What is a softstarter and where is it used?

A softstarter is a device that gradually increases the voltage to a motor instead of giving the full voltage directly. The softstarter allows for smooth acceleration instead of a sudden burst of power.

The smooth start reduces the strain on the motor during the power-up and shut-down phase. It extends the life of your motor and equipment by reducing the heat caused by numerous starts and stops, reducing the mechanical stress on the motor, shaft and surrounding cables and equipment.

How does a softstarter work?

A softstarter controls the amount of voltage running through the motor. An Emotron softstarter does so by limiting the torque in the motor. It uses solid-state devices called thyristors to reduce the voltage as the motor starts up and then feed full voltage as the motor has come to full speed.

DOL-start: A conventional Direct-On-Line start

Using Emotron softstarters when stopping you can benefit from the same smart principle as when starting it – a linear stop using the torque control.

There are two types of Emotron softstarters and they both work with 3-phase torque control.

It relies on three pairs of thyristors, one pair for each phase of power. These are applied gradually for each voltage phase, limiting the voltage provided to the motor. The current is reduced proportionally to the reduction in voltage. Torque, however, is proportional to the square of the voltage, so a small reduction in voltage results in a large reduction in torque. A 50% reduction in voltage gives a 50% reduction in current and a 75% reduction in torque.

Working principle of the softstarter




T2 = Torque at reduced current/voltage

T1 = Torque at locked rotor current

I2 = Reduced current

I1 = Locked rotor current

V2 = Reduced voltage

V1 = Full voltage

With The Emotron MSF softstarter the voltage to motor is controlled by means of the thyristors also after the starting phase.

The Emotron TSA softstarter is using thyristors to control the voltage to motor only during starting and stopping phases. At full speed the built-in bypass contactors are switched in to conduct the motor currents.

Emotron softstarters also contain Emotron load monitoring technology. This observes the load torque of the motor driving the process or machine. It continuously calculates the motor torque using the built-in motor model. The result is a correct and linear motor load value across the entire load range. Any deviation from the set load levels will immediately cause the monitor to initiate a warning or stop the machine. 

Where can I use a softstarter?

A softstarter can be used for any motor that requires a controlled and smooth start and stop. Emotron softstarters gives you a value added cost efficient means of controlling not only the startup and stopping phase but also observing the motor and machine load while running the motor. There are numerous applications, where the following are most common;

Softstarters for pumps

When using pumps in a process there is a risk of pressure surges. Using a soft starter and feeding the voltage to the motor in a gradual progression greatly reduces this risk to a pump application and water hammering in the surrounding piping. The internal load monitor will also detect e.g. cavitation or if a pump is running dry.

Softstarter for conveyor belts 

A conventional Direct-On-Line start could put a lot of stress to the mechanical components of the conveyor belt. Conveyor belts are easily damaged if subject to a sudden start. A single jerk could result in a misaligned belt. By installing a soft starter, the conveyor belt will start up more smoothly by applying motor torque gradually. This can save many hours downtime. The Emotron Load monitor function will give possibility to catch e.g. a misaligned belt or other mechanical problems.

Softstarters for fans and blowers

Fans with belt drives also face a potential risk of slipping due to a jerky start. Using an Emotron softstarter eliminates this problem by applying a smooth torque controlled start. The internal load monitor in the Emotron softstarter will notice if a fan filter is starting to be blocked.

Softstarters for compressors

Softstarters are ideal for making a smooth start of compressors, using Emotron linear torque control at start to avoid mechanical shock for compressor, motor and transmissions. The built-in phase reversal alarm prevents starting compressor in wrong direction.

The internal load monitor in the Emotron softstarter will notice if a compressor is idling, allowing for energy savings by stopping.

Softstarters for crushers, mixers, mills, band saws 

All applications where jamming might become an issue, such as a crusher or mill, will benefit from a softstarter. Applications with demands for extremely short braking times will benefit from the strong vector brake or reverse current braking for high inertia loads available in an Emotron softstarter.The built-in monitor will give a constant load indication.


Why should I use a softstarter from Emotron?

A softstarter allows you to lower your power consumption during starting. Instead of using full voltage from the start, the gradual voltage increase means that less power is used. The Emotron softstarter also makes it possible to adjust starting time.

The softstarter reduces the risk of a power surge, such that when maximum voltage rushes into the motor, it may overload the supply circuits (like fuses/breakers/cables) and put the motor at risk of a power surge. This risk is greatly reduced with the help of a softstarter building up voltage gradually.

The Emotron softstarter also contains a load monitor function, which warns if it detects a deviation from set load levels, signaling or even stopping any equipment experiencing trouble. This gives you protection against dry running, blocked pump, broken belt.

Your Emotron softstarter will improve overall operational efficiency and extend the lifetime of your critical equipment.