Softstart Question

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Mcamnl

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Can multiple motors be started with a single softstart?
We are putting together is a system that is going to be controlling 3 cooling fans. What our controls guy wants to do is use one softstart to feed 3 manual motor starters. Each MMS will then be tied to a fan motor. The softstart will be sized according to the total FLA of the 3 motors.
I don't have much experience with softstarts yet and want to make sure we do this right.
Thanks for your help.
 
Mcamnl said:
I think I found my answer here.
http://www.galco.com/circuit/start_ac.htm
Click to expand...
LOL, that "Tech Tip" is a direct excerpt, including the drawing, of a white paper I wrote for another manufacturer on the subject. They left out other details but it covers most of the important stuff.

As long as everyone is OK with the fact that the starters MUST all start at the same time with the soft starter, no problem. The only thing you should avoid is starting one, then adding in one or more of the others. A) the soft starter will of course NOT soft start any subsequent add-in, and B) the high DI/DT from starting the subsequent motors across the line may do incremental damage to the SCRs in the soft starter. So that means if one of your manual motor starters trips off line, no problem, but to re-start it, you would shut them all down, close the MMS and then restart them all.
 
Jraef said:
As long as everyone is OK with the fact that the starters MUST all start at the same time with the soft starter, no problem.
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I have seen a few schemes where one soft start was used for more than one motor, starting at different times.
One motor would be run up to speed and then the soft start was bypassed and disconnected from the motor so it was then available for another start on a different motor.
I have a vague recollection that we may have done that ourselves but it would have been a long time ago. The reason for such a system was to save money on component cost. Soft starts and power electronics in general has greatly reduce in relative terms so the economic case for "sharing" a soft start is no longer in the least compelling - if it ever was.
 
From the link
...start multiple motors in parallel provided the motors are mechanically connected to the same load...

How literal should we take that?

1. Same shaft?
2. Same piece of equipment/enclosure, but separate loads? (say three fans in a unit...)
3. Three entirely separate fans/locations?
4. Any of the above?
 
The system is designed to have all three start at the same time and if one motor faults out then the other two will shut down. All three motor starters will need to be good for the system to start.
 
wasasparky said:
From the link
...start multiple motors in parallel provided the motors are mechanically connected to the same load...

How literal should we take that?

1. Same shaft?
2. Same piece of equipment/enclosure, but separate loads? (say three fans in a unit...)
3. Three entirely separate fans/locations?
4. Any of the above?
Click to expand...
Any of the above. Just as long as they start and run at the same time. As I said, that excerpt was taken from a much longer paper dealing with the multiple motor issue. ONE of the applications is where they are both on the same shaft, as in belted together. But I have done many systems, especially cooling fans, where a lot of small fans are started and run together.

Generally its a good idea to keep controls for a machine on or associated with that machine, but that's just good engineering practice, not a technical requirement.
 
Besoeker said:
... One motor would be run up to speed and then the soft start was bypassed and disconnected from the motor so it was then available for another start on a different motor.
Click to expand...
I have dis-proven the economics of this time and time again for low voltage soft starters. If it is done correctly, the cost of the necessary switching and isolation components and logic is always more expensive than multiple soft starters. It may have worked out if, as you said, it was a long time ago when solid state soft starters were more expensive, but it hasn't been that way for at least 10 years that I know of. Whenever someone proposes it to me and shows me that they can save money, I always find a glaring omission of switching / isolation devices and/or a circuit protection devices.

It does sometimes work for MV soft starters because the cost is so much higher. But even then, to be economically viable you need to sacrifice the ability to work on a unit that is shut down while the others continue operation.
 
Jraef said:
It may have worked out if, as you said, it was a long time ago when solid state soft starters were more expensive, but it hasn't been that way for at least 10 years that I know of.
Click to expand...
Hence my comment:
the economic case for "sharing" a soft start is no longer in the least compelling - if it ever was.
Click to expand...
 
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