For The Right Rotary Soft Starter Or Frequency Converter?

The decision between frequency inverters and soft starters is often not easy. Users should consider a number of factors in order to effectively control the motors used. In addition to the type of application, this also includes mechanical requirements for the system and the costs. Compliance with applicable standards, the reliability of the drive system and the improvement of energy efficiency should also be included in the decision-making process.

An application-specific decision should be made as to whether it is worth using soft starters or frequency converters. When it comes to high-performance applications, a soft starter is often the more economical solution if it does not require continuous control of acceleration, torque and speed. Other advantages are the compactness of the soft starters, which makes them very space-saving, and their low purchase costs. These aspects in particular are the reason why soft starters are so popular in many industries and areas of application.

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Frequency converter: more expensive, but saves more energy

In contrast, a frequency converter is initially the more expensive option, but in the long term it will bring about a significantly greater reduction in energy costs if the production process requires variable process parameters. It supports users in complying with EU efficiency regulations and enables enormous savings in operating costs over the lifetime of a system through significantly lower energy consumption. Another advantage that speaks for the choice of a frequency converter is the possibility of speed control with uniform acceleration over the entire operating range of the motor. As a result, the motor can deliver high torque not only during startup, but also at all speeds.

Soft starters and frequency converters prevent excessive torque

Starting up a rotary motor to its operating speed at full mains voltage results in a high inrush current. Since the torque of three-phase asynchronous motors is basically analogous to that of the motor start-up curve, excessive torque can occur during start-up, which can shorten the lifespan of the components in the mechanical drive train and ultimately lead to premature damage.

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Both soft starters and frequency converters can prevent this and can be used to protect high-quality systems and to extend the service life of electric motors and mechanical components such as couplings and bearings of the driven machine. This is made possible in both cases by the reduced clamping voltage, which ensures a gentle torque-controlled start-up in the electric motors. The difference between the two technologies lies in the type of motor control.

Soft starts limit motor acceleration to nominal speed

Soft starters are particularly suitable for applications in which a controlled, torque-limited motor acceleration down to the nominal speed is necessary. By using power semiconductors such as thyristors, which ensure that the voltage at the motor terminals is reduced during the startup process, soft starters limit the inrush current and thus the inrush torque due to the reduced motor voltage. In this way, they reduce the momentary loads on the mechanical drive train compared to direct switching on and uncontrolled engine run-up. By continuously increasing the voltage at the motor terminals, a soft starter enables controlled acceleration up to the nominal speed.

Applications include 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 high inrush currents have to be limited to avoid interference with the power grid. In these applications, the soft starter helps the motors to start in a controlled manner and avoids torque peaks and other mechanical loads that often occur when the devices are started up normally. In addition, soft starters are also suitable as limiters for pressure waves or water hammer, which can lead to strong vibrations in pipes or systems. This avoids sudden changes in the flow rate.

Supplementary information on components for the DC network

In addition to the question of whether a frequency converter or a soft starter is more suitable for controlling an electric motor in an application, it also depends on the type of power network. Many companies are now involved in the redesign of the power supply for industrial plants via a smart, open DC network. This could further increase the efficiency of drive systems and thus help users save even more energy. DC voltage is also an issue at WEG. For example, the CWF11 frequency inverters are available in DC design. And that has advantages: With AC voltage, frequency converters generally have a conversion loss of 2.2% to 3.0%. However, if they are connected to a DC network, the losses are between 1.1% and 1.5%.

Frequency inverters regulate speed permanently

The frequency converter, on the other hand, is used when permanent speed control is required. This type of application includes, for example, fans, pumps or compressors, for which a number of general speed affinity laws apply. In contrast to the soft starter, the frequency converter converts the stationary mains frequency and mains voltage into a variable frequency-proportional output voltage, which means that it works reliably both when accelerating and decelerating and in operating states with constant or variable speed. Frequency converters also play an important role in the control of process speeds in industrial plants and manufacturing facilities such as mixers, grinders or crushers. They also have another advantage: ease of use and flexibility. In this way, they can be individually adapted to many applications.

Ecodesign

New ecodesign requirements for motors and frequency converters

For many planners and designers, it is particularly important to significantly reduce the energy consumption and thus the operating costs of drive systems. The frequency inverters are particularly suitable for this purpose. Their decisive advantage is the increase in energy efficiency through speed control of the asynchronous motors. With the help of frequency converters, such as the CFW series from WEG, the energy efficiency of electric motors can be significantly improved because the peak energy consumption and thus the power is reduced. The energy consumption can be reduced between 40% and 60%, since a process variable adaptation such as pressure or volume flow is realized with the increase and decrease in the speed, and mechanical, lossy actuators are therefore no longer required. In addition, frequency inverters are also suitable for applications such as elevators or rotary tables, where the speed can be variably adjusted and a smooth movement must be generated.

Improve energy efficiency

Thanks to the smooth start-up and the integrated protective functions, soft starters such as the SSW900 from WEG also ensure energy-efficient, reliable operation of electric motors. The high-end device with integrated PLC offers fully programmable methods for controlling asynchronous motors and a completely flexible torque control. The soft starter allows both direct connection of motors in star or delta connection as well as strand connection (root disconnection with six connected conductors). This makes it possible to switch the three motor strings directly into the three thyristor pairs. This means that the soft starter only has to carry 58% of the rated motor current (corresponds to the phase current) and that smaller devices can be used for the same output. Thanks to the integrated bypass contactor, the SSW900 extends the service life, makes good use of the space and reduces the power loss in the control cabinet. (ud)

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