Application of variable frequency power supply

The correct selection of variable frequency power supply is crucial for the normal operation of the electronic control system of mechanical equipment. When selecting a variable frequency power supply, we must first consider the type of mechanical equipment, load torque characteristics, speed regulation range, static speed accuracy, starting torque and the requirements of the use environment, and then decide which control method and protective structure of the variable frequency power supply is most suitable. The so-called suitability is to achieve the best cost-effectiveness of the variable frequency power supply application under the premise of meeting the actual process production requirements and use occasions of the mechanical equipment.

First point: Select the variable frequency power supply with appropriate control mode according to the load characteristics

There are many types of variable frequency power supplies on the market now, and their functions are becoming increasingly powerful. The performance of variable frequency power supplies is becoming more and more a decisive factor in the quality of speed regulation performance. In addition to the "innate" conditions of the manufacturing process of the variable frequency power supply itself, the control method used for the variable frequency power supply is also very important. The following table summarizes the performance characteristics of various variable frequency power supply control methods in recent years.

In summary, by selecting different control methods for asynchronous motor variable frequency control, speed regulation characteristics with different performance characteristics can be obtained.

Second point: Select the protection structure of the variable frequency power supply according to the installation environment

The protection structure of the variable frequency power supply must be adapted to its installation environment, which requires consideration of factors such as ambient temperature, humidity, dust, pH, corrosive gases, etc. This is of great importance to whether the variable frequency power supply can operate long-term, safely, and reliably. Most variable frequency power supply manufacturers can provide the following commonly used protection structures for users to choose from:

(1) Open type IP00, which protects the human body from touching the live parts inside the variable frequency power supply from the front. It is suitable for installation on the screen, panel, and rack in the electric control cabinet or electrical room, especially for centralized use of multiple variable frequency power supplies, but it has high requirements for the installation environment.

(2) Enclosed IP20, IP21, this type of protective structure of the variable frequency power supply has an outer cover all around and can be mounted on the wall of a building. It is suitable for most indoor installation environments.

(3) Sealed type IP40, IP42, which is suitable for industrial sites with poor environmental conditions.

(4) Sealed type IP54, IP55, it has a dustproof and waterproof protective structure, suitable for industrial site environments with poor environmental conditions, water spray, dust and certain corrosive gases.

In practice, people often divide production machinery into three types according to the different load torque characteristics: 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. At any speed, TL always remains constant or basically constant, and the load power increases linearly with the increase of load speed. Friction loads such as conveyor belts, mixers, extruders, and feed mechanisms of mechanical equipment, as well as gravity loads such as cranes, hoists, and elevators are all constant torque loads.

When the variable frequency power supply is used to drive a constant torque load, the output torque at low speed must be large enough and have sufficient overload capacity. If it is necessary to run at a low speed for a long time, the heat dissipation capacity of the standard cage asynchronous motor should be considered to avoid excessive temperature rise of the motor.

2 Constant power load

The characteristic of this type of load is that the required torque TL is roughly inversely proportional to the speed n, but their product, that is, the power, remains approximately constant. The spindle of metal cutting machine tools and rolling mills, paper machines, winders and unwinders in film production lines, etc., are all constant power loads.

The constant power property of the load should be in terms of a certain speed change range. When the speed is very low, due to the limitation of mechanical strength, TL cannot be increased infinitely and it will be transformed into a constant torque property at low speed. The constant power area and constant torque area of ​​the load have a great influence on the selection of the transmission scheme. When the motor is in constant flux speed regulation, the maximum allowable output torque remains unchanged, which belongs to constant torque speed regulation; while in weak flux speed regulation, the maximum allowable output torque is inversely proportional to the speed, which belongs to constant power speed regulation. If the range of the constant torque and constant power speed regulation of the motor is consistent with the constant torque and constant power range of the load, that is, the so-called "matching" case, the capacity of the motor and the capacity of the variable frequency power supply are both minimum.

3 Fluid loads

The torque of this type of load is proportional to the square of the speed, and the power is proportional to the cube of the speed. Various fans, water pumps and oil pumps are typical fluid loads.

Fluid loads can save a lot of electricity by adjusting the air volume and flow rate through variable frequency power supply speed regulation . Since the power demand of fluid loads increases too quickly at high speeds, which is proportional to the cube of the load speed, such loads should not be operated at over-power frequency.