VFD versus Soft Start

[linus2003]

I am new to the water industry. Would like to know more about the differences between VFD and soft start. Terms that are frequenly used in the industry. Trying to get a layman's perspective on this.

 

[BigInch]

In a nutshell,

Consider soft start to save on starting current draws when you start up large electrical equipment.

Consider VFD/VSD ONLY when you have variable flow requirements that can range from 50%-90% of BEP flow (70 to 90% best range) and the pump still produces enough head to move water in your pipe system, as opposed to a soft starter with a constant speed pump.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[semo]

In laymans terms:

Softstart runs a motor at one constant speed; however, it ramps the speed up/down slowly during start/stop to save electrical costs.

VFD can operate the motor over a wide range of speeds as well as ramping the speed up/down during start and stop. This allows a pump to more accurately match the demand as well as save electrical costs.

 

[bimr]

A soft start is a reduced voltage motor controller for medium voltage ac motor applications. The unit controls the motor start-up by delivering an adjustable amount of initial voltage and current to the motor, and then slowly increases the voltage and current to 100 percent. The acceleration ramp time for a typical start-up is 30 seconds or less and is adjustable to allow the motor to smoothly accelerate the load. At the end of the start cycle the unit will switch in a bypass contact, placing the unit across the line. The motor is then running at a constant speed.

A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive.

http://en.wikipedia.org/wiki/Variable-frequency_drive