I've heard from some electricians that frequency converters can save electricity, but I've never understood why they can save electricity, or how much they can save, and whether high frequency or low frequency saves more.
Some people may 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, will the frequency converter still save electricity? And how much will it save?
2. If the torque of these two motors does not reach the rated torque of the motors (the frequency and speed are still the same at 50HZ), how much electricity can the one with the frequency converter save?
3. Under the same conditions, how much can be saved under no-load conditions, and which of these three conditions saves more?
So the answer is:
It is 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 doesn't change much, so even with voltage reduction, the energy savings are minimal. 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.
Let's go back to the three questions we just asked:
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? How much can it save?
A: In this situation, the frequency converter can only improve the power factor, not save electricity.
2. If the torque of these two motors does not reach the rated torque of the motors (the frequency and speed are still the same at 50HZ), how much electricity can the one with the frequency converter 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 these three conditions saves more?
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. The closed-loop concept in the text is too narrow. 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.
1. Variable frequency drives (VFDs) do not save electricity everywhere; in many situations, using VFDs may not necessarily save electricity.
2. As an electronic circuit, the frequency converter itself also consumes power (approximately 3-5% of its rated power).
3. It is true that frequency converters operate at the mains frequency and have energy-saving functions. However, this is conditional:
First, it must be a high-power load, specifically a fan/pump type load;
Second, the device itself has a power-saving function (software supported).
Third, long-term continuous operation. These are the three conditions that demonstrate the energy-saving effect.
Aside from that, whether it saves electricity or not is irrelevant and meaningless. Claiming that inverters are energy-efficient when operating at power frequency without any preconditions is exaggeration or commercial hype. Knowing the facts will allow you to skillfully utilize it to your advantage. It's crucial to pay attention to the usage scenarios and conditions for proper application; otherwise, you risk being blindly followed, easily deceived, and ultimately "tricked."
4. When the motor is operated using a frequency converter, what are the starting current and starting torque of the motor?
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 with a longer starting time). 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.
5. Why does the frequency converter stop when large motors in the same factory start running together?
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.
6. Are there any restrictions on the installation direction when installing the 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.
7. Is it permissible to directly connect the motor to a frequency converter with a fixed frequency without using soft start?
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 to 7 times the rated current) will flow through it, and the motor will not start because the frequency converter cuts off the overcurrent.
8. What issues should be considered when a motor operates at a frequency exceeding 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 motors with medium or large capacity, especially 2-pole motors, it is necessary to discuss with the manufacturer carefully when operating at 60Hz or above.
9. 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.
10. 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.
11. 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 inverters with built-in regenerative braking (FR-K), if the braking losses are also taken into account, the power consumption will increase, which must be considered in the design of the control panel, etc.
12. 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.
13. 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.
14. 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.
15. How long is the lifespan of a frequency converter?
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.
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