The selection of the inverter should be considered according to the type of controlled object, speed regulation scale, static speed accuracy, starting torque, etc., so that it is easy to use and economical together with the satisfaction of process and production requirements.
1. Inverter and the motor to be operated
(1) The number of poles of the motor. Generally, the number of poles of the motor is not more than 4, otherwise the capacity of the inverter should be appropriately increased.
(2) Torque characteristics, critical torque, and accelerated torque. In the case of equal motor power, the inverter specification can be reduced compared to the high overload torque form.
(3) Electromagnetic compatibility. In order to reduce the disturbance of the main power supply, add a reactor to the central circuit or inverter input circuit, or install a pre-isolation transformer. Generally, when the distance between the motor and the inverter exceeds 50m, the reactor and filter should be connected in the center of them or the shielded protective cable should be selected.
2. Selection of inverter box structure
In order to adapt the box structure of the inverter to the conditions, it is necessary to consider the factors such as temperature, humidity, dust, pH, and corrosive gas. There are several common structures:
(1) Open IP00 type. It has no chassis and can be installed in the electric control box or on the screen, plate and rack in the electrical room, which is especially suitable for the use of multiple inverters, but the environmental conditions are high.
(2) Closed IP20 type. It is suitable for general use, and can have a small amount of dust or a small amount of temperature and humidity.
(3) Sealed IP45 type. It is suitable for the environment with poor industrial site conditions.
(4) Closed IP65 type. It is suitable for situations where environmental conditions are poor, and there is water and dust Egypt must corrosive gases.
3. Selection of inverter power
The relationship curve between the inverter load rate β and the power η is visible: when β = 50%, η = 94%; When β = 100%, η = 96%. Although the β has doubled, the η change is only 2%, but it is also considerable for medium to high power (a few hundred kilowatts to several thousand kilowatts) electric motors. The system power is equal to the product of the power of the inverter and the power of the motor. Starting from the power point of view, the following points should be paid attention to when selecting the power of the inverter.
(1) When the power of the inverter is equivalent to the power of the motor, it is most suitable to facilitate the operation of the inverter in the high-power state.
(2) If the power classification of the inverter and the power classification of the motor are not together, the power of the inverter should be as close as possible to the motor
and should be slightly greater than the power of the motor.
(3) When the motor is frequently started, braked or started under heavy load and is more frequent, a larger inverter can be selected to facilitate the inverter to run for a long time and safely.
(4) After testing, the practical power of the motor does have a surplus, and the inverter whose power is less than the power of the motor can be considered, but it is necessary to pay attention to whether the instantaneous peak current will form an overcurrent maintenance action.
(5) When the power of the inverter and the motor are not together, it is necessary to adjust the setting of the energy-saving program accordingly to achieve a higher energy-saving effect.
4. Confirm the capacity of the inverter
Reasonable capacity selection is itself a way to save energy and reduce consumption. Based on the available materials and experience, there are three more concise methods.
(1) The motor power verification method is practiced. The practical power of the motor is determined first, and the capacity of the inverter is selected in order to select it.
(2) Formula method. Set the safety factor to be 1. 05, then the capacity of the inverter pb is:
pb = 1. 05 pm/ hm ×cosφ,kW
where pm – motor load, kW
hm – motor power, kW
After calculating the PB, select the specific specifications according to the inverter product catalog.
When one inverter is used for multiple motors, the influence of the starting current of at least one motor should be considered to prevent the inverter from overcurrent tripping.
(3) Motor rated current method. The inverter capacity selection process, in practice is an inverter and the motor of the best matching process, the most common, also safer is to make the capacity of the inverter greater than or equal to the rated power of the motor, but in the practice of matching to consider the difference between the practical power of the motor and the rated power, usually the equipment selection can be large, and the practice needs to be small, so it is reasonable to select the inverter according to the practical power of the motor, to prevent the selection of the inverter is too large, so that the investment increases. For light load classes, the inverter current should generally be 1. 1 In ( In is the rated current of the motor) to select, or according to the maximum motor power indicated by the manufacturer in the product and the output power rating of the inverter.
5. Mains power
(1) Power supply voltage and fluctuation. Special attention should be paid to adapt to the low voltage maintenance setting value of the inverter (generally set to 0.8-0.9Un at the factory), because in practical use, the possibility of low grid voltage is greater.
(2) Frequency fluctuation and harmonic disturbance of the main power supply. This disturbance adds to the heat loss of the drive system, resulting in increased noise and reduced output.
(3) The power consumption of the inverter and motor itself during operation. In the design of the main power supply of the system, the power consumption factors of both should be taken into account.
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