We've always said that frequency converters save electricity, and with so many people saying it, it's become accepted. But does it actually save electricity?
If two identical motors are both operating at a 50Hz power frequency, one using a frequency converter and the other not, and both their speed and torque are at their rated values, can the frequency converter still save electricity? And how much can it save?
In fact, in this situation, frequency converters can only improve the power factor, not save electricity. This is because frequency conversion doesn't save electricity everywhere; in many situations, using frequency converters doesn't necessarily save electricity. Furthermore, the frequency converter itself also consumes electricity (approximately 2-5% of its rated power). Additionally, while it's true that frequency converters have energy-saving functions when operating at the mains frequency, this is contingent on the device itself having energy-saving capabilities (software support), meaning it must match the requirements of the entire system or process.
Claiming that frequency converters are energy-efficient when operating at industrial frequency without any preconditions is either exaggeration or commercial hype. Understanding the underlying principles will allow you to skillfully utilize them for your benefit. However, it is crucial to pay attention to the application scenarios and conditions to ensure correct use; otherwise, you risk being blindly followed, easily deceived, and ultimately cheated.
We often have the following misconceptions when using frequency converters:
Myth 1: Using a frequency converter can save electricity.
Some documents claim that variable frequency drives (VFDs) are energy-saving control products, giving the impression that using a VFD can save electricity.
In fact, the reason why variable frequency drives (VFDs) can save electricity is because they can regulate the speed of motors. If VFDs are considered energy-saving control products, then all speed control devices can also be considered energy-saving control products; VFDs are simply more efficient and have a slightly higher power factor than other speed control devices.
Compared to DC speed control systems, DC motors are more efficient and have a higher power factor than AC motors. Digital DC speed controllers are comparable in efficiency to frequency converters, and sometimes even slightly more efficient. Therefore, the claim that using AC asynchronous motors and frequency converters is more energy-efficient than using DC motors and DC speed controllers is incorrect, both theoretically and practically.
Myth 2: The capacity of the frequency converter is selected based on the rated power of the motor.
Compared to electric motors, variable frequency drives (VFDs) are more expensive. Therefore, it is very meaningful to reasonably reduce the capacity of VFDs while ensuring safe and reliable operation.
The power of a variable frequency drive refers to the power of the 4-pole AC asynchronous motor to which it is applicable.
For motors of the same capacity, different pole numbers result in different rated currents. As the number of poles increases, the rated current also increases. Therefore, the capacity selection of a variable frequency drive (VFD) cannot be based solely on the motor's rated power. Furthermore, for retrofit projects that did not previously use VFDs, the VFD's capacity selection should not be based solely on the motor's rated current. This is because motor capacity selection must consider factors such as maximum load, safety margin, and motor specifications, often resulting in a significant safety margin. Industrial motors frequently operate at 50%–60% of their rated load. Selecting the VFD capacity based solely on the motor's rated current would leave too much of a safety margin, leading to economic waste without improving reliability.
In designs that utilize frequency converters from the outset, selecting the converter capacity based on the motor's rated current is perfectly acceptable. This is because the converter capacity cannot be chosen based on actual operating conditions at this stage. However, to reduce investment, in some cases, the converter capacity can be initially determined, and then selected based on the actual current after the equipment has been running for a period of time.
Myth 3: General-purpose electric motors can only be operated at speeds reduced by frequency converters below their rated speed.
Classical theory holds that the upper limit of frequency for general-purpose electric motors is 55Hz. This is because when the motor speed needs to be adjusted to above the rated speed, the stator frequency will increase to above the rated frequency (50Hz). At this point, if constant torque control is still applied, the stator voltage will rise above the rated voltage. Therefore, when the speed range exceeds the rated speed, the stator voltage must be kept constant at the rated voltage. In this case, as the speed/frequency increases, the magnetic flux decreases, thus the torque under the same stator current will decrease, the mechanical characteristics will soften, and the motor's overload capacity will be significantly reduced.
Therefore, the 55Hz upper limit for the frequency of general-purpose electric motors is conditional:
(1) The stator voltage must not exceed the rated voltage;
(2) The motor is operating at its rated power;
(3) Constant torque load.
Under the above conditions, theory and experiments have shown that if the frequency exceeds 55Hz, the motor torque will decrease, the mechanical characteristics will become softer, the overload capacity will decrease, the iron loss will increase sharply, and the heat generation will be severe.
