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.
For squirrel-cage motors, the capacity of the variable frequency drive (VFD) should be selected based on the principle that the rated current of the VFD is greater than or equal to 1.1 times the maximum normal operating current of the motor. This will maximize cost savings. For heavy-load starting, high-temperature environments, wound-rotor motors, synchronous motors, and other similar conditions, the capacity of the VFD should be appropriately increased.
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.
In the two-stage grinding system of a Φ24m×13m cement mill at a cement company in Inner Mongolia, there is a domestically produced N-1500 type O-Sepa high-efficiency air classifier equipped with a Y2-315M-4 type motor with a power of 132kW. However, an FRN160-P9S-4E type frequency converter was selected, which is suitable for a 4-pole motor with a power of 160kW. After being put into operation, the maximum operating frequency is 48Hz, and the current is only 180A, less than 70% of the motor's rated current, indicating that the motor itself already has considerable margin. Furthermore, the selected frequency converter is one size larger than the driven motor, resulting in unnecessary waste and failing to improve reliability.
The No. 3 limestone crusher at the Chaohu Cement Plant in Anhui Province uses a 1500×12000 plate feeder as its feeding system. The drive motor is a Y225M-4 AC motor with a rated power of 45kW and a rated current of 84.6A. Before the frequency converter speed regulation modification, tests revealed that the average three-phase current of the plate feeder drive motor during normal operation was only 30A, only 35.5% of the motor's rated current. To save investment, an ACS601-0060-3 frequency converter was selected. This converter has a rated output current of 76A and is suitable for a 4-pole, 37kW motor, achieving good results.
These two examples, one positive and one negative, illustrate that for retrofit projects that did not previously use frequency converters, selecting the frequency converter capacity based on actual operating conditions can significantly reduce investment.
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 has the effect of improving 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.
A cement plant's centrifugal fan drives a Y280S-4 motor with a rated power of 75kW, a rated voltage of 380V, and a rated current of 140A. Before the variable frequency drive (VFD) upgrade, with the valves fully open, testing revealed a motor current of 70A, only 50% load, a power factor of 0.49, active power of 22.6kW, and apparent power of 4607kVA. After the VFD upgrade, with the valves fully open and running at rated speed, the average current of the three-phase power grid was 37A, leading to the assumption of a power saving of (70-37)÷70×100%=44.28%. This calculation, while seemingly reasonable, still calculates energy saving based on apparent power. Further testing revealed that the power factor was now 0.94, the active power was 22.9kW, and the apparent power was 24.4kVA. Clearly, the increase in active power did not save power but actually increased energy consumption. The increase in active power is due to the consideration of inverter losses, but not the savings in line losses and transformer copper losses. The key to this error lies in failing to account for the impact of improved power factor on current reduction, assuming the power factor remains constant, thus exaggerating the inverter's energy-saving effect. Therefore, active power, not apparent power, must be used when calculating energy-saving effects.
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.
As shown in Figure 1, curve (1) represents the air pressure-air volume (H-Q) characteristic of the fan at constant speed; curve (2) represents the network resistance characteristic (valve fully open). When the fan is working at point A, the output air volume is Q1. At this time, the shaft power N1 is proportional to the area of the product of Q1 and H1 (AH1OQ1). When the air volume decreases from Q1 to Q2, if the valve is adjusted, the network resistance characteristic changes to curve (3). When the system changes from the original operating point A to the new operating point B, the air pressure increases instead, and the shaft power N2 is proportional to the area (BH2OQ2). N1 and N2 are not much different. If the speed control method is adopted, the fan speed decreases from n1 to n2. Then the air pressure-air volume (H-Q) characteristic is as shown in curve (4). Under the same air volume Q2, the air pressure H3 is greatly reduced, and the power N3 (equivalent to the area CH3OQ2) is significantly reduced, resulting in a significant energy saving effect.
