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What is the working principle of frequency converter?

What is the working principle of frequency converter?

2023-09-26

Frequency converter , as the name suggests, is a device that can change the frequency. It is an AC power supply whose frequency and voltage can be adjusted to output . It is mainly used to regulate the speed of asynchronous motors . Before the advent of frequency converters, speed regulation of asynchronous motors was very troublesome because the speed and torque were not in a straight line. For example, speed regulation through a slip head, or simple step-down speed regulation, etc., were not ideal. According to the speed formula of the three-phase asynchronous motor, the speed n=60*f/p(1-s), where p is the number of pole pairs, s is the slip rate, and f is the frequency of the power supply. As long as it can output a frequency controllable AC power supply, the speed of the three-phase asynchronous motor can be changed smoothly. This is the fundamental working principle of the inverter.


However, the three-phase AC power supplied by the power plant is a sine wave with a phase difference of 120°, and the frequency is 50HZ. Before the invention of power electronic devices, it was almost impossible to change the phase frequency of the power supply, so the AC motor was invented. There has been no good speed control device for almost 100 years.

Later, high-power were invented at the end of the last century. Microcontroller embedded technology became increasingly mature, and people found a basic method for asynchronous motor speed regulation. That is to first rectify the three-phase AC voltage through diodes or thyristors into fluctuating DC , then use capacitors to filter and stabilize the voltage, turning it into stable DC, and use 6 IGBT tubes to form an inverter circuit . Through the PWM chopping method, a series of square waves with variable pulse width are output to simulate the equivalent effect of variable frequency alternating current to achieve the purpose of controlling the motor speed.

You can imagine that any curve can be simulated in segments using many straight segments. The more straight segments used in the simulation, the more accurately the curve can be described. This is actually the concept of mathematical calculus.

Although the sine wave looks very complicated, many square waves can be used to simulate it. If a square wave with equal amplitude and variable width is output, and the area of the square wave is equal to the area of the corresponding sine wave, its The effect will be equivalent to the working effect of a sine wave. By controlling the working cycle, the frequency of the output power supply can be changed. This is the so-called PWM control, because IGBT switching tubes can achieve very high-speed switching functions, such as dozens of K frequency, so that enough square waves are output, the simulated sine wave effect will be better.

However, it is not enough for the motor to simply change the frequency. The voltage must also be adjusted accordingly. Otherwise, the motor may not work properly, such as being severely heated and burned out. Mainly because the iron core of the motor is nonlinear, it requires that when the power frequency changes, the voltage must also change, so that the main magnetic flux can be controlled to be constant.
Because: main magnetic flux ≈ motor voltage ÷ (4.44*frequency*number of electronic winding turns)
Therefore, during frequency regulation, the motor voltage should also change with the change of frequency, so that the main magnetic flux can remain unchanged and avoid flux saturation or working in a weak field state.

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