Starting a motor with the help of a direct online starter will draw a heavy inrush current of about 5-7 times the Full Load Amps (FLA). It will lead to the failure of mechanical parts in the system, and the frequent inrush currents can reduce the life of the AC motor. In order to overcome such problems, VFDs and soft starters are suitable solutions for various industrial applications.
Is it better to choose a VFD instead of Soft Starter?
The controlling way of starting and stopping the three-phase industrial motors with soft starters and VFDs is similar. But still, they are different in many characteristics. The following points will help you to understand the differences between VFD and motor soft starters.
- A soft starter in a circuit starts and stops a motor smoothly, while a variable frequency drive also does the same function along with controlling AC motor through its operations.
- Variable frequency drive controls the speed of the motor by varying the frequency. A soft starter is only helpful for a smooth start and stop of AC motor.
- VFD transmits harmonics voltage to power up the motor. However, there are no harmonics produced by using the soft starter.
- VFD price is a little higher compared to the soft starter. A variable frequency drive can adjust the rotating direction of the induction motor whereas it's not practical with a soft starter.
- The soft starter is light and small in size. It is easy to be installed in the constraint space rather than VFD.
- All soft starters bypass the AC current by using an AC contactor as soon as the motor reaches its full loaded speed while VFDs drive the motor throughout all the operations.
- Variable frequency drive provides almost all protections to the motor including supply phase loss, motor phase loss, over current, over voltage, over temperature, under voltage, short circuit, motor stall, over frequency, speed error, high torque, acceleration time, deceleration time, ramp speed, multiple speed, earth fault, etc. On the other hand, soft starter only provides basic protections including over voltage, over current, short-circuit, earth fault, motor jam, motor stall, over temperature, etc.
So, both soft starter and VFD are used for safe operation, but the soft starter is suitable in the application where speed control is not important and VFD is used for speed control.