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What's the difference of variable frequency drive and soft starter

Soft starters and variable frequency drives are two different purpose products. VFD is for AC motor speed control, it's not only change the output voltage but also change the frequency; Soft starter is a regulator actually for motor starting, just changing the output voltage. Variable frequency drive has all the features of soft starters, but the price is much more expensive than the soft starter and the structure is much more complex.

Variable frequency drive is converting power supply (50Hz or 60Hz) into a variety of frequency AC power, to achieve electric motor variable speed operation control. There are several ways to define a VFD. Base on main circuit working methods, it can be divided into voltage type VFD and current VFD; Base on switching methods, it can be divided into PAM control drive, PWM control drive and high carrier frequency PWM control drive; Base on the working principle, it can be divided into V/F control VFD, slip frequency control and vector control VFD, etc.; Base on usage, it can be divided into general-purpose VFD, high-performance dedicated VFD, High-frequency VFD, single phase VFD and three-phase variable frequency drive.

Soft starter is a set of motor soft start/stop, light-load energy saving and various protection functions devices to control motors.

Soft starter uses three opposite parallel thyristors as regulator, plug it into the power source and motor stator.  When using soft starter to start the motor, the thyristor output voltage increases gradually, and the motor accelerates gradually until the thyristor is turned on completely. The motor operates at rated voltage to achieve a smooth start, reduce starting current and avoid start overcurrent trip. When the motor reaches rated RPM, the startup process is completed, the soft starter uses bypass contactor to replace thyristor to provide rated voltage to the motor, in order to reduce the thyristor heat loss, extend the soft starter service life and improve efficiency, also avoid harmonic pollution to the power grid.
I am looking for variable frequency drives and soft starters to the mining fan applications. Totally closed control board and semi conductors, ambient temperatures < 50°C.
Heat sinks outside of cabinet and forced ventilated. The cabinet can equip with auxiliary heat exchanger IP54/NEMA 12 if necessary. Power range 45 - 3000HP, voltage 400 and 4160v, frequency 50Hz.
---->> by Antti
This is an excellent article and very informative. There are many reasons for choosing a soft starter or AC Drives. Soft Starters are inherently more efficient typically 99% efficient as once the bypass contactor pulls in the Thyristors are switch off, whereas in an AC Drive the IGBT's are always switched on.

---->> by Proplas

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