Advantages of Soft Starters

Advantages of using a Soft Starter

Soft Starters are also known as Reduced Voltage Soft Starters (RVSSs). A Soft Starter is a solid-state device used with AC Motors to temporarily reduce the starting current and torque during start-up (avoiding undue mechanical stress and line voltage fluctuation). It applies a gradually increasing voltage up to the desired voltage within an adjustable ramp time. A Soft Starter thus 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. 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.

Soft Starters are often the more economical choice for applications which require speed and torque control only during motor start-up (and usually take up less space than Variable Speed Drives (VSDs)). Optimum benefits are gained when a motor duty involves frequent start or stop cycles but is still likely to be worthwhile in systems which are in continuous operation. Because of the reduced power consumption and maintenance, a Soft Starter will typically pay for itself very quickly.

Benefits of Soft Starters:

Energy Cost Reduction (Peak Demand Charges)

When AC motors start directly at full voltage, the starting current will be up to 8 times of its rated current. When using a Soft Starter, the starting current of the motor is only 2 to 3 times that of its rated current.

Improved Power Factor

Soft Starters facilitate power factor correction as an inbuilt feature which improves the Power Factor at all load cycles (and thereby reducing the kVA). When the motor is operating at less than full load, the relative reactive component of current drawn by the motor is redundantly excessive due to magnetising and associated losses. The voltage dependent losses are thus negligible with the load proportional active current component and as a result the power factor also improves side-by-side.

Improved Operating Efficiency

Soft Starters enhances motor starting duty by reducing the temperature rise in the stator windings and supply transformer which allows for an increased number of starts per hour (reduced heating of the motors). Soft Starters also allow for easy adjustment of start-up performance (current limit) to suit the specific motor and load (smooth, adjustable acceleration time without the torque/voltage/current transients). It also provides reliable performance if the load characteristics vary between starts (eg. loaded or unloaded starts).

Reduced Power Surges

For a motor with a relatively large capacity, the starting current (without a Soft Starter) will cause a sharp drop in the grid voltage and thus affecting the normal operation of other equipment in the power grid. Using a Soft Starter, the voltage fluctuation of the power grid is generally less than 10% (only slightly affects other equipment), thus reducing the burden on the electrical system, equipment, and electrical devices throughout the system. The effect on the power grid is mainly manifested in two aspects:

The impact of inrush current on power grid caused by direct starting of large motors usually results in power oscillation and makes the power grid unstable.
The starting current contains lots of harmonics, which may cause high frequency resonance with the power grid circuit parameters leading to faults such as the malfunction of relay protections and failure of automatic controls.

Reduced Torque Transients

As a result of increased loads (due to sudden changes in the direction or magnitude of the torque load), reduced torque transients could significantly decrease the lifespan of a motor. Using a Soft Starter reduces the torque load significantly during start-up and limits the associated problems.

Increased life of Contactors

A common issue associated with running an AC motor without a Soft Starter is the need to regularly replace contactors. Using a Soft Starter will reduce or even eliminate the need to replace contactors. It can also reduce the noise associated with contactors during the operation of the motor.

Reduced Maintenance

Using a Soft Starter lets the motor run more smoothly during the start-up process (uniform starting) with the gradual acceleration helping in preventing jerks and thus reducing the need for maintenance by preventing mechanical, electrical and thermal weakening of the electrical equipment (elimination of starting mechanical shock to couplings and driven equipment – gear/belt wear). Soft Starters thus protects motors, transformers, cables, and switchgear. As a result, there will also be less downtime (a further possible cost saving).

High currents cause impact on a motor’s stator coil and rotor squirrel cage bar, which may lead to faults of clamping loosening, coil deformation and breakage of squirrel cage.
Heat produced by high currents impacts the external insulation of wires which accelerates insulation ageing (reduces its service life). The mechanical force produced by high currents also causes friction between wires, further reducing the insulation life. In addition to this, the oscillation of contacts (closing of high voltage switches) causes operating over-voltage on the stator winding of a motor (could be more than 5 times that of the applied voltage) which in turn also cause damage to motor insulation. Using a Soft Starter to prevent these issues thus greatly extends the service life of the insulation.

For more information regarding Soft Starters, please refer to the following related blog posts: