time:2023-02-07 15:30:06Views:
Why use a frequency converter?
For a motor driven by alternating current, its speed is related to the structure of the motor (specifically, the number of magnetic poles of the winding), and also related to the frequency of the power supply.
Take synchronous motor as an example, its speed formula: 60f/p
f: Frequency of the power supply
p: number of pole pairs
Speed of asynchronous motor: 60f(1-s)/p
s: slip (slip), the ratio of the difference between the asynchronous motor speed n and the synchronous speed n0 to the synchronous speed. S=(n0-n)/n0
The greater the load of the asynchronous motor, the lower the speed and the greater the slip; on the contrary, the smaller the load, the higher the speed and the smaller the slip. When the asynchronous motor has a rated load, its rated speed is very close to the synchronous speed, so the slip is very small, generally 0.01 to 0.06.
After the motor is completed, its number of magnetic poles is fixed, and there is no way to change it. Even if there is a way to change, the speed still has "gears", and "stepless" speed change cannot be realized.
If you want to change the speed of the motor, you can change the frequency of the power supply.
To change the frequency, you have to use a frequency converter.
How to calculate the number of magnetic poles is determined by the connection method of the winding when the motor is designed. Simply put, when the motor is working normally, there are several pairs of magnetic NS poles in the motor. There will be specific explanations in the general "Motor and Drag" book.
Why change the speed of the motor?
A, save energy/money
The output power of the motor is related to the speed of the motor. Through frequency conversion, the motor power can be adjusted.
For example: commercial central air conditioning. During the day, there are many people working in the building, which requires a large cooling capacity. I got off work at night, but someone was on duty, so the air conditioner couldn't be turned off. At this time, by adjusting the speed of the motor that drives the air conditioner, the demand for cooling capacity of the air conditioner at different time periods can be realized, achieving the purpose of energy saving, and saving electricity bills.
What to do when there is no inverter air conditioner? One way is to use multiple air conditioning units in parallel. When the demand for air conditioners is high during the day, turn on more air conditioners. When the demand is low at night, we will stop several air conditioners. Such trouble. At the same time, the purchase cost of multiple air-conditioning units adds up to more than one inverter air-conditioning unit with the same cooling capacity.
With the same cooling capacity, the inverter air conditioner is more expensive than the fixed frequency air conditioner, mainly because the starter cabinet that drives the air conditioner is more expensive. But this price difference can be earned back by saving electricity bills, which is the price competitive advantage of inverter air conditioners.
B, to achieve low power start, reduce the impact on the grid
The moment the electrical appliance is connected to the grid, the sudden current inside the electrical appliance is 6-8 times the normal working current. If the power is large, the sudden change of current will have an impact on the grid.
The electrical appliances we usually use at home have low power, so this problem is not obvious. This problem is obvious when the power of electrical appliances is too large, such as commercial central air conditioners.
C, high speed can be obtained
Take the industry I am doing now as an example. There is a magnetic levitation centrifugal compressor. The bearing of its rotating part uses a strong magnet to lift the shaft up and suspend it in the air. When the rotating part rotates at high speed, there is no friction to the parts, the resistance is small, and there is no wear. The driven motor can drive the impeller to work at a very high speed. In order to allow the motor to rotate at high speed, a frequency converter is used to provide 200Hz power to drive the motor.
Of course, in order to make the impeller rotate at a high speed, there are other ways: to use the gear to change the speed multiple times, but in this way, the life of the last stage gear will not be long.
A compressor with a magnetic levitation structure can have a very high speed. In this way, for two compressors with the same cooling capacity, the ordinary bearing compressor driven by a 50Hz motor is much larger than the magnetic levitation one.
D, the power of the motor can be made relatively smaller
The high-speed rotating part can be slowly accelerated from standstill to the highest speed through the frequency converter. If direct start is used, the power of the motor must be very large, which means that a big cow pulls a small cart.
This is easy to understand, the time for a car to accelerate from rest to 100 kilometers. The shorter the acceleration time of the car, the larger its engine displacement.
Application Scenarios of Inverter
There are many application scenarios, and its energy-saving and money-saving characteristics lead to many application scenarios. The large application scenarios include:
A. The air-conditioning industry, mainly commercial central air-conditioning, has obvious energy-saving effects. Sales use this as a selling point to persuade customers to use inverter air conditioners;
B, EMU, high-speed rail, subway, this industry is now using this technology;
C. For high-speed elevators used in high-rise buildings, its motors must be driven by frequency conversion technology to achieve speed regulation;
D. Now solar power generates electricity, and the output power is direct current. To generate electricity on the grid, it must be converted into AC. This transformation also has to use the basic principle of the frequency converter;
E, electric vehicles, electric bicycles we ride, the motor is DC drive. Speed regulation is to make the voltage applied to the motor different by adjusting the duty cycle, so that the speed can be changed.
In Tesla's electric vehicles, the driving motor inside is AC. It is powered by a DC battery, and the DC is converted into an AC by a frequency converter, and then the motor is driven. By controlling the frequency of the inverter, the car can be accelerated and decelerated. In addition, traditional cars are decelerated by friction, which converts the kinetic energy of the car into heat energy. When an electric car is decelerating, the speed of the car tires exceeds the speed of the magnetic field of the alternating current in the stator. At this time, the motor becomes a generator, and the electricity generated is alternating current. The current is poured into the battery pack through the rectification of the IGBT freewheeling diode on the inverter, so that the deceleration is realized. At the same time, this part of kinetic energy will not be wasted, but Some of them have achieved electric energy recovery. Think about how many inverters will be needed if all the cars in the world are changed to electric, and how big the market will be!
With the development of technology, more and more application scenarios will be developed.
I found an application case on the Internet, and it can be seen that it has a wide range of applications.
The dust removal fan of the coking plant (this is a high-voltage motor), this is for energy saving and production needs. Due to the needs of the production process, dust removal is required every 20 minutes or so. The power of the fan is 560KW. If it takes 1 minute to accelerate from 0 to the required speed, it is obviously unreasonable to restart it every time dust removal is needed, and then stop it when it is used up. It cannot keep up with the needs of production, and the high-voltage motor is not allowed to be short-circuited. Time starts and stops repeatedly. The high-voltage inverter solves this problem. When there is no need for dust removal, it runs at a low speed of 15HZ. Once the dust removal command is issued, it quickly accelerates to 50HZ to meet production needs, and then continues to stand by at low speed to save energy.
The current situation of the use of frequency converters in the United States (source wikipedia "inverter" entry)
Taking the United States as an example, it is estimated that about 60-65% of electric energy is used to drive electric motors, of which 75% is used for fans, pumps and compressors whose load characteristics are variable torque. If frequency converter is used to drive, it is estimated that 18% of the energy consumed in 40 million electric motors can be saved.
Only about 3% of all installed AC motors are driven by frequency converters. But in the newly installed AC motor. About 30% to 40% will be used with frequency converters.