Myth 1: Using a frequency converter can save electricity.
Some literature claims that variable frequency drives (VFDs) are energy-saving control products, giving the impression that using any VFD will save electricity. In reality, the energy-saving effect of VFDs stems from their ability to regulate the speed of the motor. If VFDs are considered energy-saving control products, then all speed control devices can also be considered energy-saving control products. VFDs simply have slightly higher efficiency and power factor than other speed control devices.
Whether a variable frequency drive (VFD) can save energy depends on the speed regulation characteristics of the load. For loads like centrifugal fans and centrifugal pumps, torque is proportional to the square of the speed, and power is proportional to the cube of the speed. As long as the original flow control was valve-controlled and not operating at full load, switching to speed regulation will result in energy savings. When the speed drops to 80% of its original value, the power consumption is only 51.2% of the original. Therefore, the energy-saving effect of VFDs is most significant in these types of loads. For loads like Roots blowers, torque is independent of speed, i.e., a constant torque load. If the original method of regulating airflow by releasing excess air through a vent valve is changed to speed regulation, energy savings will also be achieved. When the speed drops to 80% of its original value, the power consumption is also 80% of the original. This is much less energy-saving than in applications like centrifugal fans and centrifugal pumps. For constant power loads, power is independent of speed. In cement plants with constant power loads, such as batching belt scales, the belt speed decreases when the material layer is thick and increases when the material layer is thin, provided the flow rate is constant. Variable frequency drives (VFDs) do not save energy in these types of loads.
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.
Myth 3: Using apparent power to calculate reactive power compensation energy-saving benefits
The apparent power is used to calculate the energy-saving effect of reactive power compensation. For example, in the original system, when the fan is running at full load at the power frequency, the motor current is 289A. When the variable frequency speed regulation is adopted, the power factor of the fan running at full load at 50Hz is about 0.99 and the current is 257A. This is because the filter capacitor inside the frequency converter improves the power factor. The energy saving calculation is as follows: ΔS=UI=×380×(289-257)=21kVA
Therefore, the article concludes that its energy-saving effect is approximately 11% of the single-unit capacity.
Practical Analysis: S represents apparent power, which is the product of voltage and current. When the voltage is the same, the percentage saving in apparent power and the percentage saving in current are the same thing. In circuits with reactance, 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 as active power. In this example, although the actual current is used for calculation, it is the apparent power that is calculated, not the active power. We know that the actual power consumed by the motor is determined by the fan and its load. Improving the power factor does not change the fan load, nor does it improve the fan efficiency; the actual power consumed by the fan does not decrease. After improving the power factor, the motor's operating state does not change; the motor stator current does not decrease; and the active and reactive power consumed by the motor do not change. The reason for the improved power factor is that the reactive power generated by the internal filter capacitor of the frequency converter supplies the motor's power. As the power factor improves, the actual input current of the frequency converter decreases, thereby reducing the line loss between the grid and the frequency converter, and the copper loss of the transformer. Meanwhile, the reduced load current allows the power distribution equipment supplying the frequency converter, such as transformers, switches, contactors, and conductors, to handle a larger load. It should be noted that if, as in this example, line losses and transformer copper losses are disregarded and only the frequency converter's losses are considered, then the frequency converter operating at full load at 50Hz will not only fail to save energy but will also consume more electricity. Therefore, calculating energy-saving effects using apparent power is incorrect.
Myth 4: Contactors cannot be installed on the output side of a frequency converter.
Almost all instruction manuals for variable frequency drives (VFDs) state that contactors should not be installed on the output side of the VFD. For example, the instruction manual for Yaskawa VFDs from Japan specifies that "do not connect electromagnetic switches or electromagnetic contactors to the output circuit."
The manufacturer's specification is to prevent the contactor from operating when the frequency converter has output. When the frequency converter is connected to a load during operation, leakage current can trigger the overcurrent protection circuit. Therefore, by adding necessary control interlocks between the frequency converter output and the contactor operation, ensuring that the contactor only operates when the frequency converter has no output, a contactor can be installed on the output side of the frequency converter. This solution is significant for situations with only one frequency converter and two motors (one running and one on standby). When the running motor fails, the frequency converter can be easily switched to the standby motor, and after a delay, the frequency converter can automatically start operating the standby motor. Furthermore, it allows for easy mutual backup between the two motors.
Myth 5: The application of variable frequency drives in centrifugal fans can completely replace the fan's regulating valves.
Using a variable frequency drive (VFD) to regulate the speed of a centrifugal fan to control airflow offers significant energy savings compared to controlling airflow with regulating valves. However, in some applications, VFDs cannot completely replace fan valves, requiring special consideration in the design. To explain this, let's first discuss its energy-saving principle. The airflow of a centrifugal fan is directly proportional to the first power of its rotational speed, the air pressure is directly proportional to the square of its rotational speed, and the shaft power is directly proportional to the cube of its rotational speed.
The analysis above also shows that when regulating airflow using valves, the air pressure increases as the airflow decreases; however, when using variable frequency drives (VFDs) to control airflow, the air pressure drops significantly as the airflow decreases. A significant drop in air pressure may fail to meet process requirements. A factory that introduced VFDs for centrifugal fans experienced considerable problems because they lacked valve design and relied solely on VFD speed adjustments to change the fan's operating point. Either the speed was too high, resulting in excessive airflow; or, if the speed was reduced, the air pressure failed to meet process requirements, preventing air from being blown in. Therefore, when using VFDs to save energy in centrifugal fans, both airflow and air pressure must be considered; otherwise, adverse consequences may occur.
Myth 6: 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 a general-purpose electric motor is 55Hz. 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 will decrease, 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 operates 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.
Myth 7: 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 variable frequency drive (VFD), the cable connected to the motor should be a shielded or armored cable, preferably laid in a metal conduit. Cable ends should be cut as neatly as possible, unshielded segments should be as short as possible, and the cable length should not exceed a certain distance (generally 50m). When the wiring distance between the VFD and the motor is long, high harmonic leakage current from the cable can adversely affect the VFD and surrounding equipment. The grounding wire returning from the motor controlled by the VFD should be directly connected to the corresponding grounding terminal of the VFD. The VFD'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 VFD, 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 VFD'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.
