What is a Soft Starter?

    When one thinks of starting an electric motor in an industrial application, the words “Direct on Line (DOL)”, “Star Delta”, “Auto Transformer” and “ Soft Starter” usually come to mind. A Star-Delta starter, “Auto Transformer”, and a “Soft Starter”, all help in lowering the starting current at which a motor starts. Lowering the starting current ensures the grid is protected from excess load, and the motor is less susceptible to long term damage. However, when we deal with large capacity motors running at medium and high voltages, the soft starter is the most preferred mode of starting. This can be primarily attributed to the fact that not all starting methods allow for a significant reduction in the starting current to the scale offered by the soft starter. A Soft Starter is an advanced “Motor Starter”, a device that allows for smooth motor starting, by reducing the starting current of a motor which works by reducing the voltage drop across the motor. A “soft start” ensures a smoother, safer and gradual start- up, by extending the lifespan of the motor and also lesser voltage drops on the system. By lowering the starting current, it ensures that the grid is protected from excess load, voltage drops and ensures that other consumers are not affected with the starting of a large motor.
    The soft starter technology consists of two types:

    • 1.FCMA plus Reactor based Soft Starter
    • 2.Electronics Thyristor based SCR soft starter

    Both starters can be used in different applications for different voltage ranges. Electronic soft starters are typically more popular for lower voltage i.e. up to 600 Volts and low capacity motors up to 200 KW applications, while FCMA soft starters, given their reliability and ruggedness, find greater use in the Medium Voltage Range, and higher capacity beyond 200 KW. In medium voltage range, reliability and continuation of the equipment availability i.e. MTBF is very critical. Replacement and maintenance is significantly more difficult. This makes the FCMA soft starter the best alternative in MV applications.

    FCMA plus Reactor based Soft Starter

    The FCMA soft starter is based on the principle of reactive impedance where the motor voltage increases gradually as motor accelerates to full speed. It does not generate any harmonics and therefore gives minimum starting current at site. The main advantage of the FCMA plus is its ruggedness and simple design that allows for ease of operation and maintenance. It also offers reliable quality at commercially attractive prices with a long life. The FCMA plus soft starter also allows for greater flexibility, customization and can help lower the starting current to as low as 1.2 times the full load current. Each type of soft starter is installed in a panel and comes with various switchgear components along with different protection equipment such as a CPR, isolator, fuse, VC or VCB etc.
    FCMA soft starters reduce starting current, accelerates motor to full speed and then is bypassed when the motor reaches full speed Thus ensuring that the motor continues to run regularly on full voltage. The circuit diagram below illustrates the working of the soft starter.

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Applications of Soft Starter:

Soft starters are also very popular in applications involving the reduction of the starting current in numerous pump, compressor and fan applications. Soft Starters find application in various industries, right from Water Supply, Oil and Gas (O&G), Steel, HVAC (Air-Conditioning and District Cooling), Cement and Pharmaceuticals. Soft starters find use in both high and low voltage industrial applications, and are often installed in various units of the same factory.

Electronics Soft Starter

Electronics soft starter work on the same principle as an FCMA soft starter, but use back-to-back thyristors. They are actuated during the start-up phase such that their turn-on is successively delayed less for each half cycle of the A/C supply. The delayed switching ramps up the average A/C voltage to the motor until motor sees a full-line voltage. Similar to the FCMA starter, the thyristor switching is bypassed when the motor reaches full speed/voltage. At higher voltages, the thyristors are replaced with MOSFET, IGBT; thyristors are more effective at lower voltages. The thyristor scheme must be bypassed to avoid extra power losses. When thyristors are not bypassed final voltage to motor will be 93-94 % thereby increasing current for the same power consumed, adding heat losses in the system.