Soft Starter Basics

A Soft Starter is an electrical device that controls the applied voltage to a motor to control its acceleration. In industrial settings, induction motors are commonly used because of their ability to self-start. While this is beneficial, the starting mechanism draws in high current (inrush) which may damage mechanical and electrical components. This gives way to the need for a soft starter.

A motor can be started in one of three ways:

• Applying full load voltage at short time intervals
• Applying reduced voltage and ramping it up
• Applying part winding starting through auto transformer

A soft starter has the ability to reduce the applied torque to an electric motor. It consists of electronic components specifically the thyristor whose firing angle is modified to control the output voltage. The torque is directly proportional to the square of the starting current, which is also proportional to the voltage. Therefore, by reducing the voltage, the torque and current can be controlled.

Open Control

A starting voltage is applied, irrespective of the initial current drawn by the motor. For each phase, two thyristors are connected back-to-back, conducting at a delay of 180 degrees. The delay is reduced gradually as the inrush current settles down until the voltage is ramped up to full-load settings.

Closed Loop Control

As the name suggests, one of the motor’s output is monitored, depending on which the firing angle of the thyristors are varied. This allows the output voltage to stay within allowable limits.

There are two basic components of any soft starter:

1. Power Switches

These are SCRs or thyristors that are phase controlled solid-state devices. For a three-phase motor, two SCRs are connected back-to-back per phase. It must be noted that the switching devices must have a rating of three times that of the line voltage.

 2. Control Logic

Through PID controllers or microcontrollers the gate voltage to the SCR is controlled so that accurate firing angle is maintained throughout the operation of the soft starter.

Advantages

As soft starters make use of solid-state components, they are high-efficiency compared to “low on state voltage” methods. The fact that the starting voltage follows a “ramp trend” means there is a smooth rise in the current and torque protecting the components from undergoing any sudden shock. For controlling the starting voltage of a motor, a soft starter is a wise choice compared to a full-blown frequency drive, from the economic point of view. Moreover, compared to a VFD, a Soft Starter is also smaller and can even be mounted on top of motor housings.