Selecting the inverter capacity is an important and complex issue. It is necessary to consider the matching between the inverter capacity and the motor capacity. If the capacity is too small, it will affect the effective torque output of the motor, affect the normal operation of the system, and even damage the equipment. If the capacity is too large, the harmonic components of the current will increase, which will also increase the equipment investment.
Principles for Selecting Frequency Converter 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 may require different frequency converter capacities due to different load characteristics. 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 can be seen from 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 products are selected.
(6) When the capacity of the selected frequency converter is reduced, 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 the impact of the impact current and acceleration current; while meeting the requirements of acceleration and deceleration in the production process, the acceleration and deceleration time should be set longer; and the preset value of U/f during startup should be set smaller.
Steps (methods) for selecting inverter capacity
1. Steps for selecting inverter capacity
(1) Understand the nature and variation of the load, calculate the magnitude of the load current or draw the load current diagram I=f(t);
(2) Pre-select the inverter capacity;
(3) Verify the pre-selected frequency converter, and if necessary, verify its overload capacity and starting capacity. If both pass, the capacity of the pre-selected frequency converter is selected; otherwise, start from (2) again until it passes.
Under the premise of meeting the requirements of production machinery, the smaller the capacity of the frequency converter, the more economical it is.
2. Method for selecting inverter capacity
There are many misconceptions about selecting the capacity of frequency converters in actual operation. Here are three basic capacity selection methods that complement each other.
(1) From the perspective of current
Most frequency converters can be described in terms of three aspects: rated current, available motor power, and rated capacity. The latter two are often given by frequency converter manufacturers based on standard motors produced in their own country or company, or they may decrease with the frequency converter's output voltage, making it difficult to accurately express the frequency converter's capability.
When selecting a frequency converter, the rated current of the converter is the only key quantity reflecting the load capacity of the semiconductor frequency converter. Ensuring the load current does not exceed the rated current of the frequency converter is the fundamental principle for selecting its capacity. It is important to emphasize that before determining the frequency converter capacity, the process conditions of the equipment and the parameters of the motors should be carefully understood. For example, the rated current of submersible pumps and wound-rotor motors is greater than that of ordinary squirrel-cage induction motors. The motors used in roller conveyors in the metallurgical industry not only have much larger rated currents, but they also allow short-term stall operation. Furthermore, roller conveyor drives are mostly multi-motor drives; therefore, it must be ensured that the total load current under fault-free conditions does not exceed the rated current of the frequency converter.
(2) From the perspective of efficiency
System efficiency equals the product of inverter efficiency and motor efficiency. Only when both operate at high efficiencies will the system efficiency be high. From an efficiency perspective, the following points should be considered when selecting inverter power:
1) It is most suitable when the power value of the frequency converter is comparable to that of the motor, so that the frequency converter can operate at a high efficiency value;
2) When the power rating of the frequency converter is different from that of the motor, the power rating of the frequency converter should be as close as possible to the power rating of the motor, but should be slightly greater than the power rating of the motor.
3) When the motor is frequently started and braked, or is under heavy load and operates frequently, a larger inverter can be selected to facilitate the long-term and safe operation of the inverter.
4) After testing, the actual power of the motor is indeed sufficient. It is advisable to select a frequency converter with a power rating lower than that of the motor. However, attention should be paid to whether the instantaneous peak current will cause the overcurrent protection to trip.
5) When the power of the frequency converter and the motor are different, the energy-saving program settings must be adjusted accordingly to achieve a higher energy-saving effect.
The relationship curve between the inverter load rate b and efficiency η is shown in the figure.
The curve showing the relationship between load factor and efficiency.
It can be seen that when β=50%, η=94%; when β=100%, η=96%. Although doubling β only changes η by 2%, this is still considerable for medium and high power motors, such as those ranging from several hundred kilowatts to several thousand kilowatts. System efficiency equals the product of inverter efficiency and motor efficiency; only when both operate at high efficiencies is the system efficiency high.
(3) From the perspective of calculating power
For a continuously operating frequency converter, the following three calculation formulas must be met simultaneously.
1) The load output is satisfied: PCN≥PM/η.
2) Meets the motor capacity requirement: PCN≥√3kUeIe&TImes;10-3.
3) The motor current must be satisfied: ICN ≥ kIe.
In the formula, PCN is the inverter capacity (in kVA); PM is the required motor shaft output power (in kW); Ue is the rated voltage of the motor (in V); Ie is the rated current of the motor (in A); η is the motor efficiency (usually about 0.85); cosφ is the motor power factor (usually about 0.75); and k is the current waveform compensation coefficient (since the output waveform of the inverter is not a perfect sine wave and also contains high-order harmonic components, its current should be increased, usually k is 1.05 to 1.1).
