Soft starters

Would you like to avoid suddenly occurring high starting currents when switching on an electric motor, reduce wear and tear on the motor and thus extend its service life? Find out here how a soft starter works and which criteria you can use to select such a device.

What is a soft starter?

Soft starters, soft starters or soft starters can basically be used in all electric motors. The devices are used in particular for asynchronous motors that are operated with alternating current. The starters work according to the fact that the torque developed by the electric motor is proportional to the square of the starting current and, correspondingly, proportional to the applied voltage. Thus, with a soft starter, by reducing the voltage at the moment the motor starts, you can adjust the torque and current. So, the task of these devices is to keep the motor parameters (current, voltage, torque and so on) within safe limits during the start. This is done to:

  • to relieve the engine drive train when starting
  • to limit the inrush current
  • to avoid a voltage drop in the mains voltage
  • prevent the miniature circuit breaker in the circuit from responding too quickly
  • prevent engine overheating
  • Eliminate dropouts and irregularities in the course of the mechanical drive
  • extend engine life

 

What is an asynchronous motor?

To understand the importance of soft starters, we need to know something about the motors they are used in. Electric motors run on either direct current or alternating current. When it comes to AC motors, we mainly differentiate between asynchronous motors and synchronous motors. The difference between the two motor variants lies in the detailed design and the interaction between the rotor and stators in the magnetic field generated.

Already knew?

In a synchronous motor, the rotor speed is always equal to the rotational frequency of the electromagnetic field. In an asynchronous motor, there is a difference between the rotational speeds of the rotor and the rotating magnetic field in the stator.


Asynchronous motors of small and medium power are the most commonly used electric motors and are used both in industry and in household appliances. In the industrial environment, three-phase asynchronous motors are used most frequently, for example as drives in construction, in traffic, in public utilities or as drives for water supply devices. The main problems of these engines are:

  • the high starting current, which can be many times higher than the nominal current and thus leads to problems with the performance stability
  • the inevitable difference between engine torque and load torque. When switching on, the torque is often available in fractions of a second. This sudden force can lead to failure of the drive train.

 

A soft starter avoids these problems by slowing the acceleration and deceleration of the motor. As a result, starting currents can be reduced and irregularities in the mechanical part of the drive as well as hydraulic shocks in lines and valves when starting and stopping the motors can be avoided.

What is the starting current?

The principle of operation of asynchronous motors is based on electromagnetic induction. The build-up of a back EMF from the application of a changing magnetic field during engine start-up causes transients in the electrical system.

 


Definition transient

Transients in electric motors are brief voltage peaks caused by switching on the electrical circuit.


 

These transients can affect power supplies and other connected devices. When starting, the engine accelerates to full speed. Meanwhile, the starting current can reach many times the full load current. During this time, the cables must carry more current than when they are running. The voltage drop in the system is also much greater when starting up. This becomes especially clear when starting a powerful power unit or a large number of engines at once.

This is why soft starters are increasingly being used when starting high-performance electric drives. The function of the starters is to supply voltage to the motor windings evenly from zero to nominal, which also allows the motor to accelerate smoothly to maximum speed. During the starting process, the soft starter gradually increases the applied voltage and the electric motor accelerates to the nominal speed without high torque peaks and current surges.

How a soft starter works

You have now learned about the main problems involved in starting induction motors. Soft starters counteract these problems and can be either mechanical or electrical or a combination of both. Mechanical soft starters directly counteract the sudden increase in engine speed by limiting the torque, for example, with brake pads, fluid couplings, magnetic locks or counterweights.

The electrically constructed devices gradually increase the current or voltage from an initially low level to a maximum voltage. This starts the motor smoothly and gradually accelerates it to its rated speed. These starters usually work with an amplitude control and can therefore also start up when idling or under load. Devices of the newer generation use phase control methods and also start drives with heavy starting.

Source: eibabo®, Siemens 3RW4037-1BB04 soft starterImage: Siemens 3RW4037-1BB04 soft starter

What types of soft starters are there?

Soft starters are phase controlled. Thus, three types of soft starters are used: devices with one, two and with all controlled phases. The first variant is applied to single-phase motors to provide reliable protection against overload and overheating, as well as to reduce the effects of electromagnetic interference. The devices of the second type usually contain a bypass contactor in addition to the semiconductor control board. After the motor has reached rated speed, the bypass contactor is activated and supplies DC voltage to the motor. The three-phase type is the optimal and most technically advanced solution. It provides reliable current and magnetic field strength limitation without phase distortion.

What should you consider when buying a soft starter?

The main feature of a soft starter is the current rating. This value should be 'many times' greater than the value of the current flowing through the motor winding. How high this 'multiple' is depends on the severity of the launch. When it comes to motors for fans or pumps, the starting current is about three times higher than the rated current. Certain saws or press machines are often devices with heavy starting. These are drives with a large moment of inertia. Their starting current is about five times higher than the rated current. For motors with particularly difficult starting, the starting current can be eight to ten times higher.

