Compared with the traditional AC drive system, the AC drive system that uses the frequency converter to control the speed of the AC motor has many advantages, such as energy saving, easy to realize the speed control of the existing motor, and can realize a wide range of efficient continuous Speed ​​control, easy to realize the positive and negative switching of the motor, can start the high-frequency start and stop operation, can be electrically braked, can use a frequency converter to control the speed of multiple motors, the power factor is large, The power supply capacity is small, and a high-performance control system can be formed. The following describes the main advantages of the above mentioned frequency converter speed control system.
In many cases, the purpose of using the frequency converter is to save energy, especially for fans, blowers and pump loads that are used in large quantities in the industry. Speed ​​control through the frequency converter can replace the traditional use of baffles and valves. The control of air volume, flow and lift, so the energy saving effect is very obvious.
Since the speed control operation for the purpose of energy saving does not require a high speed range and accuracy of the motor, a general-purpose inverter that is economical in terms of price is usually used.
Since the frequency converter can be regarded as a frequency-adjustable AC power supply, for an existing asynchronous motor that operates at a constant speed, it is only necessary to connect the inverter and the corresponding equipment between the grid power supply and the existing motor. The frequency converter is used to realize the speed control without major equipment modification of the motor and the system itself.
In the AC drive system using the inverter, the speed control of the asynchronous motor is realized by changing the output frequency of the inverter. Therefore, when the speed control is performed, the output frequency of the inverter can be controlled to make the motor work in a range with a small slip ratio, and the speed range of the motor is wide, and the operation efficiency can be improved. In general, the general-purpose inverter can achieve a speed range of 1:10 or higher, while the high-performance vector control mode can achieve a speed range of 1:1000. In addition, when the frequency converter is controlled by the inverter of the vector control mode, the output torque of the motor can be directly controlled. Therefore, the combination of high-performance vector control inverter and inverter-specific motor can achieve and exceed the control performance of high-precision DC servo motor in terms of control performance.
In order to realize the forward/reverse switching of the motor by using the AC drive system operated by the common grid power supply, the power supply must be commutated and switched by means of a switch or the like. When using the frequency converter for speed control, it is only necessary to change the switching sequence of the inverter device of the internal inverter circuit of the inverter to achieve the purpose of commutating the output. It is easy to realize the forward and reverse switching of the motor without special setting. Reverse the switching device.
In addition, when the motor running under the power supply of the grid is switched between forward and reverse, if the phase sequence is switched when the motor has not stopped, the current in the motor will flow more than the starting current due to the change of the phase sequence, and there will be burnt. The danger of the motor, so it is usually necessary to wait for the motor to stop completely before the commutation operation. In the AC speed control system using the frequency converter, the motor can be accelerated according to the ramp function by changing the output frequency of the frequency converter, thereby achieving the purpose of limiting the acceleration current. Therefore, it is easier to form an automatic control system with other equipment when using the inverter for speed control.
For an AC drag system that operates with an ordinary grid power supply, since the starting current of the motor is large and there is a power loss proportional to the starting time, the motor cannot be started and stopped at a high frequency. For the AC speed control system using the frequency converter, since the start and stop of the motor are all in the low speed zone and the acceleration and deceleration process is relatively gentle, the power consumption and heat generation of the motor are small, and the frequency can be increased. Stop running.
The above characteristics of the variable frequency speed control system can be used for the purpose of energy saving by using a conveyor belt and a moving table of the AC drag system. This is because, in the conveyor belt and the moving table that use the asynchronous motor for constant speed driving, the motor is always in the working state, and after the speed control is performed by the inverter, the motor can be started and stopped at a high frequency. The conveyor belt or moving table is only stopped when there are goods or workpieces, thus achieving energy saving.
Since the speed control of the motor in the inverter drive system is performed by changing the output frequency of the inverter, when the output frequency of the inverter is reduced below the frequency corresponding to the actual speed of the motor, the mechanical energy of the load will be converted into The electrical energy is fed back to the power grid and forms an electrical brake. In addition, some inverters also have a DC braking function, that is, when braking is required, a DC voltage can be applied to the motor through the inverter, and the current generated by the voltage can be used for braking.
