View source for Motor soft starter

{{Short description|Device used with AC electrical motors}}
[[File:Carlo Gavazzi RSBT Soft Starter 1.png|alt=|thumb|upright|Compact soft starter for a 3 phase machine, 15kW/10HP]]
A '''motor soft starter''' is a device used with [[AC motor|AC electrical motor]]s to temporarily reduce the load and [[torque]] in the [[powertrain]] and electric current surge of the motor during start-up. This reduces the [[mechanical stress]] on the motor and shaft, as well as the [[electrodynamics|electrodynamic]] stresses on the attached power cables and [[electricity distribution|electrical distribution network]], extending the lifespan of the system.<ref name=Siskind>{{cite book | first=Charles S. | last=Siskind | year=1963 | title=Electrical Control Systems in Industry | publisher=McGraw-Hill, Inc. | location=New York | isbn=978-0-07-057746-6 | page=[https://archive.org/details/electricalcontro0000unse/page/150 150] | url=https://archive.org/details/electricalcontro0000unse/page/150 }}</ref>{{rp|150}}

It can consist of mechanical or electrical devices, or a combination of both. Mechanical soft starters include [[clutch]]es and several types of [[coupling]]s using a [[fluid coupling|fluid]], magnetic forces, or steel [[Lead shot|shot]] to transmit torque, similar to other forms of [[torque limiter]]. Electrical soft starters can be any control system that reduces the torque by temporarily reducing the [[voltage]] or [[electric current|current]] input, or a device that temporarily alters how the motor is connected in the [[electric circuit]].

==Operating principles==
[[File:Digital Soft Starter with Motor Protection and In Built By Pass.jpg|thumb|Digital Soft Starter]]
Whenever the armature of an [[electric motor]] is moving, both the ''motor action'' and ''generator action'' are occurring simultaneously; the electromagnetic force produced by generator action opposes the desired motor action and effectively creates a variable motor resistance which increases with motor speed. When a voltage is applied to the motor, this resistance dictates the current drawn by the motor. At rest, the resistance is relatively low, so the starting or ''inrush'' current can be high if the full line voltage is applied to the motor. Compared to DC motors, AC motors tend to have significantly higher stator resistance and correspondingly lower inrush current.<ref name=Siskind/>{{rp|24}}

Nevertheless, across-the line starting of induction motors is accompanied by inrush currents up to 7-10 times higher than running current, and higher efficiency motors can experience inrush currents 10-15 times running current. In addition, starting torque can be up to 3 times higher than running torque. The starting torque transient can create a sudden mechanical stress on the machine, which leads to a reduced service life. Moreover, the high inrush current stresses the power supply, which may lead to voltage dips. As a result, lifespan of sensitive equipment may be reduced.<ref name="Siskind" /> Another common side-effect, especially in residential installations, is voltage sag in the site's [[power supply]] created by the high inrush current, visible as flickering lights.

A soft starter continuously controls the motor's voltage supply during the start-up phase. This way, the motor is adjusted to the machine's load behavior. Mechanical operating equipment is accelerated smoothly. This lengthens service life, improves operating behavior, and smooths work flows. Electrical soft starters can use [[solid state (electronics)|solid state]] devices to control the [[Electric current|current]] flow and therefore the voltage applied to the motor. They can be connected in [[series circuits|series]] with the line voltage applied to the motor, or can be connected inside the delta (Δ) loop of a [[Y-Δ transform|delta-connected motor]], controlling the voltage applied to each winding. Solid state soft starters can control one or more [[three-phase electric power|phases]] of the voltage applied to the induction motor with the best results achieved by three-phase control. Soft starters controlled via two phases have the disadvantage that the uncontrolled phase will always shows some current unbalance with respect to the controlled phases. Typically, the voltage is controlled by reverse-[[parallel circuits|parallel]]-connected [[silicon-controlled rectifier]]s ([[thyristor]]s), but in some circumstances with three-phase control, the control elements can be a reverse-parallel-connected SCR and [[diode]].<ref>{{cite web |url=http://machinedesign.com/engineering-essentials/soft-starters|title= Soft starters |publisher=machinedesign.com|date= 2014-07-16 }}</ref><ref>{{Cite web |title=Reliance Electric GV3000 Drive 50V4160 {{!}} Automation Industrial |url=https://50v4160.com/blog |access-date=2024-01-02 |website=50v4160.com |language=en}}</ref>

