How to select a frequency converter based on load characteristics

With the rapid development of inverter technology, variable frequency speed regulation technology has become the mainstream in various motor application solutions. The application of general-purpose inverters in the industrial field has greatly saved resources for enterprises, reduced enterprise costs, and protected production equipment. So how should enterprises choose the right general-purpose inverter?

Correctly selecting the inverter is very important for the normal operation of the transmission control system of mechanical equipment, so as to avoid equipment failure and unnecessary losses caused by wrong selection. First of all, the purpose of selecting the inverter should be clear, and then the appropriate inverter should be selected according to the equipment type, load characteristics, speed regulation range, control method, use environment, protective structure and other requirements. In this way, the purpose of both meeting the production process and achieving economic benefits can be achieved.

There are many types of production machinery, with different performance and process requirements, and their torque characteristics are also different. Therefore, before applying the inverter, you must first understand the nature of the load carried by the motor, that is, the load characteristics, and then choose the inverter and motor. There are three types of loads: constant torque load, fan pump load and constant power load. Different types of loads should choose different types of inverters.

(1) Constant torque load: It can be divided into friction load and potential load. The starting torque of friction loads usually requires about 150% of the rated torque, and the braking torque usually requires about 100% of the rated torque. Therefore, the inverter should be selected with constant torque characteristics, and the starting and braking torques are relatively large, and the overload time and overload capacity are large. Potential loads usually require large starting torque and energy feedback functions, and can quickly achieve forward and reverse rotation. The inverter should be selected with four-quadrant operation capability.

(2) Fan and pump loads: Fan and pump loads are typical square torque loads. The load is very small at low speed and is proportional to the square of the speed. The combination of a general-purpose inverter and a standard motor is the most suitable. This type of load does not require high performance from the inverter, only economy and reliability. Therefore, a inverter with U/f=const control mode is sufficient.

(3) Constant power load: Constant power load refers to a load where the torque is inversely proportional to the speed, but the power remains constant, such as a winder, machine tool, etc. When using a frequency converter for clothing with constant power characteristics, the following issues should be noted: the output voltage of the frequency converter is controlled at a fixed value within the frequency range above the industrial frequency, so the torque generated by the motor and the load torque have opposite tendencies, and the combination of a standard motor and a general frequency converter is difficult to adapt, so a special design is required.

How to select a frequency converter based on load characteristics

The load of the inverter seems to have many types, such as extrusion, winding, hanging objects, blowing, etc. In fact, the load can be roughly divided into ① friction load; ② gravity load; ③ fluid load; ④ inertia load. Mechanical load is generally divided into three types of load characteristics: ① constant torque load, ② square torque load, ③ constant power load; In order to facilitate everyone to understand the mechanical load characteristics and torque characteristics, the following table is specially made.

The load characteristics and the relationship between the motor output power and speed are as follows; For the five types of constant power, constant torque, square torque, decreasing power, and negative torque,

① For constant torque loads, such as extruders, mixers, conveyor belts, in-plant transport trams, lifting mechanisms, etc., if ordinary functional inverters are used, to achieve constant torque speed, the method of increasing the capacity of the motor and inverter is often adopted to improve the low-speed torque; it is more ideal to use a high-function inverter with torque control function to achieve speed regulation of constant torque loads. Because this inverter has a large low-speed torque, a large static mechanical characteristic hardness, is not afraid of load impact, and has the characteristics of an excavator.

② For production machinery that requires high precision, good dynamic performance, and fast speed response, such as papermaking machines, injection molding machines, rolling mills, etc., high-performance general-purpose inverters with vector control or direct torque control should be used.

③ For constant power loads, such as lathes, planers, blowers, etc., since there is no inverter with constant power characteristics, constant power can be achieved by relying on the U/f control method.

④ For fans and pumps, since the load torque is proportional to the square of the speed, the load torque is small at low speed, and diangon.com can usually choose a dedicated or general-purpose inverter. It must be pointed out that some general-purpose inverters can be applied to all three types of loads, so although the price of general-purpose inverters is a little higher, it is worth the money. See the figure below.

The motor power is proportional to the product of the speed and torque. Even for motors of the same power, different load properties will require different inverter capacities. The inverter capacity required for square torque loads is lower than that for constant torque loads. Therefore, the inverter and motor must be combined into a variable frequency speed regulation system, and the technical parameters of the two must meet the requirements to meet the load torque requirements under low and high speed conditions. For example, the type of inverter should be selected according to the load requirements.

(1) For constant torque loads, such as extruders, mixers, conveyor belts, in-plant transport trams, lifting mechanisms, etc., if ordinary functional inverters are used, to achieve constant torque speed regulation, the method of increasing the capacity of the motor and inverter is often used to increase the low-speed torque; if a high-function inverter with torque control function is used to achieve constant torque load speed regulation operation. It is more ideal. Because this inverter has a large low-speed torque, a large static mechanical characteristic hardness, is not afraid of load impact, and has the characteristics of an excavator. (2) For production machinery that requires high precision, good dynamic performance, and fast speed response, such as papermaking machines, injection molding machines, rolling mills, etc., high-performance general-purpose inverters with vector control or direct torque control should be used.

(3) For constant power loads, such as lathes, planers, blowers, etc., since there is no inverter with constant power characteristics, constant power can be achieved by relying on the U/F control method.

(4) For fans and pumps, since the load torque is proportional to the square of the speed and the load torque is small at low speed, a dedicated or general-purpose inverter can usually be selected. Friendly reminder: Some general-purpose inverters can be applied to all three types of loads. The following points should also be noted to achieve a reasonable match between the inverter and the motor to achieve ideal speed regulation and energy-saving operation. The following issues should be noted in the configuration of the two.

① Since the power output by the inverter often contains high-order harmonics, it will increase the total loss of the motor. Even if it runs at the rated frequency, the output torque of the motor will be reduced. For example, when the speed is adjusted above or below the rated frequency, the rated output torque of the motor cannot be fully utilized. If the rated torque output is always required regardless of the speed, a larger capacity motor should be used with reduced capacity.

② From the perspective of efficiency (i.e. energy saving), the following points should be noted.

a. It is most appropriate when the inverter power value is equal to the motor power value, so that the inverter can run at a higher efficiency.

b. When the power classification of the inverter is different from that of the motor, the power of the inverter should be as close to that of the motor as possible, but should be slightly larger than that of the motor.

c. When the motor is frequently started, braked, or started under heavy load and works frequently, a higher-level inverter can be selected to facilitate long-term and safe operation of the inverter.

d. When the actual power of the motor is sufficient, you can consider using a frequency converter with a power smaller than the motor power, but pay attention to whether the instantaneous peak current will cause overcurrent protection to operate.

e. When the power of the inverter and the motor are different, the setting of the energy-saving program must be adjusted accordingly to achieve a higher energy-saving effect. When the inverter works in a constant U/F mode, the motor starting torque is proportional to the frequency, so the starting torque is extremely small when starting at a low frequency. For example, at 10Hz, the output torque of a Y series motor is about 50% of the rated torque, so when selecting the type of motor, pay special attention to the change of low-frequency starting torque. When starting with load, the problem of static friction torque should be considered, and the motor must have a large enough starting torque to ensure load starting.