Since the general motor operates at the rated frequency and the magnetic circuit is nearly saturated, the increase of the magnetic flux will inevitably lead to excessive saturation of the magnetic circuit of the motor, and the excitation component of the stator current will increase sharply, thus reducing the power factor of the motor and the load of the motor. Capability, in severe cases, damage the motor due to overheating of the windings. On the contrary, when it rises above the rated frequency, it falls below the rated value. Under the rated stator current, the output torque of the motor decreases, and the motor is not fully utilized, resulting in waste. Therefore, it is necessary to ensure that the air gap magnetic flux is constant when adjusting the speed. When the speed is below the fundamental frequency, keep U= constant, that is, constant flux frequency control, which is a constant torque speed control mode; when the fundamental frequency is above the speed regulation, because U can't exceed the rated voltage Ue, when f increases upward, U Increasingly, the 5 is reduced, which is equivalent to the weak magnetic speed regulation of the DC motor, and belongs to the constant power speed regulation mode. The control characteristics of frequency conversion speed regulation. The frequency converter is in the form of AC-DC-AC, and is mainly composed of a rectifier, an inverter, a human-machine dialogue unit, a control loop and control terminals. The rectifier on the grid side is a three-phase bridge rectifier circuit for converting the input three-phase AC into DC power; the inverter on the motor side is a three-phase bridge inverter circuit composed of six semiconductor main switches, and the main switch can be pressed The command of the control loop is regularly turned on and off to obtain the three-phase AC output of the corresponding frequency; the human-machine dialog unit is composed of a digital, liquid crystal display and operation keys for displaying various types of information and changing screens, changing data, setting frequencies, etc. The control circuit consists of a frequency and voltage calculation circuit, a voltage and current detection circuit of the main circuit, a speed detection circuit of the motor, a drive circuit that amplifies the control signal of the operation circuit, and a protection circuit of the inverter and the motor. Used to control the operation of the inverter. The control terminal includes an analog (0 to 10V/4 to 20mA) input terminal and a digital input/output terminal.
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