Another way to limit motor starting current is a series [[inductor|reactor]].  If an air core is used for the series reactor then a very efficient and reliable soft starter can be designed which is suitable for all types of 3 phase induction motor [ synchronous / asynchronous ] ranging from 25 kW 415 V to 30 MW 11 kV.  Using an air core series reactor soft starter is very common practice for applications like pump, compressor, fan etc.  Usually high starting torque applications do not use this method.

==Applications==
Soft starters can be set up to the requirements of the individual application. Compared to variable-frequency drives, soft starters require very few user adjustments. Some soft starters also include a "learning" process to automatically adapt the drive settings to the characteristics of a motor load, to reduce the power inrush requirement at the start. In pump applications, a soft starter can avoid pressure surges that could lead to ''[[water hammer]]''. Conveyor belt systems can be smoothly started, avoiding jerk and stress on drive components. Fans or other systems with belt drives can be started slowly to avoid belt slipping as well as air pressure surges. Soft starters are seen in electrical R/C helicopters, and allow the rotor blades to spool-up in a smooth, controlled manner rather than a sudden surge. In all systems, a soft start limits the inrush current and so improves stability of the power supply and reduces transient voltage drops that may affect other loads. 
<ref name=Bartos21st>{{cite journal|last=Bartos |first=Frank J. |title=AC Drives Stay Vital for the 21st Century |journal=Control Engineering |date=2004-09-01 |url=http://www.controleng.com/article/CA450388.html |access-date=2008-03-28 |url-status=dead |archive-url=https://web.archive.org/web/20080917014756/http://www.controleng.com/article/CA450388.html |archive-date=September 17, 2008 }}</ref><ref>{{cite conference | first = Robert E. | last = Eisenbrown | title = AC Drives, Historical and Future Perspective of Innovation and Growth | book-title = Keynote Presentation for the 25th Anniversary of The Wisconsin Electric Machines and Power Electronics Consortium (WEMPEC) | pages = 6–10 | publisher = WEMPEC | date = 2008-05-18 | location = University of Wisconsin, Madison, WI, USA | url = http://www.wempec.wisc.edu/keynotes.htm | access-date = 2008-03-28 | archive-url = https://web.archive.org/web/20070818235707/http://www.wempec.wisc.edu/keynotes.htm | archive-date = 2007-08-18 | url-status = dead }}</ref><ref>{{cite journal  | last =Jahns   | first =Thomas M. |author2=Owen, Edward L. | title =AC Adjustable-Speed Drives at the Millennium: How Did We Get Here? | journal =IEEE Transactions on Power Electronics | volume = 16 | issue = 1 | pages =17–25 | date =January 2001  | doi =10.1109/63.903985 | bibcode =2001ITPE...16...17J | url =http://digital.library.wisc.edu/1793/11120 }}</ref>

==See also==
* [[Adjustable-speed drive]]
* [[Braking chopper]]
* [[Brushed DC electric motor#DC motor starters|DC motor starter]] section of [[Electric motor]]
* [[Electronic speed control]]
* [[Korndorfer starter]]
* [[Motor controller]]
* [[Space Vector Modulation]]
* [[Thyristor drive]]
* [[Variable-frequency drive]]
* [[Variable-speed air compressor]]
* [[Vector control (motor)]]

==References==
<references/>
{{electric motor}}
{{DEFAULTSORT:Motor Soft Starter}}
[[Category:Control engineering]]
[[Category:Electric motors]]
[[Category:Electromagnetic components]]
[[Category:Electric power systems components]]
[[Category:Power electronics]]