How does soft start work? working principle

 Soft Starter:

 SoftStarters :A soft starter is another form of reduced voltage starter for AC.  induction motors.  The soft starter  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. 

Fig: Soft starter control pannel ABB

 

Voltage Control Voltage control is achieved by means of solid state Ac.  switches in series with each phase These switches comprise 
1 x Triac per phase 
1 x scR and 
1 x Diode reverse parallel connected per phase.

 DIFFERENT WAYS OF CONNECTING THE SOFT STARTER:

 There are two different ways of connecting the soft starter in- line.  which is the most common and Note that only a few types of softstarters can actually be connected Inside  Delta 

In-line connection:

fig:soft starter connection inline

This is easily the most common way to connect the softstarter.  All three phases are connected in a series with the overload relay,  the main contactor and devices used just like the diagram below.  The selected devices for Inline connection must be chosen to cope with the rated motor current. Example:  100 A motor requires a 100 A softstarter,  100 A main contactor etc.

Inside Delta connection:

fig:soft starter connection in side

 

 The Inside Delta connection makes it possible to place the softstarter in the delta circuit and in the way it can easily replace an existing-starter.  Star delta, 
When the softstarter is Inside Delta it will only be exposed to 58%(1/3)  of the In-line current Therefore it is possible to downsize the devices in order to achieve a more cost-effective solution 
Example:  A 100 A motor requires a 58 A softstarter,  a 58 A main contactor if placed in the delta  circuit,  etc.  
A motor used for an Inside Delta connection must be able to delta connect during a continuous run
 

Location of the main contactor:

When using the softstarter Inside Delta there are two options for the main contactor in the delta circuit or outside Both locations will stop the motor but in alternative A,  the motor is still consider to be under tension. In alternative B the main contactor must be chosen according to the rated current of the motor while the contactor in alternative A can be chosen according to 58%(1N3) of the rated carrent.

Advantages of Soft Start:

  let us recollect few reasons why it is preferred over other methods.

• Improved Efficiency: The efficiency of soft starter 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.
• Controlled acceleration: Motor acceleration is controlled smoothly.
• Low Cost and size: This is ensured with the use of solid state switches.