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Old 11-07-2019, 11:37 PM   #1
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Default Multiple motors on one soft start

We have a customer that has a hammer mill powered by a 125hp motor. They are considering changing that to a roller mill powered by 3 40hp motors. The existing hammer mill has a soft starter in a SqD MCC. It seems to me I have 2 options to do this swap
1. There is an empty section in the MCC. I could get 3 40hp buckets. I would also have to pull new motor feeders. I like this one as it is straightforward. We don't need softstarters so it's just a size 3 motor starter.
2. Use the existing softstart and motor feed and put in overloads for each motor. This seems to be the more economical way of doing it but adds complexity. Sometimes they may only want to run 2 of the 3 roller motors so the load on the starter would vary. Are there reasons to shy away from this? I'm more inclined to go route #1 but I would consider this from a cost saving standpoint.

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Old 11-08-2019, 01:38 AM   #2
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Option 1 would be my choice
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Old 11-08-2019, 05:55 AM   #3
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Option 2 means they will likely be pulling a safety disco on the motor or motors they won’t be running, not really a good idea to me. If you can remodel the MCC with the buckets and doors, and they are still available, this is what I’d do.
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Old 11-08-2019, 06:55 AM   #4
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option one.
I would also look at staggering the start sequence of the motors by a few seconds using the plc or timer relays to act as a poor mans softstart
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Old 11-08-2019, 06:55 AM   #5
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Option 2 means they will likely be pulling a safety disco on the motor or motors they won’t be running, not really a good idea to me. If you can remodel the MCC with the buckets and doors, and they are still available, this is what I’d do.
@paulengr mentioned the same thing a few days ago, with option 2, the disconnects may not last long...

https://www.electriciantalk.com/f146...6/#post5258454

If you save them a few bucks, they'll remember it a while, probably until the next time they have to spend some money. You do something creative that causes down time, you didn't save them $1, and they'll remember "YOUR IDEA" it forever.
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Old 11-08-2019, 07:49 AM   #6
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I don't think the disconnects would be operated under load but I can't guarentee they won't. If they only run two motors they have to go to the machine and open the one set of rollers and they would open the disconnect then. However, there is the chance of someone forgetting and running out to open or close the switch. I'm still not overly fond of that setup. The whole place is run on a PLC so staggering start times is no problem.

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Old 11-08-2019, 12:20 PM   #7
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I don't think the disconnects would be operated under load but I can't guarentee they won't. If they only run two motors they have to go to the machine and open the one set of rollers and they would open the disconnect then. However, there is the chance of someone forgetting and running out to open or close the switch. I'm still not overly fond of that setup. The whole place is run on a PLC so staggering start times is no problem.
That makes sense, if the three are always started together by the PLC, the switches wouldn't be used as starters much, I don't see how it would be an issue real soon anyway.



Of course if they need to make some change to the PLC logic so the motors don't all start at once, they'll be back to option one anyway.
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Old 11-08-2019, 12:27 PM   #8
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That makes sense, if the three are always started together by the PLC, the switches wouldn't be used as starters much, I don't see how it would be an issue real soon anyway.



Of course if they need to make some change to the PLC logic so the motors don't all start at once, they'll be back to option one anyway.
The switches will not be used as starters at all, other than if someone forgets and runs out to throw it on while the others are running. The grinding process is all automated.
The automation change is needed if I go to 3 separate starters. If I do option 2 there would be fewer changes needed.

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Old 11-09-2019, 12:02 AM   #9
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If someone opens or closes one of the disconnects, in option 2 while running, it is very likely to destroy the soft start.

A "band aid" work around can be to wire an auxiliary contact in series on all three disconnects to break the soft start enable.

Never mind. You say they may not always run all three motors, so the auxiliary contact would create another problem.

Go with option 1. It has way less possible problems.

I have never been a fan of multiple motors on VFDs or soft starts. It seems like a problem waiting to happen if the planets are not aligned properly.
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Old 11-09-2019, 02:05 AM   #10
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If someone opens or closes one of the disconnects, in option 2 while running, it is very likely to destroy the soft start.

A "band aid" work around can be to wire an auxiliary contact in series on all three disconnects to break the soft start enable.

Never mind. You say they may not always run all three motors, so the auxiliary contact would create another problem.

Go with option 1. It has way less possible problems.

I have never been a fan of multiple motors on VFDs or soft starts. It seems like a problem waiting to happen if the planets are not aligned properly.
why would opening or closing a disconnect on a soft start destroy the soft start. Most of them have bypass contractors so the soft start wouldn't even be online. Disconnects probably not going to be happy but the soft start should be fine. (still don't like option 2).
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Old 11-09-2019, 12:21 PM   #11
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We get put in similar positions with our industrial and ag customers trying to save a dime upfront just to spend a dollar later.



