I've always heard people say that frequency inverters can save electricity, and I've come to accept it because so many people say it. But I've never understood why frequency inverters can save electricity, or by how much, and whether high-frequency or low-frequency inverters save more. I also have the following questions:
1. 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? If so, how much?
2. If both motors operate at less than their rated torque (while maintaining the same frequency and speed of 50Hz), how much electricity will the motor with the inverter save?
3. Under the same conditions, how much can be saved under no-load conditions, and which of the three conditions saves the most money?
A: It's an undeniable fact that frequency converters can save electricity, sometimes by more than 40%, but in other cases, it can be more wasteful than not using a frequency converter at all!
Inverters achieve energy savings by reducing voltage under light loads. When driving a torque load, the speed change is minimal, so even with voltage reduction, the energy savings are negligible. However, the situation is different in a fan environment. When a smaller airflow is needed, the motor slows down. We know that fan energy consumption is proportional to the 1.7th power of the speed, so the motor torque drops sharply, resulting in significant energy savings. If we were to use this in an oil well, the use of a braking resistor on the return stroke would waste a lot of energy, making it even more energy-intensive.
Of course, if the environment requires speed regulation, the energy-saving effect of the frequency converter is quite significant. In situations where speed regulation is not required, the frequency converter will not save electricity, but can only improve the power factor.
1. 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? If so, how much?
A: In this situation, the frequency converter can only improve the power factor, not save electricity.
2. If both motors operate at less than their rated torque (while maintaining the same frequency and speed of 50Hz), how much electricity will the motor with the inverter save?
A: If automatic energy-saving operation is used, the frequency converter can reduce the voltage at this time, which can save some electricity, but the energy saving is not significant.
3. Under the same conditions, how much can be saved under no-load conditions, and which of the three conditions saves the most money?
A: Even when a driven load is unloaded, it doesn't save much energy.
For example, regarding the concept of "closed-loop control," I think there's room for discussion. Closed-loop control isn't limited to speed sensor feedback. Frequency control in vector control is closed-loop control, specifically internal closed-loop control within the device. V/F control is open-loop control. Additionally, PID controller feedback control of physical quantities like temperature, pressure, and flow rate all fall under the category of closed-loop control. And all of these can be achieved through frequency converter adjustments. The concept of closed-loop control shouldn't be interpreted so narrowly.
For example, the explanation of the concept of braking is like nonsense, playing word games, and it's as if it says nothing at all.
2. Variable frequency drives (VFDs) do not always save electricity; in many situations, using VFDs may not necessarily save electricity.
3. As an electronic circuit, the inverter itself also consumes power (approximately 3-5% of its rated power). A 1.5 horsepower air conditioner consumes about 20-30W, equivalent to a light bulb left on continuously.
3. It is true that frequency converters have energy-saving functions when operating at the mains frequency. However, this is conditional: first, the load must be high-power and be a fan/pump type; second, the device itself must have energy-saving functions (software support); and third, it must operate continuously for a long period. These are the three conditions for demonstrating energy-saving effects. Beyond these conditions, whether or not it saves energy is meaningless. Claiming that frequency converters save energy when operating at the mains frequency without these conditions is exaggeration or commercial hype. Understanding the reasons will allow you to skillfully utilize it for your benefit. It is crucial to pay attention to the application environment and conditions to ensure correct application; otherwise, you risk being blindly followed and deceived.
1. What is a frequency converter?
A frequency converter is a control device that uses the switching action of power semiconductor devices to convert power frequency power into electrical energy of another frequency.
2. What are the differences between PWM and PAM?
PWM is an abbreviation for Pulse Width Modulation, a modulation method that adjusts the output and waveform by changing the pulse width of a pulse train according to a certain pattern.
PAM is an abbreviation for Pulse Amplitude Modulation, which is a modulation method that adjusts the output value and waveform by changing the pulse amplitude of a pulse train according to a certain rule.
3. What are the differences between voltage-source and current-source voltage ...
The main circuit of a frequency converter can be broadly divided into two categories: voltage-type frequency converters convert DC voltage to AC voltage, and the DC circuit filter is a capacitor; current-type frequency converters convert DC current to AC current, and the DC circuit filter is an inductor.
