If two identical motors work at 50HZ power frequency, one uses a frequency converter and the other does not, and the speed and torque are both at the rated state of the motor, can the frequency converter save power? How much can be saved?
Answer: In this case, the frequency converter can only improve the power factor and cannot save electricity.
1. Frequency conversion cannot save electricity everywhere, and there are many occasions where frequency conversion may not necessarily save electricity.
2. As an electronic circuit, the frequency converter itself also consumes power (about 2-5% of the rated power)
3. It is a fact that frequency converters operate at the power frequency and have energy-saving functions. But his prerequisite is:
Firstly, the device itself has energy-saving function (software support), which matches the requirements of the entire system or process;
Secondly, long-term continuous operation.
Besides, it doesn't matter whether it saves electricity or not, it's meaningless. If it is said that the frequency converter operates energy-saving without any preconditions, it is an exaggeration or commercial speculation. Knowing the whole story, you will cleverly use him to serve you. Be sure to pay attention to the usage situation and conditions in order to apply it correctly, otherwise it will be blindly following, believing easily, and being deceived.
We often have the following misconceptions when using 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 the most 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, the price of frequency converters is relatively 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 the electric motor should consider factors such as maximum load, surplus coefficient, and motor specifications. Often, the surplus is large, and industrial motors often operate at 50% to 60% of the 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: General motors can only operate at a reduced speed using frequency converters 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, resulting in a decrease in torque at the same stator current, softening of mechanical characteristics, and a significant reduction in the overload capacity of the motor.
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.
The author believes that 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 loss 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.
Application example:
The chain bucket conveyor in a certain factory has a constant torque load, and due to the increase in production, its motor speed needs to be increased by 20%. The motor model is Y180L-6, with a rated power of 15kW, a rated voltage of 380V, a rated current of 31.6A, a rated speed of 980r/min, an efficiency of 89.5%, a power factor of 0.81, an operating current of 18-20A, a maximum operating power of 7.5kW under normal conditions, and a load rate of 50%. After installing the CIMR-G5A4015 frequency converter, the operating frequency is 60Hz, the speed is increased by 20%, the maximum output voltage of the frequency converter is set to 410V, the operating current of the motor is 12-15A, which decreases by about 30%, and the temperature of the motor winding decreases significantly.
Misconception 4: 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.
If the secondary air temperature gauge of a cement plant suddenly shows abnormal readings: the indicated value is significantly low and fluctuates greatly. It has been running very well before this. Checked thermocouples, temperature transmitters, and secondary instruments, no issues were found. What are the relevant? When the instrument was moved to another measuring point, it operated completely normally. However, when similar instruments from other measuring points were replaced here, the same phenomenon also occurred. Later, it was discovered that a new frequency converter had been installed on the motor of cooling fan No. 3 in the grate cooler, and it was only after the frequency converter was put into use that the secondary air temperature gauge showed abnormal readings. Stop the frequency converter and immediately restore the secondary air temperature gauge to normal; Restarting the frequency converter, the secondary air temperature gauge showed abnormal readings again. After repeated testing several times, it was determined that the interference from the frequency converter was the direct cause of the abnormal display on the secondary air temperature gauge. The fan is a centrifugal ventilator, which originally used valves to adjust the air volume, but later changed to variable frequency speed regulation to adjust the air volume. Due to the large amount of dust and harsh environment on site, the frequency converter is installed in the MCC (Motor Control Center) control room. For the convenience of construction, the frequency converter is connected to the lower side of the main contactor of the fan, and the output cable of the frequency converter uses the power cable of the fan motor. The power cable of the fan motor is a PVC insulated non steel armored sheathed cable, and is laid parallel to the secondary air temperature meter signal cable in different bridge layers of the same cable trench. It can be seen that it is precisely because the output cable of the frequency converter does not use armored cables or be laid through iron pipes that interference phenomena occur. This lesson should be given special attention to renovation projects that did not originally use frequency converters.
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.