soft starter

A Soft Starter is a little bit of a "catch all" term, meaning that several devices may be described that way. But in general, a Soft Starter is something that starts a motor at a reduced voltage for a short period of time. Unfortunately, this also means reduced torque which can be a problem.

A variable frequency drive, on the other hand, provides a higher voltage for higher torque but at a lower frequency that ramps up to the desired frequency. It has the advantage of running a motor slower or faster so that it is unnecessary to change gears or belts, etc.

A lot of manufacturing devices use a VFD because it is a popular approach to begin at a lower speed and then take cost out by increasing speed. Not always a good idea because higher speeds always expose any shortcoming in the design of a process.

Most motor starters are of the Direct On Line type (DOL) which is the simplest and most economical starting mode. However, if the load does not allow a large starting torque or the line supplying the power does not withstand a current of up to 10 times the nominal (10xIn) you cannot use a DOL starter. In this case, an alternative would be to use a Star-Delta starter, which reduces by 3 the peak starting current. However, Star-Delta can only be used whenever the starting load is zero or its torque does not exceed 1/3 of In, and the line supplying the power can withstand the peak starting current.

An example: http://www.schneider-electric.ae/sites/uae/en/products-services/automation-control/products-offer/range-presentation.page?c_filepath=/templatedata/Offer_Presentation/3_Range_Datasheet/data/en/local/automation_and_control/tesys_u.xml&p_function_id=9&p_family_id=141&p_range_id=682&f=F13%3AAutomation%20and%20Control~!F23%3AMotor%20%26%20Motion%20Control~!NNM1:Motor+Starters~!NNM2:On-Off+Motor+Starter~!NNM3:TeSys+U

If this is not the case, then the only solution is to use a Soft Starter. The Soft Starter will supply the motor with progressively increasing voltage. Soft Starters are most suitable for "turbine machines" such as centrifugal pumps and fans. An example of use is in water pipes to avoid the hammer effect that can break the elbows in a water canalisation.

An example: http://www.schneider-electric.ae/sites/uae/en/products-services/automation-control/products-offer/range-presentation.page?c_filepath=/templatedata/Offer_Presentation/3_Range_Datasheet/data/en/local/automation_and_control/altistart_48.xml&p_function_id=1&p_family_id=100&p_range_id=689&f=F13%3AAutomation%20and%20Control~!F23%3AMotor%20%26%20Motion%20Control~!NNM1:Motion+%26+Drives~!NNM2:Soft+Starters~!NNM3:Altistart+48

A Variable Speed Drive is able to start a motor the same way as a Soft Starter but it is also able to increase and reduce voltage in order to control the motor speed at will. Using a variable Speed Drive allows, for example, to vary the speed of a pump in order to increase or reduce pumping according to the need.

An example: http://www.schneider-electric.ae/sites/uae/en/products-services/automation-control/products-offer/range-presentation.page?c_filepath=/templatedata/Offer_Presentation/3_Range_Datasheet/data/en/local/automation_and_control/altivar_71.xml&p_function_id=1&p_family_id=223&p_range_id=1155&f=F13%3AAutomation%20and%20Control~!F23%3AMotor%20%26%20Motion%20Control~!NNM1:Motion+%26+Drives~!NNM2:Standard+Drives~!NNM3:Altivar+71

A Variable Speed Drive is usually much more expensive than a Soft Starter.