Some people may have the following questions:
1. If two identical motors work at 50HZ power frequency, one uses a frequency converter and the other does not, and the speed and torque are both at the rated state of the motor, can the frequency converter save power? How much can you save?
2. If the torque of these two motors does not reach the rated torque state of the motor (the frequency and rotation speed are still the same as 50HZ), how much power can the one with a frequency converter save?
It is an indelible fact that frequency converters can save power. In some cases, they can save more than 40% of the power, but in some cases, they are also more wasteful than not connecting to frequency converters!
The frequency converter achieves energy saving by reducing voltage with a light load. As the rotational speed of the drag torque load does not change much, even if the voltage is reduced, the energy saving is very weak. However, when used in a fan environment, it is different. When a small amount of air is required, the motor will reduce its speed. We know that the energy consumption of the fan is proportional to the 1.7 power of the rotational speed, so the rotational speed of the motor will decrease sharply, with significant energy saving effects. If we use it on oil wells, we will waste a lot of electric energy and even more electricity because of the use of braking resistors on the return journey.
Of course, if the environment requires speed regulation, the energy-saving effect of frequency converters is still relatively obvious. The frequency converter will not save power in situations where the speed is not adjusted, but can only improve the power factor.
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Back to the first two questions, let’s take a look:
1. If two identical motors work at 50HZ power frequency, one uses a frequency converter and the other does not, and the speed and torque are both at the rated state of the motor, can the frequency converter save power? How much can you save?
Answer: In this case, the frequency converter can only improve the power factor, and cannot save electricity.
2. If the torque of these two motors does not reach the rated torque state of the motor (the frequency and rotation speed are still the same as 50HZ), how much power can the one with a frequency converter save?
Answer: If automatic energy-saving operation is used, the frequency converter can operate at reduced voltage at this time, which can save some electric energy, but the power saving is not obvious.
3. Under the same conditions, how much can be saved in the no-load state, and which of the three states saves more?
Answer: The no-load state of the drag type load does not save much electric energy.
For example, about “closed-loop control”. I think there is room for discussion. The concept of closed loop in this article is too narrow. Closed loop control is not just about speed sensor feedback. The frequency control during vector control is a closed-loop control, and it is also a closed-loop control within the device. V/F control belongs to open loop control. In addition, there are physical quantities such as temperature, pressure, flow, and other PID regulator feedback control, which are all closed-loop control categories. And they can be adjusted through frequency converters. The concept of closed-loop control should not be interpreted so narrowly.
Another example is the concept of braking, which is interpreted like nonsense. Playing with a word game means saying nothing.
1. Frequency conversion is not universally available to save power, and there are many fields where frequency conversion is not necessarily used to save power.
2. As an electronic circuit, the frequency converter itself also consumes power (approximately 3-5% of the rated power).
3. It is a fact that the frequency converter operates at power frequency and has a power saving function. However, his prerequisite is:
First, high power and fan/pump type load;
Second, the device itself has a power saving function (software support);
Third, long-term continuous operation. These are the three conditions that reflect the power saving effect.
In addition, there is no point in saving electricity or not. To say that frequency converter power frequency operation is energy-saving without preconditions is exaggerated or commercially hyped. Knowing the truth, you will skillfully use him to serve you. It is important to pay attention to the use situation and conditions for correct application, otherwise it will be blind obedience and credulity and “deceived”.
4. When using a frequency converter for operation, what is the starting current and starting torque of the motor?
The frequency converter is used for operation. As the motor accelerates, the frequency and voltage are increased accordingly, and the starting current is limited to below 150% of the rated current (125% to 200% depending on the model). When starting directly with a power frequency power supply, the starting current is 6-7 times, which will cause mechanical and electrical shocks. It is possible to start smoothly (starting time becomes longer) using a frequency converter drive. The starting current is 1.2 to 1.5 times the rated current, and the starting torque is 70% to 120% of the rated torque; For frequency converters with automatic torque enhancement, the starting torque is above 100% and can be started with full load.
