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Soft starting application
Soft starting application
Motor and Machine

Since the three-phase motor was invented over 100 years ago, it has continually established itself for driving all types of machines, and today, represents the classic drive concept.

If motors are directly powered-up direct-on-line, the machine is subject to a torque peak which shortens its lifetime. The high inrush current stresses the supply and causes voltage dips. This has a negative impact on the function of sensitive loads connected to the supply.

In the Past: Correct motor starting - a difficult issue!

The demand for solutions to provide satisfactory starting characteristics, which would eliminate the undesirable secondary effects, accompanied the three-phase motor as it established itself in the market. Concepts were developed using electrical devices in the supply as well as mechanical techniques.

An overview of the various methods electrical starting types is shown in the following table. The characteristics of each method of starting can be seen by examining each current- or torque characteristic.

With the exception of the electronic soft starter, all the above methods have their own specific disadvantages. Significant step changes in torque and/or high currents/current peaks can be seen in the above characteristics.

  • Step changes in torque result in high mechanical stressing of the machine, higher service costs, more wear.
  • High currents/current spikes result in high power company tariffs (peak current calculation) and high line supply- or generator load.

If the supply is overloaded as a result of a high current, voltage dips occur, which can have a negative impact on the other loads.

Motor starting today: Electronic soft starters!

A smooth torque ramp-up as well as a reduced starting current are required. The electronic soft starter permits this:

- a starting technique, which sets new standards.

The electronic soft starter continuously controls the three-phase motor voltage supply when starting. This means that the motor is adapted to the load characteristics of the driven machine.

Mechanical equipment is accelerated with especially low stressing of the system. Lifetime, operating characteristics
and processes are positively influenced, i.e. negative effects are eliminated,
such as:

- reduced stressing of gearwheel teeth
- pressure surges in piping systems are reduced
- 'V' belts no longer slip causing high noise and wear
- transport equipment such as conveyors start slowly without any jerking

The accepted world standard for electronic soft starters is IEC 947-4-2 which corresponds to EN 60947-4-2 in Europe.

Available types of soft starters

Two types of soft starters are generally available:
· Electronic soft starters for basic tasks:

  • This type is used at low output powers.
  • These soft starters have proven themselves for simple applications where smooth, jerk-free starting is important.

· Electronic soft starters for more complex applications:

  • the output range extends up to 800 kW (compact units) and for plant/
  • system technology up into the MW range.
  • The units always have an adjustable current limiting.
  • Other important features of these devices are:
    • monitoring functions (supply, device)
    • motor protective functions (if required, optional)
    • parameterizing adjustments to optimize the starter-machine function
    • control command
    • monitoring output contacts
    • e.g. for:
      • controlling an external bypass contactor (eliminating power loss in continuous operation),
      • operating messages and signals,
      • faults (e.g. supply, soft starter, motor).

Electronic soft starters are the motor starters for today and the future!

In conformance with the standards

Leading manufacturers of electronic soft starters have formed a working group in the German Electrical and Electronics Manufacturers Association (ZVEI). The manufacturer’s declaration of conformance documents that the standard is maintained, therefore securing a high level of quality.

The electronic soft starter and its applications

Flexible adaptation to the type of load:

High-quality electronic soft starters can be adapted to the requirements of the particular application. The overview below provides a summary of the loads, operating mode (standard- or heavy starting duty), as well as the advantages when using electronic soft starters. For further assistance, typical maximum starting currents are listed, which are necessary in conjunction with a soft starter for acceleration up to nominal operation.!

Load Starting duty Advantages when using 
electronic soft starters
Typical starting current I
(% of Ie-Motor)
Centrifugal pump Standard Prevents pressure surges 
Extends the lifetime of piping systems, valves etc.
Reciprocating pump Standard Prevents pressure surges
Extends the lifetime of piping systems, valves etc.
Fan Standard Lower stressing on belts/gearboxes 300
transport system
Standard Jerk-free starting
More favorably-priced belt materials can be used
Circular saw,
belt saw
Standard or heavy duty starting
Reduced starting current 300
Mixer Standard Reduced starting current 350
Mill, crusher Heavy duty starting Reduced starting current 400...450
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