How do Soft Starters Work?

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How do Soft Starters Work?

Soft Starters are electrical devices connected between a power supply and an AC motor which regulates the amount of current fed to the motor (allow the motor to use a different start-up method). Soft Starters are used for motors that initially pulls an excess of current (also known as having a large inrush current). The name Soft Starter commonly refers to electronic solid-state drives, which simply means a drive which uses semiconductors.

The purpose of a Soft Starter is thus to reduce the strain put on the motor during the typical power-up phase of a motor. To accomplish this, a Soft Starter will slowly and gradually begin applying increasing voltages to the motor which allows for a smooth acceleration of power instead of a sudden and violent burst of power (that could potentially cause damage to the motor and the machine as a whole).

Soft Starters are like Variable Speed Drives (VSDs) but cannot vary the speed as VSDs do. While unable to change the motor speed, Soft Starters do increase both efficiency and safety when used and is popular in high-inertia systems which must be brought to full speed gradually. Depending on which specific Soft Starter model you choose, some have the capability to adjust the starting voltage and the length of time it takes until the motor is fully powered up.

Essentially, a Soft Starter works by controlling the amount of voltage running through the motor’s circuits. It does this by limiting torque in the motor. This in turn 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. They are commonly used in three pairs (or TRIACS) to account for each phase of the motor, as 3-Phase motors typically require soft starting. Upon start-up, each phase will run through each TRIAC before reaching the motor. The Thyristors will reduce the voltage (and therefore the current) and will allow an attenuated signal to pass to the motor. The current is monitored until the motor reaches full speed, where the Thyristors are then bypassed by connecting the motor directly to the power source via the contacts (also known as powering the motor ‘across-the-line’).

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

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