Driving motors through frequency converters has become an irreversible trend. In actual use, due to the unreasonable matching relationship between frequency converters and motors, some problems often occur. When selecting a frequency converter, you should fully understand the load characteristics of the equipment driven by the frequency converter.

We can divide production machinery into three types: constant power load, constant torque load, and fan and water pump load. Different load types have different requirements for frequency converters, and we should match them reasonably according to the specific situation.

1. Constant power load

The torque required by machine tool spindles, rolling mills, paper machines, and coilers and uncoilers in plastic film production lines is generally inversely proportional to the rotational speed and is a constant power load. The constant power property of the load should be within a certain range of speed changes. When the speed is very low, it will change to a constant torque load at low speed due to the restriction of mechanical strength. When the motor adjusts speed with constant magnetic flux, it is constant torque speed regulation; while when it is flux weakened, it is constant power speed regulation.

2. Fan and pump loads

Fans, water pumps, oil pumps and other equipment rotate with the impeller. As the rotational speed decreases, the torque decreases according to the square of the rotational speed. The power required by the load is proportional to the cube of the speed. When the required air volume and flow rate decrease, the frequency converter can be used to adjust the air volume and flow rate through speed regulation, which can greatly save electricity. Since the power required at high speed increases too quickly with the speed, fans and pumps should not be allowed to operate beyond the industrial frequency.

3. Constant torque load

The torque always remains constant or essentially constant at any speed. When the frequency converter drives a load with constant torque, the torque at low speed must be large enough and it must have sufficient overload capacity. If it is necessary to run at a low speed and at a stable speed, the heat dissipation performance of the motor should be considered to prevent the motor from burning due to excessive temperature rise.

Issues you should pay attention to when choosing an inverter

1. When a power frequency motor is driven by a frequency converter, the motor's current will increase by 10-15% and the temperature rise will increase by about 20-25%.

2. When using a frequency converter to control a high-speed motor, more high-order harmonics will be generated. These high-order harmonics will increase the output current value of the frequency converter. Therefore, when selecting an inverter, it should be one gear larger than that of an ordinary motor.

3. Compared with ordinary squirrel cage motors, wound motors are prone to overcurrent tripping problems, so an inverter with a slightly larger capacity than usual should be selected.

4. When using a frequency converter to drive a gear reduction motor, the range of use is limited by the lubrication method of the gear rotating part. When the rated speed is exceeded, there may be a risk of running out of oil.

5. Use the motor current value as the basis for selecting the inverter, and the motor rated power is only for reference.

6. The inverter output contains rich high-order harmonics, which will reduce the power factor and efficiency performance of the motor.

7. When the inverter needs to run with a long cable, the impact of the cable on performance should be considered, and special cables should be selected if necessary. To compensate for this problem, the frequency converter should amplify the selection by one or two gears.

8. Special occasions such as high temperature, frequent switching, and high altitude will cause rainfall to exceed the capacity of the inverter. It is recommended that the inverter be selected by the first gear.

9. Compared with industrial frequency power supply, when the frequency converter drives a synchronous motor, the output capacity will be reduced by 10~20%.

10. For loads with large torque fluctuations such as compressors and vibrators and peak loads such as hydraulic pumps, you should fully understand the power frequency operation and choose a larger frequency converter.