Soft starters and frequency converters are two completely different products. Frequency converters are used where speed regulation is required; their output changes both voltage and frequency. Soft starters are essentially voltage regulators used to start motors; their output only changes voltage, not frequency. Frequency converters have all the functions of soft starters, but they are much more expensive and have a much more complex structure.
The main differences between the two are:
1. Soft start is a method of trimming and disrupting the sine wave; it's very easy, as simple as water flowing downhill.
2. Frequency converters strive to obtain an adjustable sine wave, which is very difficult! It's as difficult as water flowing uphill.
II. The characteristics of frequency converters are:
1. A frequency converter is a device that changes the frequency of a three-phase sinusoidal alternating current.
2. Since converting direct current to a sinusoidal alternating current waveform is an irreversible process, or rather, impossible, the only way to achieve the frequency conversion of the sinusoidal waveform is to use a PWM (Pulse Width Modulation) carrier wave with an average voltage waveform that is sinusoidal.
3. The average voltage waveform is a sinusoidal PWM (Pulse Width Modulation) carrier wave, which is achieved through the switching action of the fully controlled IGBT . The actual output of the frequency converter is a periodically varying rectangular wave with varying width.
4. The asynchronous motor filters out a sinusoidal waveform from the PWM pulse width modulation carrier wave and generates the constant rotating magnetic field required for motor operation;
5. By changing the frequency to alter the synchronous speed of the asynchronous motor, the purpose of variable frequency speed control of the AC motor can be achieved.
III. Differences in Technical Specifications:
1. A thyristor AC voltage regulating circuit is a circuit that obtains an AC current with an adjustable average voltage by adjusting the conduction angle of the thyristor to cut the waveform of a sinusoidal AC current;
2. This type of AC voltage regulation circuit is used in load circuits such as automatic heating temperature control circuits and stage lighting circuits, where the AC waveform is independent and only the average voltage matters.
3. However, some people, even so-called experts, use this thyristor AC voltage regulating circuit for starting asynchronous AC motors, and euphemistically call it soft starting;
4. Because asynchronous AC motors operate under a three-phase symmetrical sinusoidal AC waveform, they possess a constant rotating magnetic field;
5. If an AC current whose waveform is chopped by adjusting the conduction angle of the thyristor is used in an asynchronous AC motor, a constant rotating magnetic field cannot be generated;
6. Using this AC waveform with an asynchronous motor completely destroys the motor's starting performance, resulting in low starting torque, high starting current, and no soft-start function as people imagine.
7. All soft-start motors start only when the voltage is close to full, resulting in a longer starting time, a larger starting current, and a greater impact on the power grid;
8. Autotransformer reduced voltage starting, star-delta starting, water resistance reduced voltage starting, etc., for asynchronous motors, all reduce voltage without changing the sine waveform, and are all good and effective starting devices, superior to soft starting;
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