Soft Starter Working Principle-Advantages

A soft starter is one type of reduced voltage starter for induction motors.  The soft starter is similar to a primary resistance or primary reactance starter. In that, it is in series with the supply to the motor.  The current into the starter equals the current out. The soft starter employs solid state devices to control the current flow and therefore the voltage applied to the motor. In theory, soft starters can be connected in series with the line voltage applied to the motor or can be connected inside the delta loop of a delta-connected motor,  controlling the voltage applied to each winding.

Soft Starter panel diagram is shown below diagram:


Soft starter working principle

A soft starter does not change the frequency or the speed like a drive. Instead, it ramps up the voltage applied to the motor from the initial voltage to the full voltage.

Initially, the voltage to the motor is so low that it is only able to adjust the play between the gear wheels or stretching driving belts, etc to avoid sudden jerks during the start. Gradually, the voltage and the torque increase so that the machinery starts to accelerate. One of the benefits of this starting method is the possibility to adjust the torque to the exact need, whether the application is loaded or not. Using a soft starter will reduce the starting current and thereby avoid voltage drops in the network. It will also reduce the starting torque and mechanical stress on the equipment, resulting in a reduced need for service and maintenance. Just as for a drive, the soft starter can perform a soft stop, eliminating water hammering and pressure surges in pumping systems and avoiding damage to fragile material on conveyor belts.

Single-line-block-diagram-soft starter

A soft starter consists of only a few main components. These are the thyristors that can regulate the voltage to the motor and the printed circuit board assembly (PCBA) that is used to control the thyristors. In addition to this, there are the heat sink and fans to dissipate the heat, current transformers to measure the current, and sometimes display and keypad and then the housing itself. It is more and more common to offer integrated by-pass contacts in the main circuit minimizing the power loss in normal operation. Depending on the model of the soft starter, it can be equipped with a built-in electronic overload relay (EOL) eliminating the need for an external relay, PTC input, field bus communication possibilities etc.

Working of Soft Starter

soft starter circuit using thyristors:


A soft starter  anti-parallel thyristors

Start: The thyristors allow part of the voltage through at the beginning and then increase it, according to the set ramp time for the start.

Stop: The thyristors are fully conducting and when soft stopping, they decrease the voltage according to the set ramp time for a stop

A soft starter consists of a number of anti-parallel thyristors; two in each phase. These thyristors are semiconductor components that normally are isolating but by sending a firing signal, they can start to conduct, allowing the voltage and the current to pass through.

When performing a soft start, a firing signal is sent to the thyristors so that only the last part of each half period of the voltage sinus curve passes through.

Then during the start, the firing signal is sent earlier and earlier allowing a bigger and bigger part of the voltage to pass through the thyristors. Eventually, the firing signal is sent exactly after passing zero, allowing 100% of the voltage to pass through.

By allowing more and more of the voltage to pass through the thyristors, this can be seen as a ramping up of the voltage from something called the initial voltage to the full voltage.

When performing a soft stop, the opposite happens. At first, the full voltage is allowed to pass through the thyristors and as the stop proceeds, the firing signal is sent later and later allowing less and less of the

voltage to pass through until the end voltage is reached. Then no more voltage is applied to the motor and the motor stops.

Since the voltage to the motor is reduced during the start, both the current and the torque will also be decreased. In fact, if the voltage is decreased to 50% of the full voltage, the current will be decreased to about 50% of the maximum current at that speed and the torque will be decreased to about 25% of the maximum torque.

Ways of connecting the starter to motor

Inline connection :

This connection is most common. All three phases of the motor are connected in series with the main contactor, overload relay, and soft starter.

Inside Delta:

The Inside Delta connection makes it possible to place the starter in the delta circuit and in that way it can easily replace an existing Y/D-starter order to achieve a more cost-effective.

When using the soft starter Inside Delta, there are two options for the connecting main contactor; inside the delta circuit or outside the delta circuit. Both locations will stop the motor but in the inside delta circuit, the motor is still considered to be under tension.

In the outside delta circuit, the main contactor must be selected according to the rated current of the induction motor, while the contactor in the Inside delta circuit can be chosen according to 58% (1/√3) of the rated current

Advantages of the soft starter as follows

  • These are the main benefits of using a soft starter
  • The inrush current is reduced so that voltage drops on the network are avoided.
  • The torque is reduced which will decrease the mechanical stresses on the equipment and lead to a reduced need for service and maintenance and also to a longer life of the equipment.
  • Finally, by using a stop ramp, water hammering is avoided in pump systems, which will further reduce the stress on the equipment.

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