How does soft start work? working principle

 Soft Starter:

 Soft Starters :A soft starter is another form of reduced voltage starter for 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 1: Soft starter control pannel ABB

 

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 with this starting method is the possibility to adjust the torque to the exact need, whether the application is loaded or not. Using a softstarter 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 reduced need for service and maintenance. Just as for a drive, the softstarter can perform a soft stop, eliminating water hammering and pressure surges in pumping systems and avoiding damage to fragile material on conveyor belts. 

Fig 2.Single line diagram for a softstarter

A softstarter 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 softstarter, it can be equipped with a built-in electronic overload relay (EOL) eliminating the need for an external relay, PTC input, fieldbus communication possibilities etc.

Soft starter functionality and working:

Start: The thyristors let 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 stop

A softstarter consists of a number of anti-parallel thyristors; two in each phase. These thyristors are semiconductor components which 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 send 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.

These are the main benefits of using a softstarter:
 

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.

 Torque Control:
 Normally, a softstarter performs a start and a stop by ramping up or down the voltage linearly. However, a linear change of the voltage does not necessarily give a linear change of the torque or of the speed. This is where torque control comes in. With a torque ramp, it is not the voltage that is ramped up or down linearly, it is the torque. This is done by using a regulation loop where the torque is calculated by measuring both the voltage and the current. This torque is then compared to the required torque and the voltage is adjusted so that the torque is changed in the required way. Torque control is especially useful for stopping pumps where a sudden decrease of the speed may lead to water hammering
and pressure surges that can cause tremendous

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

  
Different applications of soft starter :

1) Centrifugal fan:

 The key to solve the problems with slipping belts is to reduce the starting torque of the motor during start.By using an softstarter the voltage is reduced to a low value at the beginning of the start. Then gradually the voltage is increased in order to start up the fan. The softstarter provides the ability to adjust the settings to fit any starting condition, both unloaded and fully loaded starts. Using a softstarter will also greatly reduce the high inrush current when starting the motor, and thereby avoid voltage drops in the network. Some softstarters have built-in underload protection which will detect the reduced
current caused by a broken belt, and stop the motor to prevent damage

Selection of a suitable Softstarter:

A fan usually has a big flywheel with a big moment of inertia making it a heavy duty
start. Select a softstarter one size larger than the motor kW size. Since the big flywheel of a fan will cause a long slow down period before the fan stops, a stop ramp should never be used for this kind of application.
 
Recommended basic settings:
Start ramp: 10 sec.
Stop ramp: 0 sec.
Initial voltage: 30 %
Current limit: 4 * Ie

2) Centrifugal pump:

By using a  softstarter the voltage is reduced during the start sequence with the result that the motor torque is reduced. During the start sequence the softstarter
increases the voltage so that the motor will be strong enough to accelerate the pump to the nominal speed without any torque or current peaks. Also during the stop sequence the softstarter is the solution. A softstarter using a normal voltage ramp will for sure reduce
the problems with water hammering but in many pump systems this is still not good enough. The solution is to use a softstarter with torque control in order to reduce the torque and stop the motor in the most optimal way in order to totally avoid water hammering. In addition, some softstarters are equipped with underload protection to detect pumps running dry, with kick start to start blocked pumps and with locked rotor protection to prevent damage caused by pumps being jammed while running. 

Selection of a suitable softstarter:
 
A pump usually has a very small pumpwheel with a low moment of inertia. This makes the pump a normal start so the softstarter can be selected according to the kW rating. If more than 10 starts per hour are performed it is however recommended to upsize the softstarter one size.
 
Recommended basic settings:
 
Start ramp: 10 sec.
Stop ramp: 10 - 20 sec.
Initial voltage: 30 %
Stop mode: Torque control
Current limit: 3.5 * Ie

3) Compressor: 

 By using an softstarter it is possible to limit the starting torque to a level suitable for all different applications. The result is less stress on couplings, bearings and
no slipping belts during start. The maintenance cost will be reduced to a minimum. When using a softstarter the starting current received is approx. 3 to 4 times the
rated motor current.

Selection of a suitable softstarter

 
A compressor is usually a normal start and then the softstarter can be selected according to the motor kW size. If the compressor is a heavy duty start, the softstarter should be upsized one size. The same applies if more than 10 starts per hour are performed, upsize one size.
 

Recommended basic settings:

 
Start ramp: 5 sec.
Stop ramp: 0 sec.
Initial voltage: 30 % (piston compressor)
40 % (screw compressor)
Current limit: 3.5 * Ie

Comparision between different starting methods:

Type of problem	Type	of	starting	method
	Direct on line	Star-Delta start	Drives	soft starter
Slipping belts and heavy wear on bearings	No	Medium	Yes	Yes
High inrush current	No	Yes	Yes	Yes
Heavy wear and tear on gear boxes	No	No (at loaded start)	Yes	Yes
Damaged goods / products during stop	No	No	Yes	Yes
Water hammering in pipe system when stopping	No	No	Yes	Yes (Eliminated with Torque control Reduced with voltage ramp)
Transmission peaks	No	No	Yes	Yes

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.