The author believes that actual operating conditions of electric motors indicate that general-purpose electric motors can be sped up using frequency converters. Whether or not speed can be increased via frequency conversion, and by how much, depends primarily on the load driven by the motor. First, the load rate must be determined. Second, the load characteristics must be understood, and calculations can be made based on the specific load conditions. A simple analysis follows:
1. In fact, for a 380V general-purpose motor, it is permissible to operate for an extended period with the stator voltage exceeding the rated voltage by 10% without affecting the motor's insulation or lifespan. Increasing the stator voltage significantly increases the torque, reduces the stator current, and lowers the winding temperature.
2. The motor load rate is typically 50%–60%.
Industrial electric motors typically operate at 50%–60% of their rated power. Calculations show that when the motor output power is 70% of its rated power, a 7% increase in stator voltage results in a 26.4% decrease in stator current. At this point, even with constant torque control, increasing the motor speed by 20% using a frequency converter will not only prevent the stator current from increasing but will actually decrease it. Although increasing the frequency leads to a sharp increase in iron losses, the heat generated is negligible compared to the heat reduction due to the decrease in stator current. Therefore, the motor winding temperature will also decrease significantly.
3. Load characteristics vary.
Electric motor drive systems serve loads, and different loads have different mechanical characteristics. After speeding up, the electric motor must meet the mechanical characteristics requirements of the load. Calculations show that the maximum permissible 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 operating frequency. For constant power loads, the maximum permissible operating frequency of a general-purpose electric motor is mainly limited by the mechanical strength of the motor rotor and shaft; the author believes it is generally advisable to limit it to below 100Hz.
Myth 4: Ignoring the inherent characteristics of frequency converters
The commissioning of variable frequency drives (VFDs) is generally handled by the distributor and should not present any problems. Installation is relatively simple and is usually done by the user. However, some users fail to carefully read the VFD's instruction manual, do not strictly follow the technical requirements during installation, ignore the VFD's unique characteristics, treat it like any other electrical component, and rely on assumptions and experience, thus creating potential hazards for malfunctions and accidents.
According to the user manual of the frequency converter, the cable connected to the motor should be a shielded or armored cable, preferably laid in a metal conduit. The cable ends should be cut as neatly as possible, and unshielded segments should be as short as possible; the cable length should not exceed a certain distance (generally 50m). When the wiring distance between the frequency converter and the motor is long, high harmonic leakage current from the cable can adversely affect the frequency converter and surrounding equipment. The grounding wire returning from the motor controlled by the frequency converter should be directly connected to the corresponding grounding terminal of the frequency converter. The frequency converter's grounding wire should never be shared with welding machines or power equipment and should be as short as possible. Due to leakage current generated by the frequency converter, the potential of the grounding terminal will be unstable if it is too far from the grounding point. The minimum cross-sectional area of the frequency converter's grounding wire must be greater than or equal to the cross-sectional area of the power supply cable. To prevent malfunctions caused by interference, the control cable should use stranded shielded wire or double-strand shielded wire. Also, care should be taken to avoid contact between the shielded wire and other signal lines or equipment casings; wrap it with insulating tape. To avoid noise interference, the length of the control cable should not exceed 50m. Control cables and motor cables must be laid separately, using dedicated cable trays, and kept as far apart as possible. If they must cross, they should cross perpendicularly. Never place them in the same conduit or cable tray. Some users fail to strictly follow these requirements during cable laying, resulting in equipment operating normally during individual testing but experiencing severe interference during normal production, rendering it inoperable.
Special care must be taken during the routine maintenance of variable frequency drives (VFDs). Some electricians, upon discovering a VFD has tripped, immediately open it for repairs. This is extremely dangerous and could result in electric shock. Even when the VFD is not running, or even if the power supply has been disconnected, the power input lines, DC terminals, and motor terminals may still carry voltage due to the presence of capacitors. After disconnecting the switch, it is essential to wait several minutes for the VFD to discharge completely before resuming operation. Some electricians also habitually use a megohmmeter to perform an insulation test on the motor driven by the VFD as soon as a trip is discovered, to determine if the motor is burnt out. This is also very dangerous and could easily burn out the VFD itself. Therefore, insulation testing of the motor or the cable already connected to the VFD should never be performed before the cable between the motor and the VFD is disconnected.
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