Pre-magnetizing-based variable-frequency soft starting system and method for rotating magnetic field of induction motor

Abstract

The invention discloses a pre-magnetizing-based variable-frequency soft starting system for a rotating magnetic field of an induction motor. The system comprises a micro controller unit (MCU) and a pre-magnetizing magnetic field vector control circuit. The pre-magnetizing magnetic field vector control circuit consists of six power thyristors connected with the MCU; two power thyristors of each of an A phase, a B phase, and a C phase of a three-phase alternating-current power supply are connected with a three-phase asynchronous motor; and the MCU is connected with a first current transformer, a second current transformer, and a third current transformer that are arranged between the A phase, the B phase, and the C phase of the three-phase alternating-current power supply and the three-phase asynchronous motor. In addition, the invention also discloses a pre-magnetizing-based variable-frequency soft starting system for a rotating magnetic field of an induction motor. With the method and system, three-phase pre magnetizing and two-phase pre magnetizing can be realized; and thus a pre-heating effect of a stator can be realized.

Description

technical field [0001] The invention belongs to the technical field of motor frequency conversion soft start, and in particular relates to a premagnetization-based induction motor rotating magnetic field frequency conversion soft start system and a method thereof. Background technique [0002] There are two serious disadvantages in changing the voltage without changing the power frequency: 1. The starting torque is small. The starting torque of the three-phase AC induction motor is proportional to the square of the applied stator side terminal voltage. The torque is reduced by the square of the voltage reduction. When starting with heavy load or full load, the motor will enter the locked-rotor protection and fail to start; 2. The speed of the stator magnetic field cannot be changed, and the starting slip is too large; The stator magnetic field is rotating at synchronous speed, and compared with the low-speed rotor, the slip is very large. [0003] Discrete frequency conver...

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Problems solved by technology

[0002] There are two serious disadvantages in changing the voltage without changing the power frequency: 1. The starting torque is small. The starting torque of the three-phase AC induction motor is proportional to the square of the applied stator side terminal voltage. The torque is reduced by the square of the voltage reduction

When starting with heavy load or full load, the motor will enter the locked-rotor protection and fail to start; 2. The speed of the stator magnetic field cannot be changed, and the starting slip is too large; The stator magnetic field is rotating at a synchronous speed, and compared with the low-speed rotor, the slip is very large

[0003] Discrete frequency conversion has been proposed before. Discrete frequency conversion is based on power frequency three-phase power. Through the thyristor, some sine half waves are conducted and some are not conducted to change the power frequency of the input motor, so as to produce the effect of frequency conversion. This is not a problem in theory, but after simulation and experiments, it is found that the harmonic content of the power supply after discrete frequency conversion is very large. When a 15KW asynchronous motor is used with a 45% load, the harmonics will have a great influence and the motor will vibrate obviously.

[0004] The frequency converter is also used as a soft starter. Although the starting effect is better, the frequency converter is technically an AC-DC-AC structure. It first performs three-phase bridge uncontrollable rectification on the three-phase power frequency power supply, and then The DC voltage output by the rectifier is filtered, and finally the pulse width modulation (PWM) technology is used to output the controllable AC with adjustable voltage and frequency. The structure and technology of the frequency converter are complicated, and the cost is high. It is not easy to popularize and promote the use of frequency converters.

[0005] In summary, the current soft start technology has the characteristics of large starting current, large pulsating torque component, low starting torque, and high harmonic content in the starting process.

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