4. Why does the voltage and current of a frequency converter change proportionally?
The torque of an asynchronous motor is generated by the interaction between the motor's magnetic flux and the current flowing through the rotor. At the rated frequency, if the voltage is constant and only the frequency is reduced, the magnetic flux will be too large, causing magnetic circuit saturation, which can burn out the motor in severe cases. Therefore, the frequency and voltage must be changed proportionally; that is, the inverter's output voltage is controlled while changing the frequency to keep the motor's magnetic flux constant and avoid weak magnetic flux and magnetic saturation. This control method is often used in energy-saving inverters for fans and pumps.
5. When a motor is driven by a mains frequency power supply, a decrease in voltage will increase the current; for a frequency converter drive, if a decrease in frequency also causes a decrease in voltage, will the current increase?
When the frequency decreases (low speed), the current increases if the same power is output, but the current remains almost unchanged under the condition of constant torque.
6. What are the starting current and starting torque of the motor when operating with a frequency converter?
When using a frequency converter, the frequency and voltage increase accordingly as the motor accelerates, limiting the starting current to below 150% of the rated current (125%~200% depending on the model). Direct starting with a mains frequency power supply results in a starting current 6~7 times the rated current, causing mechanical and electrical shocks. Frequency converters allow for smooth starting (though starting time is longer). The starting current is 1.2~1.5 times the rated current, and the starting torque is 70%~120% of the rated torque; for frequency converters with automatic torque boosting, the starting torque is over 100%, allowing for full-load starting.
7. What does V/f mode mean?
The voltage V decreases proportionally as the frequency decreases, as explained in answer 4. The ratio of V to f is predetermined by taking into account the motor characteristics. Several characteristics are typically stored in the controller's ROM, selectable via a switch or dial.
8. How does the motor torque change when V and f are changed proportionally?
When the frequency decreases, the voltage is reduced proportionally. Since the AC impedance decreases while the DC resistance remains constant, this tends to reduce the torque generated at low speeds. Therefore, at low frequencies, given a V/f, the output voltage needs to be increased slightly to obtain a certain starting torque; this compensation is called enhanced starting. Various methods can be used to achieve this, including automatic methods, selecting a V/f mode, or adjusting a potentiometer.
9. The instruction manual states that the speed range is 60~6Hz, or 10:1. Does this mean there is no output power below 6Hz?
While the inverter can still output power below 6Hz, the minimum operating frequency is around 6Hz, taking into account factors such as motor temperature rise and starting torque. At this frequency, the motor can output rated torque without causing serious overheating problems. The actual output frequency (starting frequency) of the inverter ranges from 0.5Hz to 3Hz, depending on the model.
10. Is it possible to maintain a constant torque for general motor combinations even at frequencies above 60Hz?
Normally, this is not possible. Above 60Hz (and some modes above 50Hz), the voltage remains constant, exhibiting largely constant power characteristics. When requiring the same torque at high speeds, careful consideration must be given to the selection of motor and inverter capacities.
11. What does "open-loop" mean?
A closed-loop system uses a speed detector (PG) to feed back the actual speed to the control unit for control. A system that operates without a PG is called an open-loop system. Most general-purpose frequency converters are open-loop, although some models offer PG feedback via optional components.
12. What should be done if the actual rotational speed deviates from the given speed?
In open-loop operation, even if the inverter outputs a given frequency, the motor speed will vary within the rated slip range (1%~5%) when the motor is under load. For applications requiring high speed control accuracy and operation close to the given speed even with load variations, an inverter with PG feedback function (optional) can be used.
13. If a motor with a PG (Power Generator) is used, will the speed accuracy be improved after feedback?
Inverters with PG feedback function have improved accuracy. However, the speed accuracy depends on the accuracy of the PG itself and the resolution of the inverter's output frequency.
