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منتدي ممدوح عزت موسي MAMDOUH EZAT MOUSA FORUM

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اليومية اليومية


    Soft start

    شاطر

    ممدوح عزت موسي
    مدير عام المنتدي
    المشرف العام
    مشرف منتدي القصه
    مشرف منتدي العلوم الهندسيه
    مدير عام المنتدي  المشرف العاممشرف منتدي القصهمشرف منتدي العلوم الهندسيه

    عدد المساهمات : 350
    التميز : 9
    تاريخ التسجيل : 13/05/2010

    رد: Soft start

    مُساهمة من طرف ممدوح عزت موسي في السبت سبتمبر 04, 2010 4:56 pm

    [quote="ممدوح عزت موسي"][left]
    Soft start

    Soft start is an electronics term describing any circuit which reduces

    excess current during initial power-up.


    Introduction
    In industrial applications, almost everything uses a motor, in fact, motors may account for up to 80% of our country’s energy usage. There are generally three different ways to start a motor: full-voltage, reduced voltage, and inverter. A full voltage, across-the-line, or direct on-line (DOL) start uses a contactor, which is a heavier duty three-phase relay. Reduced voltage starting can be accomplished via several different ways: auto-transformer, wye-delta, primary resistor/reactor, or with a solid state soft starter. Inverters are generally referred to as drives. This paper focuses on solid state soft starters (referred to as soft starter only from here on): what they are, why they are used, their construction, and applications.

    [ What is a soft starter
    A soft starter is a solid state motor starter that is used to start or stop a motor by notching the voltage waveform, thereby, reducing the voltage to each phase of a motor and gradually increasing the voltage until the motor gets up to full voltage/speed all at a fixed frequency. The profile of the increase of voltage depends on the application. The voltage is reduced and controlled by 3 pairs of back-to-back silicon-controlled rectifiers (SCRs), which are a type of high speed thyristor. A soft starter takes the place of a contactor and can also take the place of an overload relay in a standard motor starting application.

    Why use a soft starter
    In general, there are two reasons to use a soft starter: the power distribution network may not be able to handle the inrush current of the motor and/or the load cannot handle the high starting torque. As a rule of thumb, a motor utilizes around 600-800% of its full load current (FLA) to start. This current is referred to as inrush current or locked-rotor current. If a large motor is on a smaller power distribution network or on a generator system, this inrush current can cause the system voltage to dip or to “brown out.” Brown outs can cause problems with whatever else is connected to the system, such as computers, lights, motors, and other loads. Another problem is that the system may not even be able to start the motor because it cannot source or supply enough current. Most industrial businesses run during the day can be fined or charged extra (Peak Demand charges) during this peak usage time for large transients caused by large horsepower (HP) motor start ups. These Peak Demand charges can add up very quickly, especially if the motor needs to be started multiple times during any given day. The inrush current can be controlled one of two ways with a soft starter: either with a current limit (discussed later) or reduced linearly with the reduced voltage, and follows this approximation:

    I
    Applications such as conveyors may not be able to handle a sudden jolt of torque from an across-the-line start. Utilizing soft starters reduces the wear and tear on belts, conveyors, gears, chains, and gearboxes by reducing the torque from the motor. The torque decreases as a square of the reduced voltage, and follows this approximation:

    Since soft starters are generally controlled and monitored by a microprocessor, a soft starter can add many features and protections fairly easily. It can offer a choice of the starting time, limited speed control, and energy savings. Power monitoring, such as three-phase current, three-phase voltage, power, power usage, power factor, and motor thermal capacity usage, can be implemented with current transformers, a voltage meter, and an internal clock. With the above implementations, protection, for the motor or the soft starter, from the items listed below (Table 1) can also be offered by stopping the firing of the SCRs, dropping out the bypass contactor (a contactor that carries the motor load after the motor is up-to-speed), and/or alerting a user via some form of communications with the microprocessor and another computer.

    Construction
    Generally, a soft starter is constructed with three pairs of SCRs reverse parallel connected to allow the current to flow to or from the motor. Soft starters can be made by controlling just one or two phases, but this page will focus on the most prevalent implementation, three-phase control. Each phase of a soft starter can be controlled with an SCR pair reverse parallel connected, an SCR/diode pair reverse parallel connected, or a triac, depending on cost and/or quality. The most prevalent switch in industry is probably the SCR pair and will also be the focus of this paper. Soft starters are used almost exclusively for starting and stopping and not during the run time because of the heat loss through the SCRs from the voltage drop across them.

