The capacity of the frequency converter is directly related to the operational reliability of the variable frequency speed control system, therefore, a reasonable capacity will ensure optimal investment. There are many misconceptions about the capacity selection of frequency converters in practical operation. Here are three basic methods for capacity selection, which complement each other.

1. From the perspective of current

The capacity of most frequency converters can be expressed from three perspectives: rated current, available motor power, and rated capacity. The latter two items are difficult to accurately express the capabilities of the frequency converter, either provided by the manufacturer of the frequency converter using standard motors produced domestically or by our company, or reduced with the output voltage of the frequency converter.

When choosing a frequency converter, only the rated current of the frequency converter is a key parameter reflecting the load capacity of the semiconductor frequency converter device. The basic principle for selecting the capacity of a frequency converter is that the load current should not exceed the rated current of the frequency converter. It should be emphasized that before determining the capacity of the frequency converter, it is necessary to carefully understand the process and motor parameters of the equipment. For example, the rated current of submersible pumps and wound rotor motors should be higher than that of ordinary cage asynchronous motors. The roller conveyor motors commonly used in the metallurgical industry not only have much higher rated current, but also allow for short-term locked rotor operation, and roller conveyor drives are mostly multi motor drives. It should be ensured that the total load current does not exceed the rated current of the frequency converter in a fault free state.

2. From the perspective of efficiency

The system efficiency is equal to the product of the efficiency of the frequency converter and the efficiency of the motor. Only when both are operating at a higher efficiency, the system efficiency is higher. From an efficiency perspective, when selecting the power of a frequency converter, the following points should be noted:

(1) When the power value of the frequency converter is equivalent to that of the motor, it is most suitable for the frequency converter to operate at high efficiency values.

(2) When the power classification of the frequency converter is different from that of the motor, the power of the frequency converter should be as close as possible to the power of the motor, but slightly greater than the power of the motor.

(3) When the electric motor is frequently started, braked, or in heavy load starting and frequent operation, a higher-level frequency converter can be selected to ensure long-term and safe operation of the frequency converter.

(4) After testing, it has been found that the actual power of the motor is indeed surplus. It is possible to consider using a frequency converter with a power lower than the motor power, but attention should be paid to whether the instantaneous peak current will cause overcurrent protection action.

(5) When the power of the frequency converter is different from that of the motor, the energy-saving program settings must be adjusted accordingly to achieve higher energy-saving effects.

3. From the perspective of calculating power

For a continuously operating frequency converter, the following three calculation formulas must be met simultaneously:

(1) Satisfy load output: Pcn ≥ Pm/η

(2) Satisfy the motor capacity: Pcn ≥ √ 3KUeIe cos π× 10-3

(3) Satisfy the motor current: Icn ≥ KIe

In the formula, Pcn is the capacity of the frequency converter (unit: kW), PM is the required output power of the motor shaft for the load (unit: kW), Ue is the rated voltage of the motor (unit: V), Ie is the rated current of the motor (unit: A), η is the motor efficiency (usually about 0.85), cos φ is the motor power factor (usually about 0.75), and k is the current waveform compensation coefficient (due to the fact that the output waveform of the frequency converter is not a complete sine wave, but contains high-order harmonic components, its current should increase slightly, usually K is about 1.05-1.1).

By inputting the system parameters, the minimum capacity of the selected frequency converter is 88KW, so 100KW is taken. The rated current is 139.26A, so 150A is taken.

According to the calculated required parameters, a Siemens MicroMaster430 (fan and water pump specialty) frequency converter can be selected. Specifically, the MM430-110K model frequency converter can be chosen, which is equipped with a motor capacity of 110kW and a rated current of 205A to meet the usage requirements. It can be selected.