Skip to content

Soft Starters – Applications and Benefits

Soft Starters in Motor Controls

Old motor controls used an across-the-line starters or some type of Wye or Delta configuration to start and stop electric motors.  Across-the-line starters can produce in-rush currents up to 7 times the normal operating motor current and 3 times the starting torque when compared to motor operating torque.

A soft starter electronically controls the voltage applied to the motor via SCR’s during starting and stopping, which in turn controls the speed and current levels. 

The Science of Soft Starters

Soft starters avoid undue mechnical stress and line voltage fluctuation by limiting the starting current and torque. Phase angle motor controls reduce the motor voltage, and can be increased from an adjustable starting voltage up to the line voltage within an adjustable ramp time. The motor adjusts to the driven machine’s load behavior because of the smooth, gentle increase in voltage. Because this method requires less stress on the machine, equipment lasts longer and works better. Soft starters cushion the life of a machine and create an efficient production flow.

Types of Soft Starters



The majority of soft starters are installed in pump and fan applications, although they can be incorporated into many other motor applications that do not require speed control (where a VFD would be the best option). 

Soft Starters as Motor Controls


Example 1

Low End soft starter application on a Roller Conveyor

The direction of rotation of an 11 kW motor has to be adjustable in order to realize both conveyance directions.

  • Allows rapid but controlled acceleration to nominal operating speed without torque surges due to the flexability of setting the voltage / speed ramp during start-up.
  • Motor starting current is reduced
  • Feeder and motor protection available via external breaker and fuses
  • Reversing made possible with a reversing contactor

Low End Soft Starter Application on a Roller Conveyor


Example 2

Mid Range soft starter application for pumping applications:

With a rating of 200 kW, these soft starters can be used in the production of sheet parts, to drive the respective hydraulic systems for the presses.

  • Able to adjust current limiting set points to limit the current in-rush to better match the capabilities of the distribution transformer
  • Motor overload protection provided by integrated adjustable current settings within the soft starter and selectable trip classes
  • Adjustable voltage / speed ramp rates for acceleratioin and deceleration to elimate torque surges

Mid Range Soft Starter


Example 3

high end soft starter for a milling machine with dynamic DC braking

For the production of motor blocks, the required bores are drilled in the motor’s aluminum block by means of a milling head. Due to the milling head’s high inertia, shutdown of the 15 kW motor is subject to long stopping times, which cause long downtimes for tool changes and set-up operation.

  • Provides torque control and dynamic DC braking functionality
  • Belt slippage during start-up is elimated by the use for torque control and adjustable torque limits
  • Allows optimum acceleration control to bring the milling head up to nominal operating speed as soon as possible
  • High level of current limits to reduce motor current to a set maximum level
  • Optimum setting of dynamic DC braking functions to shut the milling head down as quickly as possible
  • Superior motor and device over-load protection provided by integrated functions within the soft starter.

High End Soft Starter Application


Soft Starters as Versatile Motor Controls

As you can see, soft starters can be adapted to serve a wide variety of applications in motor controls. The advantages of using soft starters as part of your suite of motor controls are clear: longer equipment life and smoother production flow, which translates into time and cost savings.

You may already be using soft starters for your pumps and fans, but if you're interested in learning more about the unique solutions soft starters can offer, our experienced staff can evaluate your situation and recommend the best motor controls solution.

Speak to an Expert

You may also be interested in reading:


Blog comments