    A standard assembly of a soft starter uses one SCR pair per phase and once the voltage gets to within approximately 1.1V of full voltage (depending on the voltage drop across the SCR) a bypass contactor (internal or external to the soft starter), running parallel to the SCR pairs, pulls in. Once pulled in the SCRs stop firing. Typically, the bypass contactor is much smaller than compared to what is needed for a full voltage start as the contacts only need to be able to handle the full load current of the motor. Since the mechanical contacts cannot handle the inrush current, the SCRs must be sized correctly to handle the motor’s locked-rotor current. The transition from SCRs to bypass should also be near full speed to minimize the jump in current.

    Control
    A soft starter reduces the voltage by “notching” the applied sinusoidal waveform. A notch is a non-technical term for the zero voltage area in the middle waveform. As the notch decreases in size, the Vrms increases along with Irms. An initial voltage, determined by the user, is ramped up to full voltage by varying the firing angle depending on the preset profile of the soft starter. Soft starters can be controlled via open-loop or closed-loop control. All control schemes listed monitor back EMF of the motor as to not become unstable.
    Open-Loop
    An example of open loop control is the voltage ramp; the voltage ramps from an initial voltage to full voltage in a linear fashion without regards to the load. Pump start is another form of open-loop control. The pump starter’s firing circuit ramps up the voltage with a profile that allows the speed/torque to ramp in a more efficient manner and helps protect against water hammering, a common problem in pump applications.

    Closed-Loop
    Applications such as current limit use feedback from the motor or the line current/voltage to change the firing angle of the SCRs as necessary, hence closed-loop.

    Applications/Topologies
    Soft starters can be made for a reversing application by adding two extra SCR pairs that switch two phases. For example, line phase “b” is connected to load phase “c” and vice versa. L2 (“b”) is connected to T3 (“c”) and L3 (“c”) is connected to T2 (“b”) in Fig. 17. A delta configuration motor can also be controlled with a soft starter, but it will see more current than a line connected motor. As a way to get around the larger current switching, a soft starter can be wired “inside the delta”. Wiring in this configuration will allow the soft starter to control a larger motor than even line connected by a √3 advantage. For instance, an “inside the delta” soft starter can switch a 277A load versus a line connected soft starter needs to be able to switch 480A to control the same rated motor load. A disadvantage of “inside the delta” is that it requires six leads coming from the motor which can be an added expense with larger HP motors. In contrast, kick starting and low speed ramps are some other applications that can be implemented via different programming of SCR firing angles.




    Need for soft start
    Many electronic devices will consume a large initial current when first turned on (i.e. Inrush current) that can cause voltage fluctuations and affect the performance of other circuits connected to a common power supply. The source for this problem is often large capacitors with very low Input impedance. To counteract this issue, components can be added in series to throttle back the current initially as the device comes online.

    Soft starters are also used to start some types of lamps. A tungsten filament has a positive temperature coefficient of resistance: a cold filament has a smaller resistance by a factor of 8-10 than a hot filament, and allows a large inrush of current. This inrush coupled with uneven filament wear causes local temperature overshoot in hotspots during startup, further evaporating the thinner filament sections. While soft start has little effect on GLS lamp life, it can make a sizeable difference to a halogen lamp's life.

    High initial current can cause damage to other components such as semiconductors if they are not rated for the initial high current of loads such as filament lamps, motors or capacitors.

    Soft starts are sometimes used on larger equipment as well, such as electric motors in various applications. The current drawn by an electric motor during a start can be 2 to 10 times the normal operating current, and this can exceed the supply's ratings if not controlled.

    Finally soft start is widely used on hand held tools to prevent the tool being jerked out of position when switched on. Jumping of electric drills was a widespread issue in the 1970s when soft start was usually not fitted.

    Soft starting methods
    There are various ways to implement soft starting.

    The most popular for appliances is triac control which ramps up duty cycle over several cycles.

    A method used with some motors is to start with windings in series, switching them to parallel when partial speed has been reached. This is commonly known as Y-Δ start.

    Increasing supply impedance has a semi soft start effect on motors, with initial current still being above run current, but by a much reduced amount. This method is not widely used.

    Current limiting and voltage ramp-up in electronic supplies are common methods of soft starting low voltage loads.

    Logic Controller Application
    This soft start is provided by a dedicated output of a programmable logic controller. They can be used to start motors where Variable-frequency drive (VFD) are not used. This allows a cheaper starting option for motors that will run at a constant speed.[1









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