The above analysis also shows that when regulating the air volume with a valve, the air pressure increases as the air volume decreases; while when using a variable frequency drive (VFD) to control the air volume, the air pressure drops significantly as the air volume decreases. A significant drop in air pressure may fail to meet process requirements. Specifically, if the operating point is within the area enclosed by curves (1), (2), and the H-axis, simply relying on the VFD will not meet the process requirements; it needs to be combined with valve regulation. A factory that introduced a VFD for centrifugal fan application suffered greatly because it lacked valve design and relied solely on the VFD to change the fan's operating point. Either the speed was too high, resulting in excessive air volume; or the speed was reduced, but the air pressure failed to meet process requirements, preventing air from being blown in. Therefore, when using a VFD to save energy in centrifugal fans, both air volume 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 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 accelerated 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. Various load characteristics
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.
Application examples:
Our factory's chain bucket conveyor operates under constant torque load conditions. Due to increased production, the motor speed needs to be increased by 20%. The motor is model Y180L-6, with a rated power of 15kW, rated voltage of 380V, rated current of 31.6A, rated speed of 980r/min, efficiency of 89.5%, power factor of 0.81, operating current of 18-20A, and a normal maximum operating power of 7.5kW at a load rate of 50%. After installing a CIMR-G5A4015 variable frequency drive, the operating frequency is 60Hz, the speed is increased by 20%, the maximum inverter output voltage is set to 410V, the motor operating current is reduced by approximately 30% (12-15A), and the motor winding temperature decreases significantly.
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 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.
For example, a secondary air temperature gauge at a cement plant suddenly exhibited abnormal readings: the indicated value was significantly low and fluctuated wildly. It had been operating perfectly before. Inspection of the thermocouples, temperature transmitters, and secondary instruments revealed no problems. Moving the instruments to other measuring points resulted in normal operation, but similar instruments from other measuring points exhibited the same phenomenon. It was later discovered that a variable frequency drive (VFD) had been newly installed on the motor of the No. 3 cooling fan in the grate cooler, and the abnormal reading only appeared after the VFD was put into operation. Stopping the VFD immediately restored the temperature gauge reading to normal; restarting the VFD caused the abnormal reading again. This repeated experiment several times confirmed that interference from the VFD was the direct cause of the abnormal reading. The fan was a centrifugal fan, originally regulated by valves, but later converted to VFD speed control. Due to the high dust levels and harsh environment at the site, the VFD was installed in the MCC (Motor Control Center) control room. For ease of construction, the frequency converter was connected below the main contactor of the fan, and its output cable used the fan motor's power cable. This fan motor's power cable was a PVC-insulated, unarmored cable, laid parallel to the secondary air temperature gauge signal cable in a different cable tray layer within the same cable trench. It is evident that the interference occurred because the frequency converter's output cable was not armored or laid in a conduit. This lesson should be given special attention in retrofit projects that did not previously use frequency converters.
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 very dangerous and could result in electric shock. This is because 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 starting operation. Some electricians also have a habit of immediately using a megohmmeter to perform an insulation test on the motor driven by the VFD upon discovering a trip, in order 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 cables already connected to the VFD should never be performed before the cable between the motor and the VFD is disconnected.
Special attention must be paid when measuring the output parameters of a frequency converter. Since the inverter's output is a PWM waveform containing high-order harmonics, and the motor torque mainly depends on the effective value of the fundamental voltage, the output voltage measurement should primarily focus on the fundamental voltage value. A rectifier-type voltmeter is recommended, as its measurement results are closest to those measured by a digital spectrum analyzer and have an excellent linear relationship with the inverter's output frequency. To further improve measurement accuracy, an RC filter can be used. Digital multimeters are susceptible to interference and have significant measurement errors. The output current needs to be measured as a total effective value, including the fundamental and other high-order harmonics. Therefore, a moving-coil ammeter is commonly used (under motor load, the effective value of the fundamental current and the effective value of the total current are not significantly different). When using a current transformer for ease of measurement, it may saturate at low frequencies; therefore, an appropriate capacity current transformer must be selected.
