variable frequency drive
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What is a variable frequency drive consists of?

Variable frequency drive mainly consists of main circuit and control circuit.

Main circuit provides power conversion to asynchronous motor with adjustable voltage and variable frequency, the VFD main circuit can be divided into two types: voltage type is converted DC voltage source to AC power, DC line filter is capacitance. Current type is converted DC current source to AC current, the DC line filter is the inductor. It consists of three parts, the rectifier converters frequency power to DC power, the smoothing circuit absorbs voltage pulse generated by converter and inverter, the inverter converts DC power into AC power.

1) Rectifier: convert AC frequency power to DC power supply. Use two transistors constitute reversible inverter, the reversible power direction can make it regeneration operation.

2) Smoothing circuit: it contains 6times frequency pulse voltage in the rectifier's DC voltage, and the ripple current generated by the inverter also make DC voltage changes. In order to suppress voltage fluctuations, use inductors and capacitors to absorb ripple voltage (current).

3) Inverter: its contrast with the rectifier, the inverter is converting DC power into desired frequency AC power, obtaining three phase AC power output by six switching devices on/off in certain time.

Control circuit provides control signal to the asynchronous motor power supply (voltage and frequency adjustable) main circuit, which has the arithmetic circuit of frequency and voltage, the voltage and current detection circuit of the main circuit, the speed detecting circuit of the motor, the driving circuit of the operation control signal amplifier, and the protection circuit of the inverter and motor.

1) Arithmetic circuit: compare external speed or torque command with the detection circuit signal of the current and voltage, to determine the inverter output voltage and frequency.

2) Voltage and current detection circuits: detect voltage and current with isolation to the main circuit.

3) Driving circuit: a circuit to drive the main circuit, it is isolated from the control circuit to make the main circuit on/off.

4) Speed detection circuit: treat the signal of the speed detector which mounted on the induction motor shaft as the speed signal, send it to the arithmetic circuit to make the motor operation as the command speed.

5) Protection circuit: detecting main circuit voltage, current, etc. to stop or inhibit voltage and current values to avoid inverter or induction motor damaged in overload or overvoltage abnormality.

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