Please note

A soft start takes time and excess energy is converted into heat. To repeat the starting process, the starters must cool down. Therefore, if your process requires frequent switching on and off, choose a soft starter for heavy or extra heavy starting (even if your machine doesn't actually need it).


Decide on a device that controls the required number of phases. In addition, a soft starter works according to a preset program. In other words: The device increases the voltage to the nominal value within a certain time. An integrated control unit with feedback function allows you to control this process and compare voltage and torque or the differences between rotor and stator.

If necessary, pay attention to the ability of the starter to work when accelerating or braking. This would require an additional, integrated auxiliary contactor that bridges the main circuit so that it can cool down. This prevents phase asymmetry and overheating of the motor windings. With some models, you can set certain parameters manually using a rotary potentiometer on the device or digitally using a microcontroller. Consider whether you need additional functionality or features. These include:

  • a certain type of protection
  • the presence of power saving modes
  • restart capability
  • Working at reduced speed

 

 


TIP

A properly selected soft starter can double the service life of electric motors and save up to 30 percent electricity.


 

With a soft starter from the eibabo® online shop, you are opting for high-quality goods from well-known manufacturers such as Eaton, Schneider, ABB or Siemens. If you want to control the speed of a machine not only at the beginning, but permanently, then the use of a frequency converter would be an alternative for you. Frequency converters are also available here in the online shop at top conditions.

 

eibabo - technology store

 

Catalogue content:


In this eibabo® catalogue Low-voltage industrial components > Soft starter you will find items from the following product groups:

Item overview:

  • Direct soft starter
  • Direct starter
  • Drive technology
  • Engine starter
  • Performance starter
  • Soft start
  • Soft starter
  • Starting device
  • Torque control


from the following manufacturers:

Manufacturer overview catalogue Soft starters:

  • ABB
  • Dold
  • Eaton
  • Frankonia
  • Kaleja
  • Rockwell
  • Schneider Electric
  • Siemens


 
Would you like to avoid suddenly occurring high starting currents when switching on an electric motor, reduce wear and tear on the motor and thus extend its service life? Find out here how a soft... read more »
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Soft starter – gentle commissioning of electric motors

Would you like to avoid suddenly occurring high starting currents when switching on an electric motor, reduce wear and tear on the motor and thus extend its service life? Find out here how a soft starter works and which criteria you can use to select such a device.

What is a soft starter?

Soft starters, soft starters or soft starters can basically be used in all electric motors. The devices are used in particular for asynchronous motors that are operated with alternating current. The starters work according to the fact that the torque developed by the electric motor is proportional to the square of the starting current and, correspondingly, proportional to the applied voltage. Thus, with a soft starter, by reducing the voltage at the moment the motor starts, you can adjust the torque and current. So, the task of these devices is to keep the motor parameters (current, voltage, torque and so on) within safe limits during the start. This is done to:

  • to relieve the engine drive train when starting
  • to limit the inrush current
  • to avoid a voltage drop in the mains voltage
  • prevent the miniature circuit breaker in the circuit from responding too quickly
  • prevent engine overheating
  • Eliminate dropouts and irregularities in the course of the mechanical drive
  • extend engine life

 

What is an asynchronous motor?

To understand the importance of soft starters, we need to know something about the motors they are used in. Electric motors run on either direct current or alternating current. When it comes to AC motors, we mainly differentiate between asynchronous motors and synchronous motors. The difference between the two motor variants lies in the detailed design and the interaction between the rotor and stators in the magnetic field generated.

Already knew?

In a synchronous motor, the rotor speed is always equal to the rotational frequency of the electromagnetic field. In an asynchronous motor, there is a difference between the rotational speeds of the rotor and the rotating magnetic field in the stator.


Asynchronous motors of small and medium power are the most commonly used electric motors and are used both in industry and in household appliances. In the industrial environment, three-phase asynchronous motors are used most frequently, for example as drives in construction, in traffic, in public utilities or as drives for water supply devices. The main problems of these engines are:

  • the high starting current, which can be many times higher than the nominal current and thus leads to problems with the performance stability
  • the inevitable difference between engine torque and load torque. When switching on, the torque is often available in fractions of a second. This sudden force can lead to failure of the drive train.

 

A soft starter avoids these problems by slowing the acceleration and deceleration of the motor. As a result, starting currents can be reduced and irregularities in the mechanical part of the drive as well as hydraulic shocks in lines and valves when starting and stopping the motors can be avoided.

What is the starting current?

The principle of operation of asynchronous motors is based on electromagnetic induction. The build-up of a back EMF from the application of a changing magnetic field during engine start-up causes transients in the electrical system.

