Soft starters are an excellent method of controlling your motor, given that the application fits the bill.
With the three prominent methods of controlling a motor in industrial applications – across the line contactor, variable frequency drive (VFD) and soft starters, where would you normally see a soft starter being applied? In most applications, a soft starter will be employed in fan and pump applications. These are applications where you have a slowly increasing torque and then maintaining a constant speed, that being the motor rated speed. The reduction in torque on startup and through ramp-up is where the soft starter shines. An application with a belt driven fan or a centrifugal pump is well served by soft starters if you do not need to continuously vary the speed. This is ideal for preventing damaging due to overloading belts and water hammering in a pump.
There are two main types of soft starters available being solid state and internal contactor.
Solid state means that the drive is always running on its electronic circuitry. This allows for numerous starts per hour. Alternatively, the internal contact soft starter, which is normally more affordable, can only perform the number of starts per hour that the internal contactor is rated for; far less than that of the solid state.
Once you have moved beyond this consideration, it’s now time to tackle the usual suspects.
With respect to overload protection, the manufacturer will always call out specific line side protection with the soft starter itself having internal motor overload protection. All the considerations for communication, control topology, motor characteristics and dimensional characteristics then come into play as you develop your specification.
When choosing a soft starter over other starting methods, it really comes down to your need to continuously vary the speed or not. From there it’s all about capability, space and costs to round out your selection and get your process up and running!