Suppliers of energy feedback units for frequency converters remind you that with the implementation of policies and the vigorous promotion of frequency conversion technology, coupled with the strong promotion of frequency converter merchants, some industrial enterprises have subconsciously equated the use of frequency converters with energy conservation and electricity saving. However, in practical use, due to different situations faced, many enterprises gradually realize that not all places where frequency converters are applied can save energy and electricity. So what are the reasons for this situation and what are the misconceptions people have about frequency converters?
Misconception 1: Using a frequency converter can save electricity
Some literature claims that frequency converters are energy-saving control products, giving the impression that using frequency converters can save electricity.
In fact, the reason why frequency converters can save electricity is because they can regulate the speed of electric motors. If frequency converters are energy-saving control products, then all speed control equipment can also be considered energy-saving control products. The frequency converter is just slightly more efficient and power factor than other speed control devices.
Whether a frequency converter can achieve power saving is determined by the speed regulation characteristics of its load. For loads such as centrifugal fans and centrifugal pumps, torque is proportional to the square of speed, and power is proportional to the cube of speed. As long as the original valve control flow is used and it is not operating at full load, changing to speed regulation operation can achieve energy saving. 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 frequency converters in such loads has a significant energy-saving effect. For loads such as Roots blowers, the torque is independent of the speed, i.e. constant torque load. If the original method of using a vent valve to release excess air volume to adjust the air volume is changed to speed regulation operation, it can also achieve energy saving. When the speed drops to 80% of its original value, the power reaches 80% of its original value. The energy-saving effect is much smaller than that of applications in centrifugal fans and centrifugal pumps. For constant power loads, the power is independent of the speed. A constant power load in a cement plant, such as a batching belt scale, slows down the belt speed when the material layer is thick under certain flow conditions; When the material layer is thin, the belt speed increases. The application of frequency converters in such loads cannot save electricity.
Compared with DC speed control systems, DC motors have higher efficiency and power factor than AC motors. The efficiency of digital DC speed controllers is comparable to that of frequency converters, and even slightly higher than that of frequency converters. So, it is incorrect to claim that using AC asynchronous motors and frequency converters saves more electricity than using DC motors and DC controllers, both theoretically and practically.
Misconception 2: The capacity selection of the frequency converter is based on the rated power of the motor
Compared to electric motors, frequency converters are more expensive, so it is very meaningful to reasonably reduce the capacity of frequency converters while ensuring safe and reliable operation.
The power of a frequency converter refers to the power of the 4-pole AC asynchronous motor it is suitable for.
Due to the different number of poles of motors with the same capacity, the rated current of the motor varies. As the number of poles in the motor increases, the rated current of the motor also increases. The capacity selection of the frequency converter cannot be based on the rated power of the motor. At the same time, for renovation projects that did not originally use frequency converters, the capacity selection of frequency converters cannot be based on the rated current of the motor. This is because the capacity selection of electric motors should consider factors such as load, surplus coefficient, and motor specifications. Often, the surplus is large, and industrial motors operate at 50% to 60% of their rated load. If the capacity of the frequency converter is selected based on the rated current of the motor, there is too much margin left, resulting in economic waste, and the reliability is not improved as a result.
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 operating current of the motor, which can maximize cost savings. For conditions such as heavy load starting, high temperature environment, wound motor, synchronous motor, etc., the capacity of the frequency converter should be appropriately increased.
For designs that use frequency converters from the beginning, it is understandable to choose the capacity of the frequency converter based on the rated current of the motor. This is because the capacity of the frequency converter cannot be selected based on actual operating conditions at this time. Of course, in order to reduce investment, in some cases, the capacity of the frequency converter can be uncertain first, and after the equipment has been running for a period of time, it can be selected based on the actual current.
In the secondary grinding system of a cement mill with a diameter of 2.4m × 13m in a certain cement company in Inner Mongolia, there is one domestically produced N-1500 O-Sepa high-efficiency powder selector, equipped with an electric motor model Y2-315M-4 with a power of 132kW. However, FRN160-P9S-4E frequency converter is selected, which is suitable for 4-pole motors with a power of 160kW. After being put into operation, the maximum working frequency is 48Hz, and the current is only 180A, which is less than 70% of the rated current of the motor. The motor itself has considerable surplus capacity. And the specifications of the frequency converter are one level larger than those of the driving motor, which causes unnecessary waste and does not improve reliability.
