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A softstarter has different characteristics to the other starting methods. It has thyristors in the main circuit, and the motor voltage is regulated with a printed circuit board. The softstarter makes use of the fact that when the motor voltage is low during start, the starting current and starting torque is also low.

During the first part of the start the voltage to the motor is so low that it is only able to adjust the play between the gear wheels or stretching driving belts or chains etc. In other words, eliminating unnecessary jerks during the start. Gradually, the voltage and the torque increase so that the machinery starts to accelerate.

One of the benefits with this starting method is the possibility to adjust the torque to the exact need, whether the application is loaded or not. In principle the full starting torque is available, but with the big difference that the starting procedure is much more forgiving to the driven machinery, with lower maintenance costs as a result.

Another feature of the softstarter is the softstop function, which is very useful when stopping pumps where the problem is water hammering in the pipe system at direct stop as for star-delta starter and direct-on-line starter. The softstop function can also be used when stopping conveyor belts to prevent material from damage when the belts stop too quickly.


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Single line diagram for a softstarter

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Design, settings and signals


A softstarter in general is built up with a few main components such as a printed circuit board (PCB), heat sink, thyristors, fans and housing (plastic or metal). The controlling circuits can be of digital type, analogue type or a combination of these. The output signal relays can be of a type with fixed function or as a free programmable type where the user can decide upon the output function.

The softstarter is sometimes equipped with a built-in electronic overload relay (EOL) replacing the conventional bi-metal relay which is normally used. A built in EOL has much better accuracy than a conventional relay, since the values are calculated electronically and this is especially useful when on intermittent duty.

The need for communication between different devices in a plant and from the devices to a control board is increasing all the time. Many of today’s softstarters are equipped with a port for such communication, which normally consists of a few fibre optic cables instead of former solutions, which often reqired hundreds of thousands of wires. Many different communication protocols exist today and some of them are more common than others, for example Modbus, Profibus, DeviceNet, Interbus-S, LON Works and so on.

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Printed circuit board is used to control the firing of the thyristors based on the current and voltage references, and also for the calculation of different values, for example the power factor, active power, etc. It can also be used for storing historical data, the event log, indicating trends and much more.

Heat sink is used to get rid of the heat in the softstarter generated by the current during the start and the continuous run. The capacity of the heat sink very much reflects the starting capacity and the operational current of the softstarter.

Fans are used to increase the cooling capacity of the heat sink. One, two or several fans can be used depending on size and design. Some smaller softstarters don’t have fans at all and the number of starts may be limited.

Housing can be made of plastic material, metal or a combination of these, and its function is to protect the inside components from

mechanical and electrical damage. It is also used to protect the components from dust and dirt. For total outside protection from dust and dirt a separate enclosure is often required since the degree of protection (IP class) of the unit itself is too low.

Thyristors are semi-conducting components connected in an anti-parallel fasion and placed in two or three phases of the main circuit. They regulate (by increasing or decreasing) the level of voltage during start and the stop ramp, as described in the picture below. During a continuous run the thyristors are conducting fully.

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Common settings


This section includes a short description of some common setting parameters available on most of the softstarters. Other settings may be available depending on the type of softstarter and manufacturer. The setting can be done either by adjusting potentiometers, changing dip switches, using a key pad, a computer or similar.


Start ramp is the time from were the softstarter start its ramp (initial voltage) until full voltage is reached. The ramp time should not be too long, as this will only result in unnecessary heating of the motor and a risk of the overload relay to trip. If the motor is unloaded the start time for the motor will probably become shorter than the set ramp time, and if the motor is heavily loaded, the start time will probably become longer.

Stop ramp is used when a soft stopping of the motor is required, for example a pump or a conveyor belt. The stop ramp is the time from full voltage until stop voltage (initial voltage) is reached. If the ramp time is set to zero the stop will be like a direct stop.

Initial voltage. Sometimes named pedestrian voltage or torque, this is the point from where the softstarter starts or stops its ramps. The torque of the motor will drop with the square of the voltage and if the voltage is set too low, for example 20 %, the starting torque will become 0.22 = 0.04 = 4 % only, and the motor will not start from the very beginning. Therefore it is very important to find a level that is just high enough to make the motor take off directly to avoid unnecessary heating.

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Current limit can be used in applications where a limited starting current is required, or at a heavy-duty start when it is difficult to achieve a perfect start with the setting of the initial voltage and the start ramp only. When the current limit is reached, the softstarter will temporarily stop increasing the voltage until the current drops below the set limit, and then continues ramping up to full voltage.


Note that this feature is not available on all softstarters.

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Current limit function in softstarter use


Step down voltage gives a special type of stop ramp. It is possible to adjust the voltage to drop to a level where the speed of the motor starts to reduce immediately at the stop command. For low loaded motors the speed will not reduce until a very low voltage is reached, but using the step down voltage function can eliminate this phenomenon and is especially useful for stopping pumps.


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Curve showing the step down voltage function



Adjustable rated motor current makes it possible to set the motor rated current on the softstarter for the used motor. This setting may affect other values as well, such as the trip level of the electronic overload relay, the level of the current limit function and so on.


Different indications


The indications on a softstarter differ very much from one type to another and also between manufacturers. Some of the most common indications are described below.

On normally indicates that the power supply is connected to the softstarter and that the unit is ready to start the motor.

Top of Ramp indicates that the start ramp is completed and full voltage is reached. If a by-pass contactor is used it will be activated at this point.

Fault indication can be of many different types. One is if there is an internal fault on the

softstarter itself, a fault on the feeding side (phase loss, blown fuse or similar) or on the motor side (motor not connected, phase missing etc.)

Overload indicates that the overload protection has tripped. The reason for a tripping overload can be too high motor current, too long starting time, too many starts after each other, wrong set overload, wrong trip class of overload or a combination of these.

Overtemperature indicates that the softstarter unit is over-heated, due to the number of starts exceeded, too high-rated current, too long starting time or similar.


Different voltages


Different named voltages are used for the softstarters. The name and use of these different voltages is stated in the IEC-standard as below

Main Voltage (Ue), which is the voltage feeding the motor and also the voltage exposed to the main circuit (thyristors) in the softstarter. 200 - 690 V are normal values.

Supply voltage (Us), which is the voltage feeding the electronic components inside the softstarter, for example the printed circuit board. Common values are 110 - 120 V or 220 - 240 V.

Control Voltage (Uc), which is the voltage for controlling the start and stop command of the softstarter. Values between 24 - 480 V exist.

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Main voltage and supply voltage to a softstarter

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Main voltage and control voltage to a softstarter


Soft starters are based on semiconductors. Via a power circuit and a control circuit, these semiconductors reduce the initial motor voltage. This results in lower motor torque. During the starting process, the soft starter gradually increases the motor voltage, thereby allowing the motor to accelerate the load to rated speed without causing high torque or current peaks.

Soft starters can also be used to control how processes are stopped. Soft starters are less expensive than frequency converters.



They do, however, share the same problem as frequency converters: they may inject harmonic currents into the system, and this can disrupt other processes.

The starting method also supplies a reduced voltage to the motor during start-up. The soft starter starts up the motor at reduced voltage, and the voltage is then ramped up to its full value. The voltage is reduced in the soft starter via phase angle. In connection with this starting method current pulses will not occur. Run-up time and locked-rotor current (starting current) can be set.