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 .


No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
2017 Lubrication Guide; Software tools; Microgrids and energy strategies; Use robots effectively
Prescriptive maintenance; Hannover Messe 2017 recap; Reduce welding errors
Safety standards and electrical test instruments; Product of the Year winners; Easy and safe electrical design
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Diagnostic functions for system safety; Specifying industrial enclosures; Effective decision support for a crisis
Transformers; Electrical system design; Selecting and sizing transformers; Grounded and ungrounded system design, Paralleling generator systems
Natural gas for tomorrow's fleets; Colleges and universities moving to CHP; Power and steam and frozen foods

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me