Electronic Soft Starter





There are some common issues associated with starting and stopping of electrical motors. Depending on requirement, different starting and stopping methods can be used.

  • Direct Online (DOL) – It is uncontrolled stating and the motor will start with maximum current and torque regardless of the load type.
  • Star Delta – Star Delta Starter reduces current and torque during start. The starting current is about one third compared to direct online starting, although it reduces the starting torque to about 25 percent. Star delta is not adjustable, so if torque is reduced too much, the motor will not start. Current peaks will happen when switching from star to delta connection.
  • Soft Starter – Soft starter eliminates the common problems associated with motor starting and stopping, including electrical surges, spikes and high inrush currents. Because it offers soft starting and stopping, a soft starter is the optimal compromise between a direct online or star delta starter and a variable speed drive in many full speed motor applications.
  • Variable speed drive (VSD) – Like a soft starter, a variable speed drive can perform soft starting and stopping of the motor. However, VSD is designed primarily to control speed of the motor, resulting in energy efficient motor operation in variable speed applications. Using VSD with sole purpose of ensuring soft starting and stopping of motors can be considered an unnecessarily advanced solution.

From the above, it is evident that, Soft starter is ideally suited optimal solution for motor starting and stopping applications.

Soft starter is thyristor-based voltage control starting device for Motors. It uses advanced control with micro-controller for phase-controlled firing of the thyristors. It is superior to conventional method of starting as it provides smooth start, reducing inrush currents considerably and offering several inherent advantages of soft starting.


Excel Soft Starter


Excel soft starter offers simple and reliable solution for starting of LV Motors which can be easily integrated into the conventional circuits of LV Motor. Soft starter adds value by securing motor reliability and increasing application productivity.


Principle of operation:


The Starting current drawn by an induction motor is directly proportional to the applied voltage. Hence, the soft starter is designed to choose lowest voltage possible (pedestal voltage) at which the motor can be started – the lowest voltage is dependent on the load on the motor.

The voltage is ramped up from this pedestal level to full voltage in the set time (selectable from 1 sec to 180 sec). Pedestal voltage and ramp time (acceleration time) can be set at site. It is also possible to provide controlled torque soft start with current limit feature (optional) and integrated with Motor Protection Device (MPD) for providing complete control and protection to the motor.




  • Soft Start facility, with initial torque setting by minimum pedestal voltage, ramp time adjustable for setting ramping time, adjustable from 1 to 180 secs.
  • Soft stop facility, ramp down adjustable from 1 to 180 sec., suitable for mainly pump applications where soft stop prevents hammering of water.
  • Can be integrated with advanced microcontroller-based Motor Protection Device for protection of motor for Overload, Unbalance, Single phasing, locked rotor, short circuit, earth fault. Alarms and Annunciation for all the faults, indication for motor running.
  • Onboard push button for start and stop and remote operation
  • In built bypass potential free relay output for switching external bypass contactor.




  • Smooth Start and Stepless gradual acceleration of motor and consequent elimination of shocks during starting
  • Reduction in peak motor current during starting, reducing the stress on the motor windings and voltage dip in the network.
  • Reduction in maximum demand of the network and reducing the overall system loading
  • Reduction in motor temperature rise during starting, improving the motor life and less maintenance
  • Suitable for inching operation with successive soft starts and less stress on the motor
  • There are no moving parts in the soft starter, which can be affected by dust, dirt or other such environmental factors, hence low maintenance requirement.
  • Reduced power consumption and hence reduced energy bills.
  • Easy installation and retrofit capacity (3 wires in and 3 wires out).


Common Applications of Soft Starter


Pumps – Eliminating water hammering effect with gradual increase in speed during starting and gradual decrease in speed during stopping.

Fans – Fans have normally high moment of inertia, which makes starting tough and current high. In soft starter the voltage is increase gradually during start, which reduces the current and removes the inrush peak.

Compressors – The current limit feature enables the motor to start securely even in weaker network, improving the availability of the equipment and system. Reducing current means, reducing the stress on the cables, network and motor.

Conveyors – The current limit feature allows the motor to start at minimum value of torque (which is directly proportional to the current). This results in least possible stress on gearboxes and couplings and no slipping belts during start. This will in-turn reduce the maintenance cost to a minimum.

Crusher and Mill – By using soft starter with current limit function based on the different load conditions during starting, the stress on the machinery is reduced and the inrush current is greatly reduced.


Technical Specification:


Rated Insulation Voltage:              600V

Rated Operating Voltage:             415/440V AC + 10%/ – 15%, 3 phase, 50 Hz

Rated control supply voltage:     230 V AC + 15%/ – 15%, 1 phase, 50 Hz

Recommended load       :               3 – Phase AC Motor, multiple motors or other loads are

not recommended except under specific circumstances.


Overload capacity            :               115% Continuous, 150% for 10 seconds and 200% for 5 seconds

Ambient Temperature   :               During Operation             : -25 degC to +60 deg.C

During Storage                  : -40 degC to +70 degC


Altitude of Installation   :               1000 meters (Above 1000m derating had to be done)

Degree of protection      :               Main Circuit – IP00

Supply and Control Circuit – IP20


Main Circuit                        :           Provided with external bypass contactor provision


Display                                  :          LED Seven Segment Display with 4 keys keypad for setting parameters


Main Settings                    :            Ramp time during start                 – 1 to 180 sec

Ramp time during stop                 – 1 to 180 sec

Pedestal (Initial) voltage                – 1 to 60%

Current limit                                       – 1.5 to 7 x rated current


Signal Relays                      :          One C/O relay for bypass contactor

Control Circuit                   :          Number of digital Inputs – 3 (Start, stop, emergency stop)

Protections                         :           Internal protection for Overtemperature and transient voltage

Voltage rating of thyristors:       1400 to 1600 PIV

Types of cooling                :           Air-cooled below 250 hp, fan cooled for 250 hp and above.

Over temperature protection:    Normally closed contact output, thermal switch provided for heatsink

overtemperature protection (external 2-way terminal provided).

SCR Protection                  :               MOV transient voltage suppressors to absorb high energy voltage transients

provided across the input supply points. RC snubber circuit is provided across

the SCR to prevent misfiring due to rapid rise of voltage.