As the most mainstream electrical control equipment for motor control today, frequency converters have many advantages, such as precise speed and voltage regulation and flexible operation methods. Although the usage of frequency converters is familiar and mastered by most electricians, the secrets of their use may not be known to everyone.
Myth 1: Using a frequency converter can save electricity.
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: Inverter capacity selection
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 rating of a VFD refers to the power of the 4-pole AC asynchronous motor it is applicable to.
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 variable frequency drive (VFD) capacity should be selected based on the principle that the VFD's rated current is greater than or equal to 1.1 times the motor's maximum normal operating current. This maximizes cost savings. For heavy-load starting, high-temperature environments, wound-rotor motors, and synchronous motors, the VFD capacity should be appropriately increased. In designs that use VFDs from the outset, selecting the VFD capacity based on the motor's rated current is perfectly acceptable. This is because the VFD capacity cannot be selected based on actual operating conditions in this case. However, to reduce investment, in some situations, the VFD 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: Using an apparent power meter
Calculate the energy-saving benefits of reactive power compensation
The energy-saving effect of reactive power compensation is calculated using apparent power. For example, in a certain literature: when the original system's fan operates at full load at the power frequency, the motor's operating current is 289A. When using variable frequency speed control, the power factor at full load (50Hz) is approximately 0.99, and the current is 257A. This is because the internal filter capacitor of 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 variable frequency drive (VFD) instruction manuals state that contactors should not be installed on the output side of the VFD. When the VFD is connected to a load during operation, leakage current will trigger the overcurrent protection circuit. Therefore, by implementing necessary control interlocks between the VFD output and the contactor operation, ensuring that the contactor only operates when the VFD has no output, a contactor can be installed on the output side of the VFD. This solution is significant for applications with only one VFD and two motors (one running and one on standby). When the running motor fails, the VFD can be easily switched to the standby motor, and after a delay, the VFD can automatically start operating the standby motor. Furthermore, it allows for easy mutual backup between the two motors.
Myth 5: Variable frequency drives in centrifugal fans
Applications that can completely replace the regulating valves of fans.
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.
Myth 6: General-purpose electric motors can only operate at their rated speed.
Speeds below a certain speed should be reduced using a frequency converter.
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.
