The energy feedback unit supplier reminds you that the correct selection of frequency converters is very important for the normal operation of the transmission control system of mechanical equipment, in order to avoid unnecessary losses caused by frequency converter maintenance due to incorrect selection. Firstly, the purpose of selecting a frequency converter should be clearly defined. Secondly, the appropriate frequency converter should be selected based on equipment type, load characteristics, speed range, control mode, usage environment, protective structure, and other requirements. In this way, the goal is to achieve both production technology and economic benefits.
1. Load torque characteristics of mechanical equipment
In practice, production machinery is often divided into three types based on the different load torque characteristics: constant torque load, constant power load, and reduced torque characteristic load. When choosing a frequency converter, the load characteristics should naturally be the basic basis.
Reduce torque characteristic load
In various fans, water pumps, and hydraulic pumps, as the impeller rotates, the resistance generated by air or liquid within a certain speed range is roughly proportional to the second power of the speed, the torque changes according to the second power of the speed, and the load power changes proportionally to the third power of the speed. This type of load is called a reduced torque load.
Constant power load
The characteristic of this type of load is that the required torque TL is roughly inversely proportional to the speed n. As the motor speed decreases, the output torque of the load actually increases. That is, within the speed range, the torque is greater at low speeds and smaller at high speeds, while the output power of the motor remains unchanged. The spindle of metal cutting machine tools, rolling mills, paper machines, coiling machines, uncoiling machines, etc. in film production lines all belong to constant power loads.
The constant power property of a load is limited to a certain range of speed changes. When the speed is very low, due to the limitation of mechanical strength, TL cannot increase infinitely and transforms into a constant torque property at low speeds. The constant power and constant torque regions of the load have a significant impact on the selection of transmission schemes. When the motor is in constant flux speed regulation, the maximum allowable output torque remains unchanged, which belongs to constant torque speed regulation; In 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 electric motor is consistent with the range of constant torque and constant power of the load, that is, in the case of "matching", the capacity of the electric motor and the capacity of the frequency converter are both minimized.
The mechanical characteristics of constant power loads are complex. When designing the system, attention should be paid to not operating asynchronous motors beyond their synchronous speed, otherwise it may cause destructive mechanical failures. The capacity of a frequency converter is usually taken as approximately times the capacity of an asynchronous motor.
Constant torque load
In a constant torque load, the load torque TL is independent of the speed n. At any speed, the load torque TL remains constant or almost constant, and the load power increases linearly with the increase of load speed. For example, friction loads such as cranes, conveyors, injection molding machines, mixers, and hoists all belong to constant torque loads. The purpose of using frequency converters to control such loads is to achieve equipment automation, improve labor productivity, and enhance product quality.
When the frequency converter drives a constant torque load, the output torque at low speed should be large enough and have sufficient overload capacity, usually 150% of the rated current. If it is necessary to operate steadily at low speeds for a long time, the heat dissipation capacity of asynchronous motors should be considered to avoid excessive temperature rise of the motors.
When designing the system, attention should be paid to appropriately increasing the capacity of asynchronous motors or increasing the capacity of frequency converters. The capacity of a frequency converter is generally taken as~times the capacity of an asynchronous motor.
2. Select an appropriate control method for the frequency converter based on the load characteristics
In addition to the manufacturing process of the frequency converter, the control method adopted by the frequency converter is also very important. The control methods of frequency converters are mainly divided into open-loop control and closed-loop control. The open-loop control method has a simple structure and reliable performance, but its speed regulation accuracy and dynamic response performance are relatively low; The closed-loop control method can perform real-time control based on changes in parameters such as flow rate, temperature, position, speed, pressure, etc. It has fast dynamic response, but sometimes it is difficult to implement and costly. Users should choose the corresponding control mode according to their own needs in order to obtain the required speed regulation characteristics.
3. Select the protective structure of the frequency converter based on the installation environment
When choosing a frequency converter, the installation environment should be considered, including factors such as ambient temperature, humidity, dust content, and corrosive gases, which are closely related to the long-term and reliable operation of the frequency converter. If its operating conditions cannot be met, corresponding protective measures must be taken.
Most frequency converter manufacturers provide the following common protective structures for users to choose from.
(1) Open type IP00, which protects the human body from touching the live parts inside the frequency converter from the front, is suitable for installation on screens, panels, and racks in electrical control cabinets or electrical rooms, especially for centralized use of multiple frequency converters, but it has high requirements for installation environment.
(2) The enclosed IP20 and IP21 frequency converters have enclosures around them and can be wall mounted in buildings. They are suitable for most indoor installation environments with minimal dust or temperature and humidity.
(3) Sealed IP40 and IP42 are suitable for industrial sites with poor environmental conditions.
(4) Sealed IP54 and IP55, with dust-proof and waterproof protective structures, suitable for industrial sites with poor environmental conditions, water spray, dust, and certain corrosive gases.
The selection of variable frequency speed regulation system on the construction site should be based on one's actual process requirements and application scenarios. The advantages and disadvantages should be weighed, and the selection should be reasonable and comprehensively considered. Only by using the frequency converter correctly and flexibly can the AC variable frequency speed control system operate safely and reliably.