What is a soft starter?

Motors can frequently require a large amount of energy when they accelerate up to full speed. A soft starter is used to reduce the inrush currents and limit torque, useful if you want to protect your equipment, extend the life of your motor and reduce motor heating with frequent starts and stops.

So, a soft starter provides a gentle acceleration up to full speed and they are used only at startup. This gradual start is achieved by ramping up the initial voltage to the motor.

We tend to utilise soft starters in applications that require speed and torque control only during startup or where there is a requirement to reduce the large startup inrush currents associated with a large motor. We can also use a soft start when a mechanical system (like conveyors, belt-driven systems, gears, and so on) needs a gentle start to relieve torque spikes and tension associated with normal startup. We can also use them in pumps to eliminate pressure surges caused in piping systems.

Electrical soft starters reduce the voltage and therefore current input to a motor. Soft starters may use solid-state devices to help control the flow of the current. One to three phases can be controlled, with three-phase control usually producing the better results. In the normal OFF state, the thyristors restrict current, but in the normal ON state, they allow current. The thyristors are controlled during ramp up, and bypass contactors are pulled in after maximum speed is achieved to increase overall efficiency. If you have applications that require speed and torque control only during motor startup, then soft starters are often the more economical choice.

If space is at a premium they are a good choice as they usually have a smaller footprint than variable frequency drives.

Drives and Automation are able to supply you with a range of AC & DC motors, drives and peripherals and are the UK agent for Sicme Orange1. We cover the whole spectrum of automation from large projects like a new paper line all the way through to a single drive replacement and retrofits.