How a Soft Starter Works
A soft starter is used to start and stop electric motors in a controlled manner.
When an electric motor is started directly (DOL), a high inrush current can occur, often several times higher than the motor’s rated current.
This can lead to mechanical stress on the installation and voltage dips in the power supply.
A soft starter prevents these issues by gradually increasing the voltage supplied to the motor.
This limits the starting current and ensures a smooth, controlled start.
Within modern industrial automation, a soft starter is therefore an essential component of reliable
drive technology and efficient motor control.
How Does a Soft Starter Work?
A soft starter controls the voltage supplied to the motor using power electronics, typically thyristors (SCRs).
During startup, the voltage is increased step by step until the motor reaches its nominal speed.
This gradual voltage ramp-up allows the motor current to increase smoothly, reducing mechanical stress on the drive system.
As a result, shocks in the system are minimized and the lifespan of mechanical components such as bearings, gears and couplings is extended.
More technical background on drive systems and motor control can be found in the
Fluxcon variable frequency drives wiki.
Soft Starter With or Without Bypass
Soft starters are available with or without an integrated bypass.
Fluxcon supplies both versions, depending on the application and system requirements.
A bypass is a contactor that bridges the soft starter once the motor reaches full speed.
During startup, the motor is controlled via the thyristors.
After reaching nominal speed, the bypass switches on and the motor is connected directly to the mains.
The advantage is that the soft starter’s power electronics are no longer loaded.
This reduces energy losses and heat generation, improving efficiency and extending component lifetime.
Soft starters without a bypass remain continuously connected through the power electronics.
This can be desirable in applications where controlled stopping or additional functions are required.
Applications of Soft Starters
Soft starters are used in installations where controlled motor starting is important.
Typical applications include:
- Pumps
- Fans
- Compressors
- Conveyors
- Mixers and agitators
In these applications, a soft starter prevents high inrush currents and mechanical shock loads.
Soft Starter or Variable Frequency Drive?
A soft starter is used when only controlled starting and stopping of the motor is required.
If speed control during operation is needed, a
variable frequency drive (VFD) is typically used.
A VFD not only limits starting current but also allows full speed control of the motor.
This makes VFDs ideal for applications requiring energy savings, process control or variable speed operation.
Selection of a Soft Starter
When selecting a soft starter, factors such as motor power, rated current and load type must be considered.
Additional aspects include start frequency, ambient temperature and application requirements.
The choice between a soft starter with or without bypass is also important.
For continuous operation and higher power ratings, a bypass is often preferred due to higher efficiency and lower heat generation.
For questions about selecting soft starters or variable frequency drives, please visit our
frequently asked questions about VFDs.
Fluxcon Product Range – Drive Technology Components
The Fluxcon product portfolio consists of electronic components for controlling electric motors.
The range includes product groups such as variable frequency drives (VFDs), soft starters, braking resistors, brake choppers, line reactors and EMI filters.
Customers choose Fluxcon based on practical engineering and reliability.
We offer an excellent price-performance ratio and fast delivery.
Fluxcon engineering provides straightforward, effective solutions.
Simple solutions are often the best.
Fluxcon values transparency and works closely with customers to find the optimal solution for each process or application.
All products are brand-independent.
For example, braking resistors can also be used with VFDs from other manufacturers.