Absolutely option 1.
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Old 11-09-2019, 01:07 PM   #12
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Thanks for the input, guys. My initial thought was to go option 1, but the manager brought up trying to use the exist starter so I wanted to check into it. They have been a good customer that doesn't usually quibble about how we do things, so I'm willing to do what I can to help them out.

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Old 11-09-2019, 01:29 PM   #13
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In option 2 ,You still would need individual ol protection to each motor right?
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Old 11-09-2019, 01:32 PM   #14
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In option 2 ,You still would need individual ol protection to each motor right?
Yes. I would use manual motor starters to take care of SC and OL protection as there is a 400A breaker in the soft start bucket.

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Old 11-09-2019, 07:12 PM   #15
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why would opening or closing a disconnect on a soft start destroy the soft start. Most of them have bypass contractors so the soft start wouldn't even be online. Disconnects probably not going to be happy but the soft start should be fine. (still don't like option 2).

First the purpose of the bypass contactor is efficiency. The soft start just does ramp up or ramp down, It closes the bypass contactor once the motor(s) are up to full power. It either fires back up and opens the bypass for ramp down or simply shuts it off for coast to stop. Some can also act as a cycloconverter for jogging or operate the contactor as a starter if the soft starter fails but those are special cases.

Unlike VFDs soft starts shouldn’t be much affected. It’s like a solid state 3 phase relay and uses the same hardware as an SSR except the soft start can control the startup current. Unlike VFDs there shouldn’t be an issue with doing this with the soft start.

There is a scheme with soft starts for multiple motors. It is typically more for large motors or emergency backup scenarios where it’s worthwhile to save on soft starts over just adding one extra contactor per motor. Use one output contactor and one bypass contactor per motor. To start a motor close the appropriate output contactor and start the motor. At speed close the corresponding bypass contactor and once closed open the output contactor.

Disconnects are not designed to close under heavy inductive loads so the disconnect schemes as far as starting goes are not a good plan. But some are available that have aux contacts that open slightly before the disconnect opens. This allows contactors or in this case a soft start to be opened. Using the previous scheme the bypass and output contactors can be interlocked to the disconnect.
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Old 11-10-2019, 01:13 AM   #16
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[quote=gpop;5259886]why would opening or closing a disconnect on a soft start destroy the soft start. Most of them have bypass contractors so the soft start wouldn't even be online. Disconnects probably not going to be happy but the soft start should be fine. (still don't like option 2).[/quo
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Old 11-10-2019, 01:19 AM   #17
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why would opening or closing a disconnect on a soft start destroy the soft start. Most of them have bypass contractors so the soft start wouldn't even be online. Disconnects probably not going to be happy but the soft start should be fine. (still don't like option 2).
If the disconnect was opened or closed during start ramp.
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Old 11-10-2019, 07:36 AM   #18
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If the disconnect was opened or closed during start ramp.

Nothing in the manual about this. Very different from wiping out precharge circuits (soft start does not have) or messing with an intentional LC balance (drives) since all we have is a solid state switch.

If it is in a start ramp with thd disconnect open, it suddenly sees the motor. We get a jump in current. At which point depending on the phase angle it either continues the start or current limits or trips on overcurrent. If it suddenly opens and we catch the phase loss it might trip or we end up open circuit which most of them recognize and trip. We may get a surge but SCRs have a nasty property called self commutation brought on by sudden current rises or excessive voltage. To prevent this all soft starts have surge arresters and RC snubbers which prevent a surge. This would be inductive kick which we must avoid in a soft start to the point that it has hardware protection.

So how is it destroyed?
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Old 11-10-2019, 07:48 AM   #19
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Motor connections to the soft starter output should not be made or broken while the soft starter is running. *The resulting sudden change in current will cause problems and possible damage to the soft starter or other equipment.

From Schneider's website.

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Old Yesterday, 04:44 PM   #20
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At 30HP you have the option of using what are called Motor Protection Circuit Breakers (MPCBs) that are in essence, an IEC version of a Manual Motor Starter, with Short Circuit protection built-in. You can get them from all of the major players now. They look like this:



This one just happens to be a 30HP 480V rated unit. So they give you the OL, SCPD and disconnect all in one. You can put 3 of them in a box on a piece of DIN rail and have external handles coming through the doors. Perfectly legit, UL listed, covers all of the bases.


The soft starter will not care that you might have varying amounts of load, EXCEPT if you have LESS than 20% of the rating of the unit within 3 seconds of giving it a Start command, because it will think there is a phase loss. Most of them measure the current for phase loss detection, so that if at least one phase has low current, it trips. ALL 3 phases having low current still satisfies that criteria. So if that happens, shorten your ramp time.
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