AC inverter is the result of the high development of microcomputer and modern power electronics technology. The microcomputer is the center of the inverter, and the power electronics form the main circuit of the inverter. As we all know, the frequency of communication electricity sent from power plants is constant, in our country it is 50 weeks per second. The synchronous speed of the AC motor
n1=60f1/p
where: n1 is the synchronous speed, and the unit is r/min; f1 is the stator frequency, and the unit is Hz; p is the number of pole pairs of the motor.
Asynchronous motor speed formula
n=n(1-s)=60f1/P(1-s)
where s is the slip rate, s= (n1-n)/n1, generally less than 3%, and is positive or close to the positive share with the current frequency fed into the motor. Therefore, changing the frequency can easily change the running speed of the motor, that is, the frequency conversion is very suitable for the speed regulation of the communication motor.
The operating principle of the inverter is related to the operating method of the inverter. General-purpose inverters are classified according to the operation method as follows:
(1) U/f control. U/f control is the control of voltage and frequency in shares, also known as constant voltage frequency ratio control. Because the load of the general-purpose inverter is mainly the motor, for the sake of the stability of the motor's magnetic field, the voltage adjustment must be accompanied by the frequency conversion. U/f control ignores the role of motor leakage impedance and is not ambitious in the operating characteristics of low frequency bands. Therefore, E/f (constant electromotive force frequency ratio) control is often used in the practice of inverters. The inverter with the U/f control method is generally referred to as the general function inverter.
(2) Slip frequency control. Slip frequency manipulation is a control method that adds slip manipulation to the E/f manipulation. From the perspective of the speed of the motor, this is a control method to confirm the output frequency of the inverter based on the actual operating speed of the motor and the slip frequency of the motor at that speed. More importantly, under the condition of E/f constant, the control of motor torque can be realized through the control of the slip frequency. Inverters that choose to be operated by slip frequencies are generally classified as multi-function inverters.
(3) Vector manipulation. Vector control is a control method designed by the inspiration that the magnetic field current and torque current of the DC motor with excellent speed regulation function can be separated from the control. Vector control selects the vector differentiation method for communicating the stator current of the motor, calculates the magnetic field weight and torque weight of the stator current and separates the control, so as to greatly improve the accuracy and function of the inverter to the motor speed and torque control. Vector-operated inverters are generally referred to as high-performance inverters.
The main engineering significance of the classification of general inverters according to the operation method lies in the adaptability of various types of inverters to loads. General-function inverters are suitable for pump loads and low-demand resistance loads, while high-performance inverters are suitable for potential loads.
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