Soft starters are electronic devices for starting motors by reducing the inrush current and torque. They use a silicon controlled rectifier (SCR) to gradually increasing the voltage to the motor allowing it to speed up smoothly until it reaches full speed.
Their linear acceleration slope delivers smooth starting and less wear on both the motor and load. Their adjustable starting torque and acceleration times make them suitable for many applications. For example, lightly loaded conveyors having a constant full-speed operation.
Other popular duties include pumps, compressors, fans, crushers, mixers and grinders. For industrial purposes they are often used in:
– Steel industries (Rolling mills and processing lines) –
– Rubber and plastic –
– Paper and pulp –
– Sugar plants –
– Water supply scheme –
– Machine tool applications –
– Cement industries –
– Textile industries –
So far, they may sound like VSDs, but there are several important differences. When applied to a suitable application, they are smaller, cheaper and more efficient than VSDs.
The benefits of soft starters
A soft starter’s smooth and uniform starting extends the life of mechanical components and reduces their maintenance. They are also smaller than equivalent VSDs. Moreover, soft starters are cheaper to buy and run than VSDs.
At full speed, soft starters use an integrated bypass contactor to carry the current. This makes them more energy-efficient than VSDs, so they run cooler. Furthermore, in bypass mode, the SCR switches off after starting so the starter generates almost zero harmonics.
In suitable applications, they are smaller, cheaper and more energy-efficient than equivalent VSDs. So, if you want to save money, before selecting a VSD, ask yourself these questions
|Q. Does the application need reduced voltage, current, and torque while starting?
A. Use either a soft starter or an AC drive for this feature.
|Q. Does the application need full torque at zero speed?
A. Use a VSD as it can provide full torque at zero speed, where a soft starter cannot.
|Q. Does the application need variable speed control once the motor is at speed?
A. Use a VSD where variable motor speed is needed. Apart from starting and stopping, soft starters run at full speed only.
|Q. Does the application need precise starting and stopping times?
A. VSDs offer a wider range of speed control than soft starters.
|Q. Is space a consideration? –
A. Soft starters can provide a smaller footprint solution.
|Q. Is the initial purchase price a consideration? –
A. Generally, soft starters are more cost-effective.
According to Schneider Electric, common applications for soft starters are for full speed use with low or medium starting torques, and those with light loads. They are also ideal for controlling inrush currents and power monitoring applications