I. The three main functions of frequency converters
1. Energy-saving effect of frequency converters
One of the biggest advantages of frequency converters is energy saving, most notably in the fan and pump industry. Since the power consumption of fans and pumps is directly proportional to the cube of their rotational speed, when external air/water demand is low, using a frequency converter to reduce the rotational speed results in significant energy savings. The energy-saving principle in other industries is roughly the same: reducing the motor speed when full-speed operation is not required achieves energy savings.
2. Soft start function of frequency converter
When a motor is directly started at the mains frequency, it will generate 7 to 8 times the motor's rated current. This increased current increases the electrical stress on the motor windings and generates heat, thus reducing the lifespan of the frequency converter. Variable frequency speed control, on the other hand, allows for zero-speed, zero-voltage starting (with the option of appropriately increasing torque).
3. The role of frequency converter control system in simplification
Variable frequency speed control can start at zero speed and accelerate evenly according to the user's needs, and its acceleration curve can also be selected (linear acceleration, S-shaped acceleration, or automatic acceleration). The operating speed can be adjusted at any time as needed and can be quickly changed according to the process. Speed changes can also be achieved through remote control PLC or other controllers.
II. Principles for Selecting Inverter Capacity
The basic principles for selecting the capacity of frequency converters are as follows.
1. Matching principle
The selection of a frequency converter should be matched to the load. This is as follows.
(1) Power matching: The rated power of the frequency converter should match the rated power of the load. It should be noted that the power requirements of the motor vary depending on the load. For example, motors with the same power will require different frequency converter capacities depending on the nature of the load. The frequency converter capacity required for square torque loads (fans) is lower than that required for constant torque loads. Usually, the frequency converter product manual directly gives the rated power or apparent power of the suitable drive motor. Therefore, for square torque loads such as fans and pumps, the appropriate frequency converter can be selected according to the motor power.
(2) Voltage matching: The rated voltage of the frequency converter matches the rated voltage of the load.
(3) Current matching: For ordinary centrifugal pumps, the rated current of the frequency converter should match the rated current of the motor; for special loads, such as deep water pumps, the performance parameters of the motor should be considered, and the frequency converter current and overload capacity should be determined based on the maximum current.
(4) Torque matching: This should be considered when there is a constant torque load or a speed reduction device.
2. Principle of Economy
Technical and economic analyses should be conducted to select a control scheme that meets application requirements and has a high performance-price ratio.
3. The principle of analyzing specific situations in specific ways
Different application scenarios should be analyzed specifically to determine the inverter capacity.
(1) Selection of motor capacity according to the inverter product manual: In the following cases, the inverter capacity can be selected according to the motor capacity required by the inverter product manual.
(2) When selecting the capacity of the frequency converter, you need to select one or two levels higher according to the instruction manual: the following situations require selecting one or two levels higher.
(3) When using the rated power of the frequency converter as the frequency converter capacity index to select the frequency converter, the influence of the number of motor poles and the rated current of the motor is not considered. Therefore, when selecting the frequency converter based on the rated power of the frequency converter, the selected frequency converter may not meet the rated current requirements of the motor.
(4) When selecting a frequency converter based on the rated current of the motor, there is usually a certain margin when the motor capacity is not considered. This is especially true for frequency converter retrofit projects, where the margin when selecting a motor is generally 40% to 50%. Therefore, when selecting a frequency converter capacity based on the rated current of the motor, the frequency converter may have an excessive margin, causing the frequency converter to operate under low load and resulting in a waste of resources.
In addition, the maximum operating current of the motor affects the motor's heating and temperature rise. For short-term overload, frequency converters generally have an overload capacity of 150% and 1 minute. Therefore, for applications with large load fluctuations, when selecting a frequency converter, the maximum operating current and overload time of the motor should be known so that the frequency converter does not exceed its rated current when the maximum operating current is reached, or the overload current when the overload time is less than 1 minute is less than 150% of the rated current.
(5) Proper matching of frequency converter and motor means that the rated current (or maximum operating current) of the motor should be less than the rated current of the frequency converter under the same power. For example, as shown in the table, for an existing 8-pole 18.5kW motor, if a frequency converter of the same power is selected, considering the large load fluctuation, ABB's ACS800 series product should be selected.
(6) When selecting a frequency converter with a reduced capacity, attention should be paid to the impact of the motor starting current and acceleration current. To this end, it is advisable to add an output reactor between the frequency converter and the motor to filter and smooth the impact current and acceleration current, thereby reducing their impact. While meeting the requirements for acceleration and deceleration in the production process, the acceleration and deceleration times should be set longer. The preset value of U/f during startup should be set smaller.
