Key points of frequency conversion control

1. Introduction to general motors

The three-phase squirrel cage AC motor is the most common type of induction motor. Its structure and characteristics are as follows:

Schematic diagram of the structure of an induction motor

Schematic diagram of the motor structure

Motor characteristics

2. Principle and composition of frequency converter

The frequency converter is a control device that can easily and freely change the speed of the AC motor. The method of changing the speed of the AC motor is as follows. The frequency converter achieves speed regulation by changing the power supply frequency of the AC motor:

The inverter is composed as follows:

(1) The diode bridge rectifier is widely used as the converter (rectifier), as shown in Figure 1, which converts the industrial frequency power supply into a DC power supply. A reversible converter can also be constructed by using two sets of transistor converters. Since its power direction is reversible, it can perform regenerative operation.

(2) Smoothing circuit The DC voltage after rectification by the rectifier contains a pulsating voltage with a frequency six times that of the power supply. In addition, the pulsating current generated by the inverter also causes the DC voltage to fluctuate. In order to suppress voltage fluctuations, inductors and capacitors are used to absorb pulsating voltage (current). When the device capacity is small, if there is a margin in the power supply and main circuit components, the inductor can be omitted and a simple smoothing circuit can be used.

(3) Inverter Unlike rectifier, inverter converts DC power into AC power of required frequency. By turning on and off six switching devices at a predetermined time, a three-phase AC output can be obtained.

(4) Braking circuit When the asynchronous motor is used in the regenerative braking area (slip is negative), the regenerative energy is stored in the capacitor of the smoothing circuit, causing the DC voltage to increase. Generally speaking, the energy accumulated by the inertia of the mechanical system (including the motor) is greater than the energy that can be stored in the capacitor. When rapid braking is required, a reversible converter can be used to feed back to the power supply or a braking circuit (switch and resistor) can be set up to consume the regenerative power to prevent the DC circuit voltage from rising.

3. Application purpose and use of frequency converter

The adjustable speed transmission composed of frequency converter and AC motor is called frequency converter transmission, and its functions and uses are as follows. They may be related to each other, but there is no clear classification in fact. This table is for reference only.