Soft starter designs, functions

High inrush currents and mechanical wear and tear are two principal causes of motor damage. Drives can protect attached mechanical components from wear and tear by slowly starting and stopping the drive train. When a variable speed drive isn’t an option, a soft starter can ease the initial impact of the motor starting and soften the blow when it shuts off. Back to Basics, June 2008


High inrush currents and mechanical wear and tear are two principal causes of motor damage. Drives can protect attached mechanical components from wear and tear by slowly starting and stopping the drive train. When a variable speed drive isn’t an option, a soft starter can ease the initial impact of the motor starting and soften the blow when it shuts off.


Soft starters are especially useful for conveyors, fans, pumps, and any equipment where starting or stopping at full speed applies too much stress or could damage the product being moved by the machine. Other motor-driven applications where soft starters can provide benefit include escalators and moving walkways in jurisdictions where such devices can automatically stop and start as needed to save energy.


How do soft starters work?

Instead of applying the full voltage available to start the motor when powered on, a soft starter ramps up the voltage according to the application.


Douglas Yates, product manager, MCG Products, North America Motion Controls, Danfoss Inc., says that while a variable speed drive, as the name suggests, changes motor speed, a soft starter cannot. Inside, he says, a pattern of solid-state switches—called silicon-controlled rectifiers (SCRs) or thyristors—open in different intervals for voltage to ramp up to full speed.


SCR-diode and SCR-SCR designs are available for three-phase motor applications. SCR-diode requires higher start current, causes more heat, and generates undesirable harmonics, Yates says; SCR-SCR provides full wave control.


Motion Control, Power Controls

Current feedback loops hellp protect the motor and provide other current-based functions.

Designs, by phase, open, closed


Designs vary further by phase and whether they open or close the control loop, Yates explains:


  • Single-phase units control start torque, but do not reduce start current. Therefore, they are inadequate for applications with frequent cycling or high inertia loads;

  • Two-phase units do not isolate all phases from the motor. This type of control requires a thermal relay or circuit breaker to protect the motor;

  • Three-phase units provide full control of all three phases, giving maximum control of current and torque;

  • Open-loop designs offer no current feedback loop. They control starting with a preselected voltage profile, and do not provide motor protection; and

  • Closed-loop designs (diagram) provide motor protection and other current functions, and allow the user to set a maximum start current level.

Features, advances

Recent soft starter advances include:


  • Smaller (solid-state) electronics or silicon-controlled rectifiers have allowed for smaller dimensions, making use easier for machine and panel builders;

  • Wide variety of currents and voltages are available (with wider voltage ranges available in specific models);

  • Better sensors for protection and diagnostics help eliminate nuisance trips;

  • Presets appropriate to various applications’ starting curves or torque ramping needs;

  • Options for closed loop control or operator indication are available, with signal relays included with some models;

  • Easier settings with knobs (no programming required);

  • Wider communications availability;

  • More mounting, enclosure, and high voltage safety interlock options; and

  • Multiple certifications and approvals.



Author Information

Mark T. Hoske is editor in chief, Control Engineering. Reach him at .


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