How Does a Soft Starter Work? How It Reduces Motor Stress

Three-phase AC motors have proven themselves for the operation of pumps, conveyor belts, compressors, and countless other drive technology applications. The direct start or the star-delta starter causes an impact on the mechanical components in the drive train. This leads to signs of wear, damage, and premature failures. On the other hand, abrupt starts lead to voltage drops which burden the power supply network and affect the surrounding components. To avoid these problems you need soft starters.

The soft starter is a kind of motor starter controlling the current applied to the motor via its thyristors. It is one of the most efficient solutions that starts and stops the three-phase induction motors.

Studies have shown that most of the motors employed in industrial applications use no form of control other than simple electromechanical switching. This results in increased machine wear as rapid acceleration causes damaging torque transients and high peak currents. The soft starter solves this problem by controlling the application of current during acceleration and deceleration.

In addition to superior starting and stopping performance, soft starters also provide a range of features. This includes areas such as:

• Metering and monitoring
• Braking
• Motor and system protection
• Communication
• Event logs

Soft starters are used to decrease the inrush current of an electrical motor for an efficient start and provide a soft stop.

During direct-on-line start, the current used by the motor is usually between 6-8 times the rated current. This is called inrush current. Inrush current mechanically and electrically forces the motor. A soft starter limits the high inrush currents. Soft starters provide a gradual ramp to stop where sudden stopping may create problems in the connected equipment.

Negative effects of inrush current and sudden stop

• Slipping belts.
• Heavy wear and tear on couplings, gearboxes, and bearings.
• High starting current.
• Pressure surges and water hammering in pipe systems.
• Damaged material and products.

How Does a Soft Starter Work?

A soft starter consists of many anti-parallel thyristors; two in each phase. Thyristors are semiconductor components. Normally they are isolated. After receiving a signal they start to conduct.

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

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.

You can see the working animation of soft starter in the video:

Soft starter wiring diagram

Standard connection (In-line connection)

The controls for isolating and protecting the motor are simply connected in series with the soft starter. The motor is connected to the soft starter with three leads.

Inside-delta connection

The wiring is similar to that of wye-delta starters. The phases of the soft starter are connected in series with the individual motor windings. The soft starter then only has to carry the phase current, amounting to about 58% of the rated motor current. Inside-delta connection can be applied only to three-phase controlled soft starters.

FAQ about softstarters

What are soft starter applications?

Soft starters can be used in many motors such as:

• Centrifugal pumps, piston pumps, heat pumps
• Fans
• Presses
• Screw compressors
• Material handling (conveyors, etc.)
• Specialist machinery (agitators, mixers, centrifuges)
• Escalators
• Bow thrusters
• Extruders
• Lathes
• Milling machines

Is VFD a soft starter?

VFD is not a soft starter but it has soft start and soft stop functions. A VFD can control the speed of the motor during the start, stop, and run cycle. 

Do soft starters save energy?

After start-up, you can bypass the soft starter, switching over to run direct-on-line. After initial start-up, running in DOL saves energy, reduces losses and cooling requirements.

What is a single-phase, two-phase, and three-phase control in soft starters?

Single-phase control

Single phase-controlled soft starters reduce torque shock at the start but do not reduce starting current. They must be used in conjunction with DOL starter.

Two-phase control
Two phase-controlled soft starters eliminate torque transients and reduce motor start current. The uncontrolled phase has a higher current than the two controlled phases during motor starting. They are suitable for all but severe loads.

Three-phase control
Two phase-controlled soft starters control all three phases, provides optimum soft-start. Three-phase control should be used for severe
starting situations.

What is the difference between the DOL starter and soft starter?

DOL is the easiest and most commonly used starting method. It is suitable for stable networks and mechanically stiff and well-dimensioned shaft systems due to the high current and torque generated during the start. DOL starting is uncontrolled, which means that the motor will start with maximum current and torque regardless of load type.

Like a direct-on-line starter, a soft starter is used to start and stop motors in full-speed applications. It eliminates common problems associated with motor starting and stopping, including electrical surges, spikes, and high inrush currents. A soft starter is an optimal solution between a direct-on-line or star-delta starter and a variable speed drive in many full-speed motor applications.