Compared with mechanical brakes, electrical brakes have many advantages, such as small size, simple maintenance, and good reliability. However, it should also be noted that since electric braking does not cause the motor to maintain a holding torque in a stationary state, it is necessary to take corresponding measures in some cases, such as simultaneous use with a mechanical brake.
High-speed drive is one of the most important advantages of inverter speed control. This is because, for a DC motor, high-speed operation cannot be performed due to factors such as a brush and a reversing ring. However, for an asynchronous motor, theoretically speaking, the rotational speed of the asynchronous motor can reach a relatively high speed due to the absence of the above-mentioned constraints.
Since the speed of the asynchronous motor is:
Where n - motor speed, r / min;
F——power frequency, HZ;
P——the number of magnetic poles of the motor;
s - slip.
When the asynchronous motor is driven by a commercial frequency power supply (50 Hz), the maximum speed of the two-pole motor can only reach 3000 r/min. In order to achieve higher speeds, a dedicated high frequency power supply or a mechanical speed increase device must be used for speed increase.
Compared with this, the output frequency of the current high-frequency inverter can reach 3000KHZ, so when the high-speed inverter is used to drive the two-pole asynchronous motor, a high speed of up to 180,000 r/min can be obtained. Moreover, with the development of inverter technology, the output frequency of high-frequency power supplies is also increasing, so it is possible to drive at higher speeds.
In addition, compared with the high-speed drive system using the mechanical speed increase device, since the high-speed drive system using the high-frequency inverter does not have mechanical devices other than the asynchronous motor, the reliability is better, and the maintenance and repair are simpler.
In the inverter speed control system, the inverter and motor can be set separately. Therefore, by properly combining with various asynchronous motors, an AC speed control system can be obtained for use in various working environments, and there is no special requirement for the frequency converter itself.
For example, in an environment with explosion-proof and anti-corrosion requirements, it is only necessary to replace the motor with a dedicated motor, and use a common inverter and install it in a normal environment outside the environment where explosion-proof and anti-corrosion requirements are required.
Since the inverter itself can be regarded as an AC power source capable of frequency regulation and voltage regulation, one inverter can simultaneously drive multiple asynchronous motors or synchronous motors, thereby saving equipment investment. For DC speed control systems, it is difficult to do this.
When a single inverter is used to drive multiple motors at the same time, if the driving target is a synchronous motor, all the motors will run at the same speed (synchronous speed), and when the driving target is an asynchronous motor with different capacity and load. , due to the cause of the slip, there will be a certain speed difference between the motors.
Because the inverter drives the asynchronous motor after AC-DC power conversion, the power factor of the power supply is not affected by the motor power factor, and is almost constant.
In addition, when the asynchronous motor is driven by the grid power supply, the starting current of the motor is 5-6 times of the rated current, and when the inverter is driven by the asynchronous motor, the output frequency of the inverter can be reduced to very low. When starting at low speed, the starting current of the motor is very small, so the capacity of the power supply at the input end of the inverter can also be relatively small. In general, the capacity of the power supply at the input of the inverter needs to be about 1.5 times the output capacity of the motor. This also means that the frequency converter can also function as a pressure reducing starter at the same time.
With the development of control theory, AC speed regulation theory and electronic technology, the inverter technology has also received full attention and development. At present, the control system composed of high-performance inverter and special asynchronous motor has reached and exceeded the performance. A DC motor servo system. In addition, since the asynchronous motor also has advantages such as strong adaptability to the environment and simple maintenance, many DC servo motors do not have the advantages, so in many applications requiring high-speed and high-precision control, such a high-performance AC speed control system is gradually replacing. DC servo system. Moreover, because the high-performance inverter has a very rich external interface function, it can be used as a component in the automatic control system to form the required automatic control system.
Since the frequency converter has the above advantages, it has been widely used in various fields.
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