I. Introduction to General Electric
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
characteristics of motor
II. Principle and Structure of Frequency Converters
A frequency converter is a control device that can easily and freely change the speed of an AC motor. The methods for changing the speed of an AC motor are as follows.
A frequency converter achieves speed regulation by changing the frequency of the AC motor power supply.
The inverter is composed of the following components:
1. Converter (rectifier)
Diode bridge rectifiers are widely used, as shown in Figure 1, to convert mains frequency power into DC power. Two sets of transistor converters can also be used to construct a reversible converter, which can operate regeneratively due to its reversible power direction.
2. Smoothing circuit
The DC voltage after rectification contains a pulsating voltage at six times the frequency of the power supply. Furthermore, the pulsating current generated by the inverter also causes DC voltage fluctuations. To suppress voltage fluctuations, inductors and capacitors are used to absorb the pulsating voltage (current). For small device capacities, if the power supply and main circuit components have sufficient capacity, the inductor can be omitted, and a simple smoothing circuit can be used.
3. Inverter
Unlike rectifiers, inverters convert DC power into AC power at the required frequency. By turning six switching devices on and off at predetermined times, a three-phase AC output can be obtained.
4. Braking circuit
When an asynchronous motor is used in the regenerative braking region (with negative slip), the regenerative energy is stored in the smoothing circuit capacitor, causing the DC voltage to rise. Generally speaking, the energy accumulated by the inertia of the mechanical system (including the motor) is greater than the energy that the capacitor can store. 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, so as to prevent the DC circuit voltage from rising.
III. Application Purpose and Use of Frequency Converters
A variable speed drive consisting of a frequency converter and an AC motor is called a frequency converter drive, and its functions and uses are as follows. These may be related to each other, and there is actually no clear classification; this table is for reference only.
