Variable frequency power supply application - Power Circuit - Circuit Diagram

The correct selection of the variable frequency power supply is crucial for the normal operation of the electronic control system of the mechanical equipment. To select the variable frequency power supply, firstly, according to the type of mechanical equipment, load torque characteristics, speed regulation range, static speed accuracy, starting torque and the requirements of the use environment, it is most appropriate to decide which control method and protection structure of the variable frequency power supply. The so-called combination is to achieve the best cost performance of variable frequency power supply application under the premise of meeting the actual process production requirements and use occasions of mechanical equipment.
The first point. According to the load characteristics, the variable frequency power supply with appropriate control mode is available. There are many kinds of variable frequency power supplies available on the market, and the functions are also increasingly powerful. The performance of the variable frequency power supply is increasingly becoming the decisive factor of the speed control performance, except for the variable frequency power supply. In addition to the "innate" conditions of the manufacturing process itself, it is also very important to control the control method of the variable frequency power supply. The following table summarizes the performance characteristics of various variable frequency power supply control methods in recent years.
In summary, the asynchronous motor variable frequency control uses different control methods to obtain the speed regulation characteristics of different performance characteristics.
Second point. According to the installation environment, the protection structure of the variable frequency power supply should be selected. The protection structure of the variable frequency power supply should be compatible with the installation environment. This should consider environmental temperature, humidity, dust, pH, corrosive gas and other factors. Long-term, safe and reliable operation is of great importance. Most variable frequency power supply manufacturers can provide the following common protection structures for users to choose:
(1) Open type IP00, which protects the human body from the front and can not touch the live parts inside the variable frequency power supply. It is suitable for installation on screens, discs and racks in electric control cabinets or electrical rooms, especially for multiple inverter power supplies. Ok, but it requires a lot of installation environment.
(2) Enclosed IP20, IP21, the variable frequency power supply of this protective structure has a cover around it, which can be installed on the wall of the building. It is suitable for most indoor installation environments.
(3) Sealed type IP40, IP42, which is suitable for occasions with poor industrial environment conditions.
(4) Closed type IP54, IP55, it has dustproof and waterproof protection structure, suitable for industrial environment with poor environmental conditions, water shower, dust and certain corrosive gases.
Third point. Load torque characteristics of mechanical equipment In practice, production machinery is often divided into three types according to the characteristics of load torque: constant torque load, constant power load and fluid load.
1 Constant torque load In this type of load, the load torque TL is independent of the speed n. The TL always remains constant or substantially constant at any speed, and the load power increases linearly as the load speed increases. Frictional loads such as conveyors, mixers, extruders, and feed mechanisms for mechanical equipment, as well as gravity loads such as cranes, hoists, and elevators, all belong to constant torque loads.
When the variable frequency power supply is dragged by a constant torque load, the output torque at a low speed is large enough and sufficient overload capability is required. If you need to run at a low speed for a long time, you should consider the heat dissipation capability of the standard cage asynchronous motor to avoid excessive temperature rise of the motor.
2 Constant power load This type of load is characterized by the fact that the demand torque TL is roughly inversely proportional to the speed n, but the product, ie the power, remains approximately unchanged. The main shaft of metal cutting machine tools and rolling mills, paper machines, coilers in the film production line, and unwinders are all constant power loads.
The constant power nature of the load should be in terms of a range of speed variations. When the speed is very low, due to the limitation of mechanical strength, TL cannot be infinitely increased, and it is converted to a constant torque property at a low speed. The constant power zone and constant torque zone of the load have a great influence on the choice of the transmission scheme. When the motor is in constant flux regulation, the maximum allowable output torque is constant, which belongs to constant torque speed regulation. In the case of weak magnetic speed regulation, the maximum allowable output torque is inversely proportional to the speed, which belongs to constant power speed regulation. If the range of constant torque and constant power speed regulation of the motor is consistent with the constant torque and constant power range of the load, the so-called "matching", the capacity of the motor and the capacity of the variable frequency power supply are the smallest.
3 fluid type load The torque of this kind of load is proportional to the square of the speed, and the power is proportional to the cube of the speed. Various fans, pumps and oil pumps are typical fluid loads.
The fluid type load regulates the air volume and flow rate through variable frequency power supply speed regulation, which can greatly save power. Since the demand power of the fluid type load at high speed is too fast, which is proportional to the cube of the load speed, such load should not be operated at a power frequency.