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 for motor starting; 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.
I. Main differences between the two
1. Soft start is a method of cutting out and destroying the sine wave, which is very easy, just like 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. Characteristics of Frequency Converters
1. A frequency converter is a device that changes the frequency of a three-phase sinusoidal alternating current.
2. Since converting direct current into 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 sine wave PWM pulse width modulation carrier, which is achieved through the switching action of the fully controlled device IGBT . The actual output of the frequency converter is a periodically changing rectangular wave with varying width.
4. The asynchronous motor filters out the sinusoidal waveform from the PWM pulse width modulation carrier and generates the constant rotating magnetic field required for motor operation;
5. And by changing the frequency to change the synchronous speed of the asynchronous motor, the purpose of frequency conversion speed regulation of AC motor is 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 the 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, which are independent of AC waveforms and only related to average voltage.
3. However, some people, even so-called experts, use this thyristor AC voltage regulation circuit to start asynchronous AC motors, and call it soft start.
4. Because asynchronous AC motors operate under a three-phase symmetrical sinusoidal AC waveform, they have a constant rotating magnetic field.
5. If the AC current, whose waveform is cut 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 on an asynchronous motor completely destroys its 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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