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Old February 13th, 2020, 06:55 AM   #16
Bit_Bucket_07
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Originally Posted by thingstodo View Post
OK - I learned something!

I don't expect that the new soft starts fixed the OTHER big issue ... 30% - 50% of rated torque on start?


I've been working with soft start drives since their inception, and I can't recall ever encountering such an absurdly small starting torque limit.
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Old February 13th, 2020, 07:27 AM   #17
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Soft Start means just that!

You normally set the initial current/torque/voltage (depending on mode or model), then the max current/torque/voltage, and it ramps it up.

IF the starter is sized properly for the application, the load will start, as the max amps can be set as high as Locked Rotor Amps...
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Old February 13th, 2020, 11:52 AM   #18
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Originally Posted by thingstodo View Post
OK - I learned something!

I don't expect that the new soft starts fixed the OTHER big issue ... 30% - 50% of rated torque on start?
Fix? Thatís the entire PURPOSE of a soft starter...
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Old February 13th, 2020, 06:17 PM   #19
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Originally Posted by Bit_Bucket_07 View Post
I've been working with soft start drives since their inception, and I can't recall ever encountering such an absurdly small starting torque limit.
Limiting the starting current limits the starting torque, doesn't it?

This is a snippet from a Rockwell white paper. I can't find anything that shows more starting torque that what is listed here.

https://literature.rockwellautomatio...p007_-en-p.pdf

@jraef and @Gene Bond - we use these on centrifugal fans and impeller pumps, since they have very low starting torque required.

I know that, in the past, some mines used a fairly complicated setup to start conveyors with MULTIPLE 250 HP motors (to get enough starting torque) and then essentially shut off all but one when the conveyor was up to speed. Is that still done?

I would think that a VFD to start up a constant torque load like a conveyor would make more sense. You can use a bypass contactor to run at full speed after it's been accelerated ... ?
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Old February 13th, 2020, 06:34 PM   #20
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Just for posterity, since this was written years ago and is dropping off of the radar, I suggest that if the subject of how and when to use various starting methods is of interest, download and save this somewhat more comprehensive White Paper. The only way I could find the link now was because I already knew the exact name, otherwise I couldn't find it on Rockwell's site.


It's called "Starting High Inertia Loads: A Comparison of Starting Options"
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Old February 13th, 2020, 07:33 PM   #21
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Quote:
Originally Posted by thingstodo View Post
Limiting the starting current limits the starting torque, doesn't it?

This is a snippet from a Rockwell white paper. I can't find anything that shows more starting torque that what is listed here.

https://literature.rockwellautomatio...p007_-en-p.pdf

@jraef and @Gene Bond - we use these on centrifugal fans and impeller pumps, since they have very low starting torque required.

I know that, in the past, some mines used a fairly complicated setup to start conveyors with MULTIPLE 250 HP motors (to get enough starting torque) and then essentially shut off all but one when the conveyor was up to speed. Is that still done?

I would think that a VFD to start up a constant torque load like a conveyor would make more sense. You can use a bypass contactor to run at full speed after it's been accelerated ... ?
We still use a lot of them on Crushers, Shredders and Grinders of >100HP.

As far as the big, long conveyors, there are still some out there on soft starters, but VFD's do most of it now, where one is the master and the others are torque followers, to share the load. The biggest issue with multiple motors on 1 load is that the motors really need to bematched if they are fed from the same power source (rather than individually controlled). Otherwise, 1 motor may wind up with the majority of the load.

I don't know if anyone still does it, but a few were using bypass contactors on VFD's if a fan was running at 100%, and then transition back if the demand lowered. It was not as good IMO as the energy saving setting on modern drives, which can reduce the voltage when the load is lighter than 100%.

That's a good paper jref. I somewhere have a similar one written by one of the guys at Saftronics from a good while back (1980?). It did not mention VFD's for obvious reasons back then (nobody made an economical and reliable VFD back then)
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Old February 13th, 2020, 08:57 PM   #22
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Quote:
Originally Posted by jraef View Post
Just for posterity, since this was written years ago and is dropping off of the radar, I suggest that if the subject of how and when to use various starting methods is of interest, download and save this somewhat more comprehensive White Paper. The only way I could find the link now was because I already knew the exact name, otherwise I couldn't find it on Rockwell's site.


It's called "Starting High Inertia Loads: A Comparison of Starting Options"
Thanks for the link. I'll add it to my library of reference material!
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Old February 14th, 2020, 07:41 PM   #23
Mike Lamond
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Quote:
Originally Posted by jraef View Post
Just for posterity, since this was written years ago and is dropping off of the radar, I suggest that if the subject of how and when to use various starting methods is of interest, download and save this somewhat more comprehensive White Paper. The only way I could find the link now was because I already knew the exact name, otherwise I couldn't find it on Rockwell's site.


It's called "Starting High Inertia Loads: A Comparison of Starting Options"
Long before that white paper, other engineers devised mechanical means to start those high inertia loads: https://www.youtube.com/watch?v=fNuI6keQXYA
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