What is Soft Starter,Working Principle,diagram,advantages

What is a soft starter, application, and working principle pdf

Soft Starters are starting devices, used for the acceleration, deceleration, and protection of the three phase electrical induction motors through the controlling applied voltage to 3 phase motor.

Induction motor is the most frequently used motor for industrial as well as domestic applications. mostly industrial motors are single phase or three-phase induction motor depends upon supply fed to it. Ac motor is become most popular because of its simple and robust construction, low maintenance and can be suitable for any working condition. 

Due to its numerous applications, the induction motor needs some starting arrangements to start softly and safely. Various starting methods are used to start induction motors like star delta starter, DOL starter, auto transformer starter, soft starter, and VFD. ( full form variable frequency drive).

In this article we are going to discuss Soft starter for three phase induction motor, soft starter circuit, soft starter working, application, advantages, block, power, control diagram, working principle, uses.

Soft Starter is another form of reduced voltage starter use for starting 3 phase induction motor. Soft Starter also called a Solid-State controller. 
Soft Starter does not change frequency as VFD. Instead it Ramp up voltage level applied to motor from initial value to full voltage.

This is the main difference between Soft Starter and VFD (variable frequency drive).

Initially applied voltage is low which is only to overcome gear wheels or stretching driving belts etc. to avoid sudden jerks during the start. Gradually voltage increase, the torque also increases and motor start to accelerate. 

Advantages of soft starter starting methods are the possibility to adjust torque as per exact need.

By the use of soft starter starting current reduced, this helps to protect the motor from taking high starting current and also prevents a huge voltage drop in supply. Soft Starter also provides a soft stop as a start. Hence it can be suitable where soft stopping is required like a conveyor belt, water pumps.

The main benefits of using a soft starter are, 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.

Soft Starter Block diagram :

Soft starter single line diagram
Soft starter single line diagram

Soft Starter contains only a few main components thyristor to regulate the voltage to the motor. In addition to this heat sink and fan for dissipate the heat to the environment.

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.

Soft Starter working principle:

soft starter working is base on thyristor or SCR firing angle.
Soft starter thyristor bank
Soft starter thyristor bank

soft starter Thyristor firing angle
Thyristor firing angle at starting

White portion= thyristor OFF

Blue portion= thyristor ON

Soft Starter contains the number of anti parallels connected thyristor. Each phase has a pair of the thyristor.

A thyristor is semiconductor devices which are normally isolation but applying firing signal to gate, they start conducting and allowing pass current and voltage through it.

At a time of start to perform soft starting firing signal is sent to thyristors so that only the last part of each half period of sinusoidal voltage wave is passed through it.

And after the start, firing signal send to earlier and earlier, to allow bigger and bigger part of voltage wave to pass through thyristor.

Eventually firing signal sends after every zero crossings to allowing 100% voltage through thyristor.
During stopping, opposite action is done.

At first, the full voltage is allowed to pass through the thyristors and as the stop initiate, 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.

Start: Thyristor allows part of voltage through it initially and after increase accordingly ramp-up time set for starting.

Stop: Thyristor is in full conducting mode when soft stop begain, voltage decreases as ramp time set for the stop.

Voltage is reduced at starting hence current and torque also decreases.

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 soft starter to motor

Soft starter connection Diagram
Soft Starter connection Diagram

Inline connection :

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

In inline for 100A motor same rating(100A) soft starter, overload relay, and main contactor required.

Inside Delta:

The Inside Delta connection makes it possible to place the soft 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 softstarter Inside Delta, there are two options for the connecting main contactor; inside the delta circuit or outside 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 Inside delta circuit can be chosen according to 58% (1/√3) of the rated current.

Advantages of soft starter :

Improved Efficiency: The efficiency of softstarter system using solid-state switches is more owing to the low on-state voltage.

Controlled startup: The starting current can be controlled smoothly by easily altering the starting voltage and this ensures smooth starting of the motor without any jerks this is big advantage of the soft starter.

Controlled acceleration: Motor acceleration is controlled smoothly using a soft starter.

Low Cost and size: This is ensured with the use of solid-state switches.


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