The device to realize variable frequency speed regulation is called frequency converter. Eimicke inverter is generally composed of rectifier, filter, drive circuit, protection circuit and controller (MCU/DSP). Firstly, the single-phase or three-phase AC power supply is filtered by rectifier and capacitor, and the DC voltage with basically fixed amplitude is added to the inverter. A rectangular pulse waveform with a certain shape is obtained at the output end of the inverter by using the switch control of the inverter power element. Here, the voltage amplitude is controlled by changing the width of the rectangular pulse; By changing the modulation period to control its output frequency, the output voltage and frequency can be controlled simultaneously on the inverter, which meets the requirements of U/f coordinated control for variable frequency speed regulation. The advantage of PWM is that it can eliminate or suppress low-order harmonics, and make the load motor run under AC voltage close to sine wave, with small torque ripple and wide speed regulation range. Source: transmission and distribution equipment network

The motor speed in PWM control mode is limited by the upper limit speed. For example, the compressor generally does not exceed 7000r/rain. The speed of compressor controlled by PAM can be increased by about 1.5 times, which greatly improves the ability of rapid growth and deceleration. At the same time, PAM can shape the current waveform when adjusting the voltage, so it can obtain higher efficiency than PWM. In addition, it has incomparable advantages in anti-interference, which can suppress the generation of higher harmonics and reduce the pollution to the power grid. After adopting this control mode of variable frequency speed regulation technology, the stator current of the motor decreased by 64%, the power frequency decreased by 30%, and the glue outlet pressure decreased by 57%. According to the motor theory, the speed of asynchronous motor can be expressed as:

n=60 f 8( 1—8)/p

F s is the motor stator frequency (i.e. grid frequency), P is the number of motor stator windings, and S is the slip. As can be seen from the above formula, as long as the slip ratio is not too large, it can be approximately considered that the rotation speed n is proportional to f s, that is to say, the continuous smooth speed regulation of AC motor can be realized by continuously and smoothly changing the power frequency. For example, a motor with a rated speed of 3000 rpm is powered by a frequency converter. If the starting frequency is set to 5HZ, the frequency converter can run at any frequency between 5 and 5-50hz, and the motor can run at any speed between 30o and 3000rpm. The motor is started by commercial power, with balanced starting, large torque and energy saving.

50HZ380V commercial power becomes DC after rectification and filtering, and then becomes AC with adjustable frequency and amplitude through inverter. In the main circuit of the inverter, the electric energy has been converted from AC to DC and then to AC, so this inverter is called AC -DC- AC inverter.