 


Definition transient

Transients in electric motors are brief voltage peaks caused by switching on the electrical circuit.


 

These transients can affect power supplies and other connected devices. When starting, the engine accelerates to full speed. Meanwhile, the starting current can reach many times the full load current. During this time, the cables must carry more current than when they are running. The voltage drop in the system is also much greater when starting up. This becomes especially clear when starting a powerful power unit or a large number of engines at once.

This is why soft starters are increasingly being used when starting high-performance electric drives. The function of the starters is to supply voltage to the motor windings evenly from zero to nominal, which also allows the motor to accelerate smoothly to maximum speed. During the starting process, the soft starter gradually increases the applied voltage and the electric motor accelerates to the nominal speed without high torque peaks and current surges.

How a soft starter works

You have now learned about the main problems involved in starting induction motors. Soft starters counteract these problems and can be either mechanical or electrical or a combination of both. Mechanical soft starters directly counteract the sudden increase in engine speed by limiting the torque, for example, with brake pads, fluid couplings, magnetic locks or counterweights.

The electrically constructed devices gradually increase the current or voltage from an initially low level to a maximum voltage. This starts the motor smoothly and gradually accelerates it to its rated speed. These starters usually work with an amplitude control and can therefore also start up when idling or under load. Devices of the newer generation use phase control methods and also start drives with heavy starting.

Source: eibabo®, Siemens 3RW4037-1BB04 soft starterImage: Siemens 3RW4037-1BB04 soft starter

What types of soft starters are there?

Soft starters are phase controlled. Thus, three types of soft starters are used: devices with one, two and with all controlled phases. The first variant is applied to single-phase motors to provide reliable protection against overload and overheating, as well as to reduce the effects of electromagnetic interference. The devices of the second type usually contain a bypass contactor in addition to the semiconductor control board. After the motor has reached rated speed, the bypass contactor is activated and supplies DC voltage to the motor. The three-phase type is the optimal and most technically advanced solution. It provides reliable current and magnetic field strength limitation without phase distortion.

What should you consider when buying a soft starter?

The main feature of a soft starter is the current rating. This value should be 'many times' greater than the value of the current flowing through the motor winding. How high this 'multiple' is depends on the severity of the launch. When it comes to motors for fans or pumps, the starting current is about three times higher than the rated current. Certain saws or press machines are often devices with heavy starting. These are drives with a large moment of inertia. Their starting current is about five times higher than the rated current. For motors with particularly difficult starting, the starting current can be eight to ten times higher.

Please note

A soft start takes time and excess energy is converted into heat. To repeat the starting process, the starters must cool down. Therefore, if your process requires frequent switching on and off, choose a soft starter for heavy or extra heavy starting (even if your machine doesn't actually need it).


Decide on a device that controls the required number of phases. In addition, a soft starter works according to a preset program. In other words: The device increases the voltage to the nominal value within a certain time. An integrated control unit with feedback function allows you to control this process and compare voltage and torque or the differences between rotor and stator.

If necessary, pay attention to the ability of the starter to work when accelerating or braking. This would require an additional, integrated auxiliary contactor that bridges the main circuit so that it can cool down. This prevents phase asymmetry and overheating of the motor windings. With some models, you can set certain parameters manually using a rotary potentiometer on the device or digitally using a microcontroller. Consider whether you need additional functionality or features. These include:

  • a certain type of protection
  • the presence of power saving modes
  • restart capability
  • Working at reduced speed

 

 


TIP

A properly selected soft starter can double the service life of electric motors and save up to 30 percent electricity.


 

With a soft starter from the eibabo® online shop, you are opting for high-quality goods from well-known manufacturers such as Eaton, Schneider, ABB or Siemens. If you want to control the speed of a machine not only at the beginning, but permanently, then the use of a frequency converter would be an alternative for you. Frequency converters are also available here in the online shop at top conditions.

 

eibabo - technology store

 

Catalogue content:


In this eibabo® catalogue Low-voltage industrial components > Soft starter you will find items from the following product groups:

Item overview:

  • Direct soft starter
  • Direct starter
  • Drive technology
  • Engine starter
  • Performance starter
  • Soft start
  • Soft starter
  • Starting device
  • Torque control


from the following manufacturers:

Manufacturer overview catalogue Soft starters:

  • ABB
  • Dold
  • Eaton
  • Frankonia
  • Kaleja
  • Rockwell
  • Schneider Electric
  • Siemens
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RECOMMENDED
3D
Siemens - 3RW4037-1BB04 - Soft starter Sirius 30kW/400V,24V - Soft starter 63A 24VAC 24VDC 3RW4037-1BB04
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Soft starter Sirius 30kW/400V,24V - Soft starter 63A 24VAC 24VDC
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