Difference between Variable Frequency Drives VFD and Soft Starter
VFD vs Soft Starters
When a motor starts, it drains three to six times its rated current from the mains. It requires a large amount of energy to accelerate to full speed. Soft starters and variable frequency drives can both be used to reduce inrush currents and limit torque, extending the life of a motor by reducing motor heating caused by frequent starts and stops. But there are a number of differences between a soft starter and a variable frequency drive.
Variable frequency drives are semiconductor devices mainly used for motor torque-speed control and protection. It can start and stop a motor smoothly. Speed torque characteristics of a motor can be controlled throughout its operation based on the application by using a VFD. The speed of the motor can be controlled from 0 to its rated speed (sometimes above the rated speed) by varying the frequency of the motor supply.
Soft starters are also semiconductor devices used to start or stop a motor. As the name indicates it offers a smooth start and smooth stop. Usually, soft starters are bypassed by a contactor-overload circuit as the motor gradually reaches its full-speed and gets back into action when the motor stops. Soft start and stop are achieved by temporarily lowering the motor terminal voltage.
Difference between Soft starters and Variable Frequency Drives
Soft starter Variable Frequency Drives
Soft starters are used to start and stop a motor smoothly. Variable Frequency Drives are used to start, stop and control a motor throughout its operation
Most of the times soft starters are bypassed as soon as the motor reaches its full speed. Variable frequency drives control a motor throughout its operation
No harmonics Harmonics are injected into the mains. Hence additional filters should be used.
Variable frequency drives and Soft starters are energy efficient devices. Selection of one of the other depends on the application, control, cost and space factors.
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