the article believes that its energy saving effect is about 11% of the stand-alone capacity.


   Actual analysis: S means the apparent power, that is, the product of voltage and current. When the voltage is the same, the apparent power saving percentage and the current saving percentage are the same thing. In a circuit with reactance, the apparent power only reflects the maximum allowable output capacity of the power distribution system, not the actual power consumed by the motor. The actual power consumed by the motor can only be expressed by active power. In this example, although the actual current is used for the calculation, 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 in power factor did not change the load of the fan, nor did it increase the efficiency of the fan, and the actual power consumed by the fan did not decrease. After the power factor is improved, the operating state of the motor has not changed, the stator current of the motor has not been reduced, and the active power and reactive power consumed by the motor have not changed. The reason for the improvement of the power factor is that the internal filter capacitor of the frequency converter generates reactive power for the consumption of the motor. As the power factor increases, the actual input current of the inverter 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 transformers, switches, contactors, wires and other power distribution equipment that supply the inverter can carry more loads. It needs to be pointed out that if the line loss and copper loss of the transformer are not saved as in this example, but the loss of the inverter is considered, when the inverter is running at 50Hz full load, not only does it not save energy, but it also consumes electricity. Therefore, it is wrong to use apparent power to calculate the energy saving effect.


   The model of the centrifugal fan drive motor of a cement plant is Y280S-4, the rated power is 75kW, the rated voltage is 380V, and the rated current is 140A. Before the transformation of frequency conversion and speed regulation, the valve was fully opened. The test found that the motor current was 70A, only 50% load, power factor was 0.49, active power was 22.6kW, and apparent power was 4607kVA. After adopting the frequency conversion speed regulation transformation, the valve is fully opened and the average current of the three-phase grid is 37A when running at the rated speed, so it is considered that power saving (70-37) ÷ 70×100%=44.28%. This calculation seems reasonable, but the energy saving effect is still calculated based on the apparent power. After further testing, the factory found that the power factor was 0.94, the active power was 22.9kW, and the apparent power was 24.4kVA. It can be seen that the increase in active power not only does not save electricity, but consumes electricity. The reason for the increase in active power is that the loss of the frequency converter is considered, but the saving of line loss and transformer copper consumption is not considered. The key to this error is that it does not consider the impact of the power factor increase on the current drop, and the default power factor remains unchanged, which unilaterally exaggerates the energy-saving effect of the inverter. Therefore, when calculating the energy saving effect, active power must be used instead of apparent power.


   Misunderstanding 4. No contactor can be installed on the output side of the inverter


  Almost all inverter manuals point out that no contactor can be installed on the output side of the inverter. For example, **Yaskawa inverter manual stipulates "Do not connect electromagnetic switch or electromagnetic contactor to the output circuit".


   The manufacturer's regulations are to prevent the contactor from operating when the inverter has output. When the inverter is connected to a load during operation, the overcurrent protection circuit will operate due to leakage current. Then, as long as the necessary control interlock is added between the output of the frequency converter and the action of the contactor to ensure that the contactor can only operate when the frequency converter has no output, and the contact can be installed on the output side of the frequency converter Device. This kind of scheme is of great significance for occasions where there is only one frequency converter and two motors (1 motor is running and 1 motor is in standby). When the running motor fails, the frequency converter can be easily switched to the standby motor, and the frequency converter can be operated after a delay, so that the standby motor can be automatically put into frequency conversion operation. And it can easily realize the mutual backup of two motors.


   Misunderstanding 5. General-purpose motors can only use frequency converters to reduce speed below their rated speed


   Classical theory believes that the upper limit of the general-purpose motor frequency is 55Hz. This is because when the motor speed needs to be adjusted to run above the rated speed, the stator frequency will increase to higher than the rated frequency (50Hz). At this time, if the constant torque is still controlled, the stator voltage will rise above the rated voltage. Then, when the speed range is higher than the rated speed, the stator voltage must be kept at the rated voltage. At this time, as the speed/frequency increases, the magnetic flux will decrease, so the torque under the same stator current will decrease, the mechanical characteristics will become softer, and the overload capacity of the motor will be greatly reduced.


   It can be seen that there are prerequisites for the upper limit of the general motor frequency to be 55Hz:

   1. The stator voltage cannot exceed the rated voltage;

   2. The motor runs at rated power;

   3. Constant torque load.


   Under the above circumstances, theory and experiment have proved that if the frequency exceeds 55Hz, the motor torque will be reduced, the mechanical characteristics will become softer, the overload capacity will decrease, the iron loss will increase rapidly, and the heat will be serious.


  The author believes that the actual operating conditions of the motor indicate that a general-purpose motor can be operated at an increased speed through a frequency converter. Can the speed be increased by frequency conversion? How much can be increased? Mainly determined by the load driven by the motor. First of all, what is the load factor? Secondly, it is necessary to understand the load characteristics and calculate according to the specific conditions of the load. A simple analysis is as follows:


   1. In fact, for 380V general-purpose motors, long-term operation with a stator voltage exceeding 10% of the rated voltage is possible, and it has no effect on the insulation and life of the motor. The stator voltage increases, the torque increases significantly, the stator current decreases, and the winding temperature decreases.


  2. Motor load rate is usually 50%~60%

   Under normal circumstances, industrial motors usually work at 50% to 60% rated power. After calculation, the output power of the motor is 70% of the rated power, and when the stator voltage is increased by 7%, the stator current drops by 26.4%. At this time, even with constant torque control, the frequency converter is used to increase the motor speed by 20%, and the stator current is not limited. Will not rise, but will fall. Although the iron consumption of the motor increases sharply after the frequency is increased, the heat generated by it is very small compared to the heat reduced by the decrease in the stator current. Therefore, the motor winding temperature will also drop significantly.


  3, various load characteristics

  The motor drive system serves the load. Different loads have different mechanical characteristics. The motor must meet the requirements of the mechanical characteristics of the load after the speed is increased. It is calculated that the allowable maximum operating frequency (fmax) at different load rates (k) for constant torque loads is inversely proportional to the load rate, that is, fmax=fe/k, where fe is the rated power frequency. For constant power loads, the maximum allowable operating frequency of general-purpose motors is mainly limited by the mechanical strength of the motor rotor and shaft. The author believes that it is generally appropriate to limit it to less than 100 Hz.