The decision between frequency inverters and soft starters is often not an easy one. Users must also take a number of factors into account to ensure effective control of the motors used. Factors to consider include the application, mechanical requirements of the system and costs. Compliance with standards, the reliability of the drive system and the improvement of energy efficiency should also be included in the decision-making process.

Whether the use of soft starters or frequency inverters is worthwhile can only be decided on an application-specific basis. If the application involves high power, a soft starter is often the more economical solution if continuous control of acceleration, torque and speed is not required. Further advantages are the compactness of the devices and their low acquisition costs. These aspects make soft starters so popular in many industries and areas of application.

In contrast, a frequency inverter may initially be the more expensive option, but in the long term it will result in a significantly greater reduction in energy costs if the production process requires variable process parameters. It helps users to comply with EU efficiency regulations and enables significant savings in operating costs by reducing energy consumption over the lifetime of a system. Another advantage of choosing a frequency inverter is the possibility of speed control with smooth acceleration. This allows the motor to deliver a high torque during start-up and at all speeds.

Variable speed control versus controlled acceleration

Running a motor up to its operating speed at full mains voltage results in a high inrush current. As the torque of three-phase asynchronous motors is generally set in line with the motor starting curve, excessive torque can occur during start-up, which can damage the mechanical components in the drive train.

Both soft starters and frequency inverters can prevent this and help to protect the systems and extend the service life of electric motors and mechanical components such as couplings and bearings of the driven machine. In both cases, this is made possible by the reduced clamping voltage, which ensures smooth starting and controlled torque in electric motors. The difference between the two technologies lies in the type of motor control.

The powerful SSW900 soft starters enable smooth starting and stopping of three-phase asynchronous motors and offer comprehensive motor protection. © WAY

Soft starters are particularly suitable for applications where controlled, torque-limited motor acceleration up to the rated speed is required. With the help of power semiconductors such as thyristors, which reduce the voltage at the motor terminals during the start-up process, soft starters limit the inrush current and thus also the starting torque. In this way, they reduce the momentary loads on the mechanical drive train compared to direct switch-on and uncontrolled motor start-up. By steadily increasing the voltage at the motor terminals, the soft starter enables controlled acceleration up to the rated speed.

Applications include, for example, conveyor belts, belt drives and escalators. These require continuous, stepless speed control and torque limitation when starting and stopping. Especially when starting large motors, the inrush currents must be limited in order to avoid repercussions on the power grid. The soft starter helps the motors in these applications to start in a controlled manner and avoids torque peaks and other mechanical stresses that often occur during normal start-up of the devices. Soft starters are also suitable as limiters for pressure waves or water hammers, which can lead to strong vibrations in pipelines or systems; they prevent abrupt changes in the flow rate.