What is meant by the term "soft start"?

[KernOily]

OK, dumb question here... what is meant by the terms "soft start" and "across the line"? This is in regard to the starting up of a 100-hp centrifugal pump.

Is special equipment or switchgear required for soft start, or can it be done with the regular motor starter/switchgear?



Thanks!
Pete

 

[d23]

74Elsinore:

Across the line or D-O-L is a contactor that instantaneously closes across the line. It's like flipping a light switch.

Soft Start as a rule refers to an electronic soft starter. A typical soft starter uses SCR's to start a motor. The SCR's limit the voltage to the motor or load. By limiting the voltage it will also limit in inrush current to the load.

Soft Start is a very loose term. In the 1950' a soft start would have been a reactor starter. Reactors are still available today.

Soft Starter could also include an auto-transformer starter etc.

Most induction motors will require anywhere from 4 to 10 times their full load amps to start. A soft starter is used to limit this high in-rush current.

There are a lot of internet sites that can explain the different types of soft starters available. You may just want to do a search.

Reliance motor use to have a lot of motor and starter info. AB did too.

Good Luck!

 

[CB2]

To add a bit:
1 Soft starters (Aka solid state starter) is a "reduced voltage starter". Available in "combination starter" configuration, with CB and OL.
2 Soft starters are programmable to "ramp" voltage up over time, 0-60 sec. Same for a "stop" if desired.
3 #2 results in a "soft" acceleration up to (or down) speed, thus a soft mechanical or hydraulic system effect.

Note that a "start" is a "start", the energy required to start the motor is basically the same, just consumed over a longer period of time.

If you are being required to provide a "reduced voltage" start by your utility, at this HP a traditional RVAT will be less cost.

 

[dadfap]

There are simpler reactive type softstarters that use a series of taps that allow the user to set the reactor winding. These are usually an oil filled reactor tank c/w current sensing relays and vacuum contactors.

You can then calculate the voltage sag required and adjust accordingly.

There are some "RULES OF THUMB" when using these beasties
1. The motor will draw 6 times the nameplate when 100% of nameplate voltage is applied.
2. The motor will draw times the nameplate when 80% of nameplate voltage is applied.
3. The motor will draw 3 times the nameplate when 60% of nameplate voltage is applied.
4. The motor will draw 2 times the nameplate when 40% of nameplate voltage is applied.

These are not as sexy as either a VFD or solid state soft starter but are robust, reliable and somewhat cheaper.

If you need any contact details for a manufacturer, just let me know.

All the best

dadfap