14. What does stall prevention function mean?
If the given acceleration time is too short, the change in the inverter's output frequency will far exceed the change in speed (electrical angular frequency), causing the inverter to trip due to overcurrent and stop operation; this is called stall. To prevent stall and allow the motor to continue running, the current magnitude must be detected for frequency control. When the acceleration current is too large, the acceleration rate should be appropriately slowed down. The same applies during deceleration. The combination of these two measures constitutes the stall function.
15. What is the significance of models where acceleration and deceleration times can be set separately, and models where both acceleration and deceleration times can be set together?
For machine types where acceleration and deceleration can be set separately, this is suitable for short-time acceleration and slow deceleration applications, or for small machine tools where strict production cycle time needs to be set. However, for applications such as fan drives, where acceleration and deceleration times are relatively long, acceleration and deceleration times can be set together.
16. What is regenerative braking?
If the command frequency is reduced while the electric motor is running, the motor will operate as an asynchronous generator and act as a brake. This is called regenerative (electric) braking.
17. Is it possible to obtain greater braking force?
The energy regenerated from the motor is stored in the inverter's filter capacitor. Due to the capacitance and voltage rating of the capacitor, the regenerative braking force of a general-purpose inverter is approximately 10% to 20% of the rated torque. If an optional braking unit is used, it can reach 50% to 100%.
18. Please explain the protection functions of the frequency converter.
Protection functions can be divided into the following two categories:
(1) Automatically perform corrective actions after detecting abnormal conditions, such as overcurrent stall prevention and regenerative overvoltage stall prevention.
(2) Upon detecting an abnormality, the PWM control signal of the power semiconductor device is blocked, causing the motor to stop automatically. Examples include overcurrent cutoff, regenerative overvoltage cutoff, semiconductor cooling fan overheating protection, and instantaneous power outage protection.
19. Why does the inverter's protection function activate when the clutch is used under continuous load?
When a clutch is used to connect a load, at the moment of connection, the motor changes drastically from an unloaded state to a region with high slip. The large current flowing through it causes the inverter to trip due to overcurrent and become unable to operate.
20. In the same factory, when large motors start together, the frequency converter stops running. Why is this?
When a motor starts, a starting current corresponding to its capacity will flow through it. The transformer on the stator side of the motor will generate a voltage drop. When the motor capacity is large, this voltage drop will have a greater impact. The frequency converter connected to the same transformer will make a judgment of undervoltage or momentary stop. Therefore, sometimes the protection function (IPE) will be activated, causing the motor to stop operating.
21. What is variable frequency resolution? What is its significance?
For digitally controlled frequency converters, even if the frequency command is an analog signal, the output frequency is given in stages. The smallest unit of this stage difference is called the frequency resolution.
Variable frequency drive (VFD) resolution is typically set between 0.015 and 0.5 Hz. For example, with a resolution of 0.5 Hz, a frequency above 23 Hz can be changed to 23.5 or 24.0 Hz, resulting in stepped motor operation. This poses a problem for applications like continuous winding control. In such cases, a resolution of around 0.015 Hz is sufficient for a 4-pole motor with a step difference of less than 1 r/min. Additionally, some models have a different given resolution than their output resolution.
22. Are there any restrictions on the installation direction when installing a frequency converter?
The internal and rear structure of the frequency converter takes into account the cooling effect, and the vertical relationship is also important for ventilation. Therefore, for unit-type inverters that are inside the panel or mounted on the wall, the vertical position is taken, and they are installed as vertically as possible.
23. Is it permissible to directly connect the motor to a frequency converter with a fixed frequency without using soft starting?
It's possible at very low frequencies, but if the given frequency is high, the conditions are similar to direct starting with a mains frequency power supply. A large starting current (6-7 times the rated current) will flow through it, and the motor will not start because the frequency converter cuts off the overcurrent.
24. What precautions should be taken when a motor is running at a frequency exceeding 60Hz?
The following precautions should be taken when operating above 60Hz.
(1) The machinery and equipment must be able to operate at this speed with sufficient capacity (mechanical strength, noise, vibration, etc.).
(2) When the motor enters the constant power output range, its output torque must be able to maintain operation (the shaft output power of fans, pumps, etc. increases proportionally to the cube of the speed, so attention should be paid when the speed increases slightly).
(3) The issue of bearing life should be fully considered.
