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Soft starters protect your motors and equipment

Soft starters start electric motors gradually, thus preventing a whole range of mechanical and electrical problems.

Sudden increases in torque due to direct, immediate starting can cause critical stresses which reduce the service life of the motor and the equipment it is driving. Electrically, high initial current draw can result in voltage drops on the power circuit which can affect other equipment connected to the same power source. This is all the more problematic because modern electronic systems are particularly sensitive to power fluctuations. Although soft starters are used to control motor starts, they can also help when the motor is turned off, to prevent it stopping too quickly. This is particularly useful in pumping systems, where it prevents hammering, which can damage the fluid circuit.

Soft starters are thus the best alternative to conventional solutions like star-delta windings, impedance starting systems and autotransformers.

How do they work?

The core of a soft starter is composed of a set of thyristors which make up the power system, controlled by a microprocessor which smoothly regulates the speed of the motor. The motor speed is linearly or quadratically proportional to the applied voltage. Since the frequency cannot be regulated, and remains that of the power supply, the only way to adjust the voltage at the motor terminals is to partialise the sine wave.

The microprocessor regulates the behaviour of the thyristors, also known as SCRs (Silicon Controlled Rectifiers), to provide 3 different starting ramps:

  • voltage ramp
  • current ramp
  • torque ramp

This partialisation of the thyristors generates heat, so that the SCRs should only be used when necessary, i.e. during the ramp. When the motor reaches its nominal speed (i.e. when the start phase has concluded), the SCRs are cut out by contactors/bypass relays.

The duration and frequency of the ramps affects the above-mentioned heating. This is why it is important to provide proper heat dissipation, which is facilitated by the soft starter’s fans (whether standard equipment or optional). The soft starter continuously monitors its own temperature so that it only operates when the temperature is acceptable.

It is clear that the intelligent control provided by the microprocessor and the SCRs ensures enormous versatility in controlling a wide range of machinery and applications.


Soft starters are complete motor control units which handle tasks previously handled by other devices in the electrical enclosure. We consider them in detail.


If the application has high friction loads at startup, the system can apply a very torque initially and then continue with the pre-configured ramp up.


Soft starters can detect and report system faults, so that they can prevent improper starting or stop the motor when required. There are various types of protection:

  • motor: thermal protection, PTC protection, locked rotor, current asymmetry, startup time-out, minimum torque and motor not connected,
  • power supply: no power supply, no phase, wrong sequence phase and out-of-range frequency,
  • additional power supply; voltage too low or mini power-outage longer than the allowed one,
  • soft starter: overtemperature, overcurrent, SCR fault, bypass switch fault, temperature sensor fault and fan fault.


The presence of a large display makes it possible to display the main electrical parameters for the purposes of system diagnostics, including: current, voltage, torque, power, power factor, motor temperature and starter temperature.


It is very important to have counters to count the number of startups and the hours of motor operation. Alarm thresholds can be set.

The display shows the temperature of the motor and the soft starter’s thyristors, while the soft starter’s status is displayed by a bar: start, bypass, stop, alarm and so on.


If it is essential to start the motor, regardless of the potential for malfunction, all protections and alarms can be inhibited (including the fire system, for instance).


The most recent versions of LOVATO Electric soft starters are equipped with:


On power-up, the soft starter launches a wizard to simplify setup. Users can configure the device in 4 simple steps:

  • language
  • nominal motor current
  • type of application
  • type of duty (normal, heavy, light) The user can customise these settings with a user friendly interface available in 6 languages.
  • NFC (Near Field Communication) and optical port.


Once these settings have been made, the soft starter automatically adjusts its operating parameters accordingly.

6 icons on the display indicates the  application, centrifugal pump, fire system pump, belt conveyor, fan, generic mixer.

NFC (Near Field Communication) and optical port

Remote control and monitoring

An optional serial communications card is available for the soft starter. This allows the device to be controlled with the LOVATO Xpress or Synergy software, and to connect to supervision/automation systems or straightforward operator panels.

XPRESS remote configuration and control software