The feeding system of the No. 3 limestone crusher at Anhui Chaohu Cement Plant adopts a 1500 × 12000 plate feeder, and the driving motor uses a Y225M-4 AC motor with a rated power of 45kW and a rated current of 84.6A. Before the frequency conversion speed regulation transformation, it was found through testing that when the plate feeder drives the motor normally, the average three-phase current is only 30A, which is only 35.5% of the rated current of the motor. In order to save investment, ACS601-0060-3 frequency converter was selected, which has a rated output current of 76A and is suitable for 4-pole motors with a power of 37kW, achieving good performance.
These two examples illustrate that for renovation projects that did not originally use frequency converters, selecting the capacity of the frequency converter based on actual operating conditions can significantly reduce investment.
Misconception 3: Using visual power to calculate reactive power compensation and energy-saving benefits
Calculate the energy-saving effect of reactive power compensation using apparent power. When the fan operates at full load at power frequency, the operating current of the motor is 289A. When using variable frequency speed regulation, the power factor at full load operation at 50Hz is about 0.99, and the current is 257A. This is because the internal filtering capacitor of the frequency converter improves the power factor. The energy-saving calculation is as follows: Δ S=UI=× 380 × (289-257)=21kVA
Therefore, it is believed that its energy-saving effect is about 11% of the single machine capacity.
Actual analysis: S represents the apparent power, which is the product of voltage and current. When the voltage is the same, the percentage of apparent power savings and the percentage of current savings are the same thing. In a circuit with reactance, apparent power only reflects the maximum allowable output capacity of the distribution system, and cannot reflect the actual power consumed by the motor. The actual power consumed by the electric motor can only be expressed as active power. In this example, although the actual current is used for calculation, the apparent power is calculated instead of the active power. We know that the actual power consumption of an electric 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 improve the efficiency of the fan. The actual power consumption of the fan did not decrease. After the power factor was increased, the operating state of the motor did not change, the stator current of the motor did not decrease, and the active and reactive power consumed by the motor did not change. The reason for the increase in power factor is that the internal filtering capacitor of the frequency converter generates reactive power, which is supplied to the motor for consumption. As the power factor increases, the actual input current of the frequency converter decreases, thereby reducing the line loss between the power grid and the frequency converter and the copper loss of the transformer. At the same time, as the load current decreases, distribution equipment such as transformers, switches, contactors, and wires that supply power to the frequency converter can carry more loads. It should be pointed out that if we do not consider the savings of line loss and transformer copper loss as in this example, but consider the losses of the frequency converter, when the frequency converter operates at full load at 50Hz, it not only does not save energy, but also consumes electricity. Therefore, using apparent power to calculate energy-saving effects is incorrect.
The centrifugal fan driving motor model of a certain cement plant is Y280S-4, with a rated power of 75kW, rated voltage of 380V, and rated current of 140A. Before the frequency conversion speed regulation transformation, the valve was fully opened. Through testing, it was found that the motor current was 70A, with only 50% load, power factor of 0.49, active power of 22.6kW, and apparent power of 46.07kVA. After adopting variable frequency speed regulation, when the valve is fully opened and the rated speed is running, the average current of the three-phase power grid is 37A, thus it is considered that energy saving (70-37) ÷ 70 × 100%=44.28%. This calculation may seem reasonable, but in essence, it still calculates the energy-saving effect based on apparent power. After further testing, the factory 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 an increase in active power not only does not save electricity, but also consumes electricity. The reason for the increase in active power is that the losses of the frequency converter were taken into account, without considering the savings of line losses and transformer copper losses. The key to this error lies in the failure to consider the impact of increasing power factor on current drop, and the default power factor remains unchanged, thus exaggerating the energy-saving effect of the frequency converter. Therefore, when calculating the energy-saving effect, active power must be used instead of apparent power.
Misconception 4: Contactors cannot be installed on the output side of the frequency converter
Almost all user manuals for frequency converters indicate that contactors cannot be installed on the output side of the frequency converter. As stated in the manual of Yaskawa frequency converter in Japan, "Do not connect electromagnetic switches or electromagnetic contactors in the output circuit".
The manufacturer's regulations are to prevent the contactor from operating when the frequency converter has output. When the frequency converter is connected to a load during operation, the overcurrent protection circuit will be activated due to leakage current. So, as long as necessary control interlocks are 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, a contactor can be installed on the output side of the frequency converter. This scheme is of great significance for situations where there is only one frequency converter and two motors (one motor in operation and one motor as backup). When the running motor malfunctions, the frequency converter can be easily switched to the backup motor, and after a delay, the frequency converter can be operated to automatically put the backup motor into frequency conversion operation. And it can also easily achieve mutual backup of two electric motors.
