With the rapid development of frequency converter technology, variable frequency speed control technology has become mainstream in various motor application solutions. The application of general-purpose frequency converters in the industrial field has greatly saved resources for enterprises, reduced costs, and protected production equipment. So how should enterprises choose a suitable general-purpose frequency converter? Below, I will introduce this in detail:
Choosing the right frequency converter is crucial for the normal operation of the transmission control system of mechanical equipment, preventing equipment failures and unnecessary losses due to incorrect selection. First, the purpose of selecting the frequency converter should be clearly defined. Then, a suitable frequency converter should be selected based on the equipment type, load characteristics, speed range, control method, operating environment, and protection structure requirements. This will achieve both production process requirements and economic benefits.
1. Load torque characteristics of mechanical equipment
In practice, production machinery is often classified into three main types based on its load torque characteristics: constant torque load, constant power load, and load with decreasing torque characteristics. Therefore, the selection of a frequency converter should naturally be based on these load characteristics.
1.1 Load with reduced torque characteristics
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 square of the speed, the torque changes with the square of the rotational speed, and the load power changes with the cube of the speed. This type of load is a torque-reducing load.
1.2 Constant power load
The characteristic of this type of load is that the required torque TL is roughly inversely proportional to the speed n. As the motor speed decreases, the output torque actually increases. That is, within the speed regulation range, the torque is high at low speeds and low at high speeds, while the motor's output power remains constant. Spindles of metal cutting machine tools, rolling mills, paper machines, and winding and unwinding machines in film production lines are all examples of constant power loads.
The constant power characteristic of a load refers to a certain range of speed variation. At very low speeds, due to mechanical limitations, torque (TL) cannot increase indefinitely, and at low speeds, it transitions to a constant torque characteristic. The constant power and constant torque regions of the load significantly influence the selection of the transmission scheme. When the motor is under constant flux speed regulation, the maximum permissible output torque remains constant, which is constant torque speed regulation; while under field weakening speed regulation, the maximum permissible output torque is inversely proportional to the speed, which is constant power speed regulation. If the constant torque and constant power speed regulation ranges of the motor coincide with the constant torque and constant power ranges of the load, i.e., under the so-called "matching" condition, both the motor capacity and the inverter capacity are minimized.
The mechanical characteristics of constant power loads are complex. During system design, care should be taken to prevent the asynchronous motor from operating beyond its synchronous speed, otherwise destructive mechanical failures may occur. Typically, the capacity of the frequency converter is 1.1 to 1.5 times the capacity of the asynchronous motor.
1.3 Constant Torque Load
In constant torque loads, the load torque TL is independent of the speed n; at any speed, the load torque TL remains constant or essentially constant, while the load power increases linearly with increasing load speed. Examples of constant torque loads include friction loads such as cranes, conveyor belts, injection molding machines, mixers, and elevators. The purpose of using frequency converters to control these types of loads is to automate equipment, improve labor productivity, and enhance product quality.
When a frequency converter drives a constant torque load, the output torque at low speeds must be large enough, and it must have sufficient overload capacity, typically 150% of the rated current. If long-term stable operation at low speeds is required, the heat dissipation capacity of the asynchronous motor should be considered to avoid excessive motor temperature rise.
When designing a system, attention should be paid to appropriately increasing the capacity of the asynchronous motor or the frequency converter. The capacity of the frequency converter is generally 1.1 to 1.5 times the capacity of the asynchronous motor.
2. Select a frequency converter with an appropriate control method based on the load characteristics.
Besides the manufacturing process of the frequency converter, the control method used is also crucial. Frequency converter control methods are mainly divided into two types: open-loop control and closed-loop control. Open-loop control is simple in structure and reliable in performance, but its speed regulation accuracy and dynamic response performance are relatively low. Closed-loop control, on the other hand, can perform real-time control based on changes in parameters such as flow rate, temperature, position, speed, and pressure, offering fast dynamic response, but it can be difficult to implement in some cases and is more expensive. Users should choose the appropriate control method based on their specific needs to obtain the required speed regulation characteristics.
3. Select the inverter's protection structure based on the installation environment.
When selecting a frequency converter, its installation environment should be considered, including factors such as ambient temperature, humidity, dust content, and corrosive gases. These factors are crucial to the long-term, reliable operation of the frequency converter. If the operating conditions cannot be met, appropriate protective measures must be taken.
Most frequency converter manufacturers offer the following common protection 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. It is suitable for installation in the control cabinet or on the panel, board or rack in the electrical room. It is especially good for multiple frequency converters to be used together. However, it has high requirements for the installation environment.
(2) Enclosed IP20 and IP21: This type of inverter has an outer cover on all sides and can be wall-mounted in buildings. It is suitable for most indoor installation environments with a small amount of dust or temperature and humidity.
(3) Sealed type IP40, IP42, which is suitable for industrial sites with poor environmental conditions.
(4) Sealed IP54 and IP55, which have dustproof and waterproof protective structures, are suitable for industrial sites with poor environmental conditions, water spray, dust and certain corrosive gases.
When selecting a variable frequency speed control system for a construction site, the choice should be based on the actual process requirements and application environment. The advantages and disadvantages should be weighed, and a reasonable selection should be made based on comprehensive consideration. Only by using the frequency converter correctly and flexibly can the AC variable frequency speed control system operate safely and reliably.
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