(4) For medium-capacity motors and above, especially 2-pole motors, it is necessary to discuss with the manufacturer carefully when operating at frequencies above 60Hz.
25. Can a frequency converter drive a geared motor?
Depending on the structure and lubrication method of the speed reducer, several issues need to be considered. For gears, a maximum speed limit of 70-80Hz is generally considered. When using oil lubrication, continuous operation at low speeds can lead to gear damage.
26. Can a frequency converter be used to drive a single-phase motor? Can it use a single-phase power supply?
The machine is basically unusable. For single-phase motors with speed controller switch start, they will burn out when operating below the speed limit.
Auxiliary windings; in capacitor-start or capacitor-operated systems, this can induce capacitor explosion. Inverter power supplies are typically three-phase, but some smaller capacity models operate on a single-phase power supply.
27. How much power does the frequency converter itself consume?
It depends on the type of inverter, its operating status, and the frequency of use, but it's difficult to answer definitively. However, inverters below 60Hz have an efficiency of approximately 94% to 96%, from which losses can be estimated. But for regenerative braking (FR-K) inverters, if braking losses are also taken into account, the power consumption will increase, which must be considered in the design of the control panel, etc.
28. Why can't it be used continuously across the entire 6~60Hz frequency range?
Generally, motors use external fans mounted on the shaft or blades on the rotor end rings for cooling. If the speed decreases, the cooling effect decreases, and therefore the motor cannot withstand the same heat generation as when running at high speed. It is necessary to reduce the load torque at low speeds, or use a combination of a high-capacity frequency converter and the motor, or use a special motor.
29. What precautions should be taken when using a motor with a brake?
The power supply for the brake's excitation circuit should be taken from the input side of the frequency converter. If the brake operates while the frequency converter is outputting power, it will cause an overcurrent trip. Therefore, the brake should only be activated after the frequency converter has stopped outputting power.
30. I want to use a frequency converter to drive a motor with a capacitor for improving the power factor, but the motor doesn't move. Please explain the reason.
The inverter's current flows into the capacitor used to improve the power factor. Because the charging current of the capacitor causes an overcurrent (OCT) in the inverter, it cannot start. As a countermeasure, please remove the capacitor and run the inverter. In addition, connecting an AC reactor to the input side of the inverter is effective in improving the power factor.
31. What is the lifespan of a frequency inverter?
Although frequency converters are static devices, they also have consumable components such as filter capacitors and cooling fans. If they are maintained regularly, they can be expected to have a lifespan of more than 10 years.
32. The inverter has a built-in cooling fan. What is the direction of the airflow? What happens if the fan breaks down?
For small-capacity models without cooling fans, and for models with fans, the airflow is from bottom to top. Therefore, avoid placing any mechanical equipment above or below the inverter installation location that might obstruct airflow. Also, do not place heat-sensitive components above the inverter. In case of fan failure, protection will be provided by fan stop detection or overheat detection on the cooling fan.
33. Since filter capacitors are consumables, how do you determine their lifespan?
As a capacitor used for filtering, its capacitance decreases slowly over time. The capacitance is measured periodically, and its lifespan is judged based on the point at which it reaches 85% of the product's rated capacity.
34. Are there any restrictions on the installation direction when installing a frequency converter?
Ideally, the contents should be primarily stored within a tray. However, the problem is that trays with a fully enclosed structure are large in size, take up a lot of space, and are relatively expensive. Solutions include:
(1) The design of the tray should be based on the heat dissipation requirements of the actual device;
(2) Increase the cooling area by using aluminum heat sinks, finned coolants, etc.;
(3) Use heat pipes.
In addition, a type of inverter with an exposed back has been developed.
35. To increase the speed of an existing conveyor belt and operate it at 80Hz, how should the capacity of the frequency converter be selected?
Assuming a base speed of 50Hz, speeds above 50Hz exhibit constant power output characteristics. For loads with constant torque characteristics, such as conveyor belts, increasing speed requires a capacity increase of approximately 1.6 times (80/50 ≈ 1.6 times). The motor capacity also increases in the same way as that of a frequency converter.
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