Two Motors on One Soft Starter

Why is it necessary?

It's cheaper, and how.

Oh, go on. Do tell.

I can imagine one or more ways to do it (depending on some related factors--do you want to run the motors simultaneously, on the same process / shaft), but it would require extra equipment and is almost surely not worth the effort.

It's done all the time. There are caveats though:

1. The motors must be the same size. Mixing motor sizes generally ends up with some sort of unsatisfactory ramp of one or the other motors, or both.
2. Both motors must start at the same time. One can stop separately if you have separate contactors, but the soft starter has no way of re-starting a second motor once it's done ramping unless the first one is isolated from it after accelerating (see #6 below).
3. You need separate Over Load Relays for each motor, but tied into the soft starter to shut it down if EITHER motor overloads. If you have separate contactors you can shut down only one however. But if you do, and then start only one motor next time, your Current Limit settings will no longer be meaningful. There are however some soft starters that allow "dual ramps" so they can be configured to use a "1 motor" and "2 motor" ramp scenario if one of the OLRs trip.
   
4. Its a good idea to over size the soft starter by about 10% of the combined FLA of the motors. Soft starter overload ratings are a numbers game played by 90% of the manufacturers, with most having very little room for error. Having two or more motors increases the possibility of unbalanced currents which the soft starter may respond to by staying in Current Limit longer, which increases the stress on the soft starter.
5. You have to pay very close attention to the Short Circuit protection of each motor. Sometimes you can get lucky and have only one SCPD ahead of the soft starter that will fit within the code limits of the smaller motors, but sometimes not. If not, you have to have a Main SCPD for the soft starter and then SEPARATE systems (fuses or CBs) for each motor circuit, which again can drive the cost up higher than just buying two soft starters. If you are looking at small motors (i.e. 63A or under), a good compromise is an IEC Motor Protective Switch that has the SCPD and OL in one.
   
6. Forget the idea of "recycling" the soft starter. That power scheme requires more control complexity and contactors than most people realize. It ALWAYS ends up more expensive than just buying two sot starters.

For schematics ->http://www.global-download.schneider-electric.com/85257578007E5C8A/all/4DD2FCCC6F198939882575780075705B/$File/60524-en%20%28web%29.pdf (see page 6)

for Characteristics->http://www.global-download.schneider-electric.com/85257578007E5C8A/all/CB0D12A9FB39E18F88257577000A1861/$File/60521-en%20%28web%29.pdf

for brief description->

http://www.global-download.schneider-electric.com/85257578007E5C8A/all/A8973E39DA7BA02D882575790007BBF4/$File/60520-en%20%28web%29.pdf