Misconception 5: The application of frequency converters in centrifugal fans can completely replace the regulating door of the fan
Using a frequency converter to regulate the speed of a centrifugal fan to control the air volume has a significant energy-saving effect compared to controlling the air volume through regulating valves. However, in some cases, the frequency converter cannot completely replace the valve of the fan, and special attention should be paid in the design. To illustrate this issue, let's start with its energy-saving principle. The air volume of a centrifugal fan is proportional to the power of its rotational speed, the air pressure is proportional to the square of its rotational speed, and the shaft power is proportional to the cube of its rotational speed.
Wind pressure air volume (H-Q) characteristics of the fan at constant speed; Curve (2) represents the wind resistance characteristics of the pipeline network (valve fully open). When the fan operates at point A, the output air volume is Q1. At this time, the shaft power N1 is proportional to the product area of Q1 and H1 (AH1OQ1). When the air volume decreases from Q1 to Q2, if the valve adjustment method is used, the resistance characteristics of the pipeline network will change to curve (3). The system operates from the original operating point A to the new operating point B, and the wind pressure increases instead. The shaft power N2 is proportional to the area (BH2OQ2), and N1 is not much different from N2. If the speed control method is adopted, the fan speed decreases from n1 to n2, and the wind pressure air volume (H-Q) characteristics are shown in curve (4). Under the same air volume Q2, the wind pressure H3 decreases significantly, and the power N3 (equivalent to the area CH3OQ2) decreases significantly, indicating a significant energy-saving effect.
From the above analysis, it can also be seen that adjusting the valve to control the air volume, as the air volume decreases, the air pressure actually increases; And using a frequency converter to control the air volume, as the air volume decreases, the air pressure drops significantly. If the wind pressure drops too much, it may not meet the process requirements. If the operating point is within the area enclosed by curve (1), curve (2), and the H-axis, relying solely on a frequency converter for speed regulation will not meet the process requirements. It needs to be combined with valve regulation to meet the process requirements. The frequency converter introduced by a certain factory, in the application of centrifugal fans, suffered a lot due to the lack of valve design and relying solely on frequency converter speed regulation to change the operating point of the fan. Either the speed is too high or the air volume is too large; If the speed is reduced, the wind pressure cannot meet the process requirements, and the air cannot be blown in. Therefore, when using a frequency converter for speed regulation and energy saving in centrifugal fans, it is necessary to consider both air volume and air pressure indicators, otherwise it will bring adverse consequences.
Misconception 6: General motors can only operate at a reduced speed using a frequency converter below their rated transmission speed
The classical theory holds that the upper limit of the frequency of a universal motor is 55Hz. This is because when the motor speed needs to be adjusted above the rated speed for operation, the stator frequency will increase above the rated frequency (50Hz). At this point, if the constant torque principle is still followed for control, the stator voltage will increase beyond the rated voltage. So, when the speed range is higher than the rated speed, the stator voltage must be kept constant at the rated voltage. At this point, as the speed/frequency increases, the magnetic flux will decrease, so the torque at the same stator current will decrease, the mechanical characteristics will become softer, and the overload capacity of the motor will be greatly reduced.
From this, it can be seen that the upper limit of the frequency of a universal motor is 55Hz, which is a prerequisite:
1. The stator voltage cannot exceed the rated voltage;
2. The motor is operating at rated power;
3. Constant torque load.
In the above situation, theory and experiments have proven that if the frequency exceeds 55Hz, the motor torque will decrease, mechanical characteristics will become softer, overload capacity will decrease, iron consumption will increase rapidly, and heating will be severe.
Generally speaking, the actual operating conditions of electric motors indicate that general-purpose motors can be accelerated through frequency converters. Can variable frequency speed be increased? How much can be raised? It is mainly determined by the load dragged by the electric motor. Firstly, it is necessary to determine what the load rate is? Secondly, it is necessary to understand the load characteristics and make calculations based on the specific situation of the load. A brief analysis is as follows:
1. In fact, for a 380V universal motor, it is possible to operate it for a long time when the stator voltage exceeds 10% of the rated voltage, without affecting the insulation and lifespan of the motor. The stator voltage increases, the torque significantly increases, the stator current decreases, and the winding temperature decreases.
2. The load rate of the electric motor is usually 50% to 60%
Generally, industrial motors operate at 50% to 60% of their rated power. By calculation, when the output power of the motor is 70% of the rated power and the stator voltage increases by 7%, the stator current decreases by 26.4%. At this time, even with constant torque control and using a frequency converter to increase the motor speed by 20%, the stator current not only does not increase but also decreases. Although the iron consumption of the motor increases sharply after increasing the frequency, the heat generated by it is negligible compared to the heat reduced by the decrease in stator current. Therefore, the temperature of the motor winding will also significantly decrease.