5. In the same factory, large motors move together and the frequency converter stops during operation. Why is this?
When the motor starts, it will flow a starting current corresponding to its capacity, causing a voltage drop in the transformer on the stator side of the motor. When the motor capacity is large, the voltage drop will also have a significant impact. The frequency converter connected to the same transformer will make a judgment of undervoltage or transient shutdown, and sometimes the protection function (IPE) acts, resulting in shutdown.
6. Is there any restriction on the installation direction when installing the frequency converter?
The structure of the inside and back of the inverter takes into account the cooling effect, and the relationship between the top and bottom is also important for ventilation. Therefore, the unit type inside the panel and hanging on the wall should be installed vertically as much as possible.
7. Is it possible to directly input the motor to a fixed frequency inverter without using soft starting?
It is possible at very low frequencies, but if the given frequency is high, the conditions for direct starting of the power supply are similar. A large starting current (6-7 times the rated current) will flow, and the motor cannot be started due to the excessive current cut off by the frequency converter.
8. What issues should be noted when the motor operates above 60Hz?
(1) The operation of machinery and devices at this speed should be fully possible (mechanical strength, noise, vibration, etc.).
(2) When the motor enters the constant power output range, its output torque must be able to maintain its operation (the output power of shafts such as fans and pumps increases proportionally to the cube of speed, so attention should also be paid to small increases in rotational speed).
(3) The problem of bearing life should be fully considered.
(4) For motors with medium capacity or above, especially 2-pole motors, it is necessary to carefully discuss with the manufacturer when operating above 60Hz.
9. Can frequency converters drive gear motors?
Depending on the structure and lubrication method of the reducer, several issues need to be noted. In terms of gear structure, the maximum limit of 70 to 80 Hz can usually be considered. When oil lubrication is used, continuous operation at low speed is related to gear damage, etc.
10. Can frequency converters be used to drive single-phase motors? Can I use a single-phase power supply?
The machine is basically unusable. For single-phase motors with governor switch starting, the auxiliary winding will be burned out when the speed range is below the operating point; For capacitor starting or capacitor operation mode, capacitor explosion will be induced. The power supply of the frequency converter is usually 3-phase, but for small capacity machines, single-phase power supply is also useful.
11. How much power does the inverter itself consume?
It is related to the model, operating state, frequency of use, etc. of the frequency converter, but it is difficult to answer. However, the efficiency of frequency converters below 60Hz is approximately 94% to 96%, and the loss can be calculated based on this. However, if the loss during braking is also taken into account, the power consumption will increase, and attention must be paid to the design of the operating panel.
12. Why can’t it be used continuously in the entire area of 6~60Hz?
Generally, motors are cooled using blades mounted on the outer fan or rotor end ring of the shaft. If the speed decreases, the cooling effect decreases, and therefore cannot withstand the same heating as high-speed operation. Therefore, it is necessary to reduce the load torque at low speeds, or use a large capacity frequency converter to combine with the motor, or use a dedicated motor.
13. What should I pay attention to when using a motor with brakes?
The power supply for the brake excitation circuit shall be taken from the input side of the frequency converter. If the brake acts while the inverter is outputting power, it will cause overcurrent cutoff. Therefore, it is necessary to stop the output of the frequency converter before actuating the brake.
14. “I want to use a frequency converter to drive a motor with capacitors for improving power factor, but the motor does not move. Please explain the reason”
The current of the inverter flows into the capacitor used to improve the power factor. Due to its charging current, the inverter cannot be started due to overcurrent (OCT). As a countermeasure, please remove the capacitor and operate it, even improve the power factor. It is effective to connect an AC reactor to the input side of the inverter.
15. How long is the life of the frequency converter?
Although frequency converters are static devices, there are also consumer devices such as filter capacitors and cooling fans. If they are regularly maintained, they are expected to have a service life of more than 10 years.
Post time: Mar-29-2023