Jump to content


Diffrence b/w VSD & Soft starter ?

  • Please log in to reply
4 replies to this topic

#1 sethar


    Junior Member

  • Full Member
  • PipPip
  • 8 posts
  • Location:karachi
  • Interests:Associate Electrical engineer

Posted 22 August 2003 - 08:16 AM

I want to know that what is the diffrence between Variable Speed Drives and Soft Starters ? Define basic diffrence in short please ?

#2 normannicklette


    Junior Member

  • Members
  • PipPip
  • 3 posts

Posted 24 August 2003 - 05:05 AM

Hi Sethar

The simplest explaination would be that a VSD is device for varying the speed of a drive either at startup, during operation or during shutdown, while a soft starter is a simpler device(similar to a VSD as far as controlling the voltage and frequency of supply) but is normally used during startup.

#3 marke


    Posting Freak

  • Moderator
  • PipPipPipPipPipPip
  • 2,651 posts
  • Gender:Male
  • Location:Christchurch, New Zealand

Posted 24 August 2003 - 05:16 AM

Hello Sethar

A soft starter comprises one or more SCR pairs connected in series with the supply to the motor. These SCRs are used to reduce the voltage applied to the motor during start. The reduced voltage results in a reduced starting current and torque.
A variable speed drive is an inverter that rectifies the incoming supply, generating a high voltage DC. This DC is then chopped up converting the DC back into AC of variable voltage and frequency. The speed of the motor is altered by altering the frequency applied to it.

Best regards,

#4 jraef


    Posting Freak

  • Moderator
  • PipPipPipPipPipPip
  • 683 posts
  • Gender:Male
  • Location:USA, California

Posted 05 September 2003 - 10:07 PM

Just to correct a slight mistake in the first response, a Soft Starter cannot change the output frequency, only the voltage. For that reason, a Soft Start cannot be used to change motor speed. It simply ramps the motor up (or down) but only operates continuously at full speed.
"He's not dead, he's just pinin' for the fjords!"



    Senior Member

  • Moderator
  • PipPipPipPipPip
  • 407 posts
  • Location:Melbourne

Posted 24 September 2003 - 02:15 AM

Hello Sethar,

In very simplistic terms, the function of each is as follows;

A soft starter is used to accelerate a motor/load from zero speed to rated speed over a user adjustable period of time. The process is achieved by controlling the voltage applied to the motor terminals during starting.

A variable frequency drive enables the user to control the speed of the motor/load. The process is achieved by varying both the voltage and frequency (Hz) of the supply to the motor.


0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users