3. There are various load characteristics
The electric motor drive system serves the load, and different loads have different mechanical characteristics. Electric motors must meet the requirements of load mechanical characteristics after acceleration. According to calculations, the maximum allowable operating frequency (fmax) for constant torque loads at different load rates (k) is inversely proportional to the load rate, i.e. fmax=fe/k, where fe is the rated power frequency. For constant power loads, the maximum allowable operating frequency of general motors is mainly limited by the mechanical strength of the motor rotor and shaft. The author believes that it is generally advisable to limit it to within 100Hz.
Misconception 7: Neglecting the inherent characteristics of frequency converters
The debugging work of the frequency converter is usually completed by the distributor, and there will be no problems. The installation of a frequency converter is relatively simple and usually completed by the user. Some users do not carefully read the user manual of the frequency converter, do not strictly follow the technical requirements for construction, ignore the characteristics of the frequency converter itself, equate it with general electrical components, and act based on assumptions and experience, laying hidden dangers for faults and accidents.
According to the user manual of the frequency converter, the cable connected to the motor should be a shielded cable or armored cable, preferably laid in a metal tube. The ends of the cut cable should be as neat as possible, the unshielded segments should be as short as possible, and the cable length should not exceed a certain distance (usually 50m). When the wiring distance between the frequency converter and the motor is long, the high harmonic leakage current from the cable will have adverse effects on the frequency converter and surrounding equipment. The grounding wire returned from the motor controlled by the frequency converter should be directly connected to the corresponding grounding terminal of the frequency converter. The grounding wire of the frequency converter should not be shared with welding machines and power equipment, and should be as short as possible. Due to the leakage current generated by the frequency converter, if it is too far from the grounding point, the potential of the grounding terminal will be unstable. The minimum cross-sectional area of the grounding wire of the frequency converter must be greater than or equal to the cross-sectional area of the power supply cable. To prevent misoperation caused by interference, control cables should use twisted shielded wires or double stranded shielded wires. At the same time, be careful not to touch the shielded network cable with other signal lines and equipment casings, and wrap it with insulating tape. To avoid being affected by noise, the length of the control cable should not exceed 50m. The control cable and motor cable must be laid separately, using separate cable trays, and kept as far away as possible. When the two must cross, they should be crossed vertically. Never put them in the same pipeline or cable tray. However, some users did not strictly follow the above requirements when laying cables, resulting in the equipment running normally during individual debugging but causing serious interference during normal production, making it unable to operate.
Special care should also be taken in the daily maintenance of frequency converters. Some electricians immediately turn on the frequency converter for maintenance as soon as they detect a fault and trip it. This is very dangerous and may result in personal electric shock accidents. This is because even if the frequency converter is not in operation or the power supply has been cut off, there may still be voltage on the power input line, DC terminal, and motor terminal of the frequency converter due to the presence of capacitors. After disconnecting the switch, it is necessary to wait for a few minutes for the frequency converter to discharge completely before starting to work. Some electricians are accustomed to immediately conducting insulation tests on the motor driven by the variable frequency drive system using a shaking table when they notice the system tripping, in order to determine whether the motor has been burned out. This is also very dangerous, as it can easily cause the frequency converter to burn. Therefore, before disconnecting the cable between the motor and the frequency converter, insulation testing must not be performed on the motor, nor on the cable already connected to the frequency converter.
Special attention should also be paid when measuring the output parameters of the frequency converter. Due to the fact that the output of the frequency converter is a PWM waveform containing high-order harmonics, and the motor torque mainly depends on the effective value of the fundamental voltage, when measuring the output voltage, the fundamental voltage value is mainly measured using a rectifier voltmeter. The measurement results are closest to those measured by a digital spectrum analyzer and have an excellent linear relationship with the output frequency of the frequency converter. If further improvement of measurement accuracy is needed, a resistive capacitive filter can be used. Digital multimeters are prone to interference and have significant measurement errors. The output current needs to measure the total effective value including the fundamental wave and other high-order harmonics, so the commonly used instrument is the moving coil ammeter (when the motor is loaded, the difference between the fundamental current effective value and the total current effective value is not significant). When considering the convenience of measurement and using a current transformer, the current transformer may saturate at low frequencies, so it is necessary to choose a current transformer of appropriate capacity.