Soft Starters

What is a Soft Starter?

Soft Starter Definition

Starting of induction motors is accompanied by inrush currents up to 7-10 times higher than running current and starting torque up to 3 times higher than running torque. Torque surges entail high mechanical stress on the machine, which results in higher service costs and increased wear. High currents and current peaks lead to high fixed costs charged by the power supply companies (peak current calculation) and to increased mains and generator loads.

A Soft Starter is a device that starts motors with reduced power supplied at start-up. Soft Starters regulate the amount of current fed to the motor to enable the AC induction motor to speed up in smaller increments, resulting in less current drawn than with a traditional motor starter. Due to decreased voltage, torque is also reduced resulting in a soft/easy start.

A Soft Starter continuously controls the three-phase motor’s voltage supply during the start-up phase. This reduces the mechanical stress on the motor and shaft, as well as the electrodynamic stresses on the attached power cables and electrical distribution network, extending the lifespan of the system.

For a more detailed explanation on this, please see our blog post: What is a Soft Starter?

Advantages of using a Soft Starter

Soft Starter Benefits

A Soft Starter is used with AC Motors to temporarily reduce the starting current and torque during start-up (avoiding undue mechanical stress and line voltage fluctuation). A Soft Starter provides a linear power slope for smooth acceleration (allows for adjusting the starting voltage and acceleration time to suit specific applications), which reduces in rush currents and excessive wear on mechanical driven components. This method requires less stress on the machine and therefore equipment lasts longer and works better. It may also benefit the electrical distribution network, using a Soft Starter could thus extend the lifespan of the entire system.

Because of the reduced power consumption and maintenance, a Soft Starter will typically pay for itself very quickly.

Below is a list of the top 7 benefits of using a Soft Starter:

Energy Cost Reduction (Peak Demand Charges)
Improved Power Factor
Improved Operating Efficiency
Reduced Power Surges
Reduced Torque Transients
Increased life of Contactors
Reduced Maintenance

Further to this Soft Starter allows for an increased number of motor starts per hour and protects motors against possible low/high voltage/current supply, short-circuits, lost phases (in/out) and 3-phase imbalances.

For a more detailed explanation on this, please see our blog post: Advantages of using a Soft Starter

Advantages of Soft Starters vs VSDs

Soft Starters are generally easier to setup than VSDs and much lower in cost – and are thus often the most economical choice when considering only the starting condition. Some disadvantages of VSDs, compared to Soft Starters, include the fact that VSDs introduce a small amount of extra heat in the motor (imperfect Sine Wave) and VSDs draw nonlinear current, known as harmonic distortion, from the power source. However, in most cases, the benefits of VSDs greatly offset these things and special VSD countermeasures exist to greatly reduce them when necessary. The main advantage of a Soft Starter over a VSD in this instance lies in its ability to bypass the power electronics in the Soft Starter once the motor is up to full speed and operating – benefits include lower Watt loss, no harmonics (except during acceleration/deceleration) and less maintenance.

For most applications, the main deciding factor between using a Soft Starter or a VSD boils down to the need for continuous speed control. If your application is going to be a set speed and across-the-line starting is not acceptable, or an application requires adjustable start and stop times for electrical or mechanical reasons, then a Soft Starter can be an excellent solution.

For a more detailed comparison on this, please see our blog post: Choosing between a VSD and a Soft Starter

For more details on using VSDs, please see our product page: Variable Speed Drives (VSDs)

Soft Starter Industry Applications

What are Soft Starters Used for?

Soft Starters are typically used in applications where:

Speed and torque control are required only during start-up (and stop if equipped with soft stop).
Reduction of large start-up inrush currents associated with large motors is required.
The mechanical system requires a gentle start to relieve torque spikes and tension associated with normal start-up (e.g., conveyors, gears, and belt-driven systems).
Pumps are used to eliminate pressure surges caused in piping systems when the fluid changes direction rapidly.

Although the majority of Soft Starters are installed in pump and fan applications, they can be incorporated into many other motor applications that do not require speed control (where a VSD would be the best option). Here are the main applications of a Soft Starter:

Steel industries (Rolling mills and processing lines)
Cement industries
Sugar plants
Paper and pulp
Rubber and plastic
Textile industries
Machine tool applications
Power sector
Water supply
Various process control application
Etc…

For a more detailed discussion on this, please see our blog post: Soft Starter Industry Applications

For other industry applications, please also see our Industry Applications page: Use Cases – VSDs and Soft Starters

How do Soft Starters Work?

Soft Starter Working Principle

A Soft Starter works by controlling the amount of voltage running through a motor’s circuits by limiting torque in the motor. This allows the Soft Starter to reduce the voltage and allows it to gradually stop reducing the voltage to allow for a smooth progression of current.

To reduce the inrush of current and ramp-up to full speed, solid-state Soft Starters typically use components known as Thyristors or Silicon-Controlled-Rectifiers (SCRs) to limit the voltage to a more manageable amount for the motor as it begins to start up. These components reduce the incoming voltage to the motor and can allow operators to keep voltage constant until full speed is achieved. These components have an ON state, where they allow the current to flow, and an OFF state, where they control and limit the electrical current. When powering up a machine, these components activate, restrict the voltage, and then relax as the machine reaches full power.

Soft Starters do not only equalise the current during start-up, but also control motor torque, which keeps the motor’s heat down and reduces overall strain. Soft Starters therefore provide reliable, consistent torque at rated speeds, and provide consistent, safer power.

For a more detailed explanation on this, please see our blog post: How do Soft Starters Work?