Soft start
Encyclopedia
Soft start is a term describing any electrical circuit which reduces excess current
during initial power-up of an electric motor.
. 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.
s (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.
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)/Kilowatt (kW) 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:
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.
, 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.
All control schemes listed monitor back EMF of the motor as to not become unstable.
In contrast, kick starting and low speed ramps are some other applications that can be implemented via different programming of SCR firing angles.
) 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 lamp
s. 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 semiconductor
s 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.
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.
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
during initial power-up of an electric motor.
Introduction
In industrial applications, electric motors are used extensively and represent a significant percentage of total industrial electrical load . There are generally three different ways to start a motor: full-voltage, reduced voltage, and inverterInverter
Inverter may refer to* Inverter , a device that converts direct current to alternating current** Inverter , an air conditioner that can continuously regulate its output by altering the compressor speed in response to cooling demand** Uninterruptible power supply, which often are based on an...
. A full voltage, across-the-line, or direct on-line (DOL) start uses a contactor
Contactor
A contactor is an electrically controlled switch used for switching a power circuit, similar to a relay except with higher current ratings. A contactor is controlled by a circuit which has a much lower power level than the switched circuit....
, 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
Solid state
Solid state may refer to:In science:* Solid-state chemistry* Solid-state physics* Solid-state laser* Solid matterIn electronics:* Solid state , circuits built of solid materials* Solid-state fan...
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 rectifierSilicon-controlled rectifier
A silicon-controlled rectifier is a four-layer solid state device that controls current. The name "silicon controlled rectifier" or SCR is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W...
s (SCRs), which are a type of high speed thyristor
Thyristor
A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...
. 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 currentInrush current
Inrush current, input surge current or switch-on surge refers to the maximum, instantaneous input current drawn by an electrical device when first turned on. For example, incandescent light bulbs have high inrush currents until their filaments warm up and their resistance increases...
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
Power outage
A power outage is a short- or long-term loss of the electric power to an area.There are many causes of power failures in an electricity network...
”. 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
Horsepower
Horsepower is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The unit was widely adopted to measure the...
(HP)/Kilowatt (kW) 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:
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
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
, 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 triacTriac
Triac may refer to:* TRIAC , an electronics component* Triac , a green vehicle* Tiratricol, a common thyroid hormone analogue used for treating thyroid hormone resistance syndrome...
, 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 currentInrush current
Inrush current, input surge current or switch-on surge refers to the maximum, instantaneous input current drawn by an electrical device when first turned on. For example, incandescent light bulbs have high inrush currents until their filaments warm up and their resistance increases...
) that can cause voltage fluctuations and affect the performance of other circuits connected to a common power supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
. The source for this problem is often large capacitors
Capacitor (component)
Practical capacitors are often classified according to the material used as the dielectric, with the dielectrics divided into two broad categories: bulk insulators and metal-oxide films .-Capacitor construction:...
with very low Input impedance
Input impedance
The input impedance of an electrical network is the equivalent impedance "seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law...
. 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 lamp
Lamp (electrical component)
A lamp is a replaceable component such as an incandescent light bulb, which is designed to produce light from electricity. These components usually have a base of ceramic, metal, glass or plastic, which makes an electrical connection in the socket of a light fixture. This connection may be made...
s. 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
Halogen lamp
A halogen lamp, also known as a tungsten halogen lamp, is an incandescent lamp with a tungsten filament contained within an inert gas and a small amount of a halogen such as iodine or bromine. The chemical halogen cycle redeposits evaporated tungsten back on to the filament, extending the life of...
's life.
High initial current can cause damage to other components such as semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
s 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
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
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
Triac
Triac may refer to:* TRIAC , an electronics component* Triac , a green vehicle* Tiratricol, a common thyroid hormone analogue used for treating thyroid hormone resistance syndrome...
control which ramps up duty cycle
Duty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....
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
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
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
Current limiting
Current limiting is the practice in electrical or electronic circuits of imposing an upper limit on the current that may be delivered to a load with the purpose of protecting the circuit generating or transmitting the current from harmful effects due to a short-circuit or similar problem in the load...
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.http://www.latticesemi.com/documents/I0191.pdfSee also
- Motor soft starterMotor soft starterA motor soft starter is a device used with AC electric motors to temporarily reduce the load and torque in the powertrain of the motor during startup...
- Inrush current limiterInrush current limiterAn inrush current limiter is a component used to limit inrush current to avoid gradual damage to components and avoid tripping the supply's fuse or circuit breaker. Negative temperature coefficient thermistors and fixed resistors are often used for this. Less often, other components are also used...
- Korndorfer starterKorndorfer starterKorndorfer starter is a technique used for soft starting of induction motors. The circuit uses a three-phase autotransformer and three three-phase switches....