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3-Phase Root Switching?

02/16/2017 10:15 AM

Trying to find the current rating for this Siemens soft starter

I noticed on this data sheet there are two different ratings.

http://www.sistemamid.com/panel/uploads/biblioteca/1/349/1259/1260/1261/5124/5126/5127/5176/5178/Datasheet_3RW4443-6BC45_EN.pdf

There is one that says 3 phase root switching?

Anyone know what that is?

Verse the top number where it just says "operating current"

I know the temp ranges changes of course but unsure of "root switching"

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#1

Re: 3phase root switching?

02/16/2017 10:20 AM

Unknown Host


Description: Unable to locate the server named "www.sistemamid.com" --- the server does not have a DNS entry. Perhaps there is a misspelling in the server name, or the server no longer exists. Double-check the name and try again.

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#3
In reply to #1

Re: 3phase root switching?

02/16/2017 10:52 AM

I got the address to work.

It details a Siemens 3RW4442-6BC45

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#13
In reply to #3

Re: 3phase root switching?

02/17/2017 12:54 AM

In this case, there is no need for the address to work, as that is the title of the PDF link.

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#2

Re: 3phase root switching?

02/16/2017 10:41 AM

I'm not reading that link (because I'm too lazy to cut and paste and you were too lazy to make it an active link), but I think I know the answer, it comes up a lot.

I believe what you are referring to as "3 phase root rating" is a translation issue. Their soft starters, and many others, can be connected in two different ways, depending on the type of motor you have. If you want to use it and connect it in the same way that you would use and connect an Across-The-Line (DOL) starter, meaning 3 wires from the starter terminals to the motor, Siemens refers to this as a "Line" connection, which means the starter handles the full starting and running current of the motor.

The alternative is that you have a motor that can be connected as if you were using Star-Delta starting, meaning 6 leads going to the motor. In that case, they provide you with a diagram that allows you to place the soft starter poles in SERIES with each motor phase coil in the Delta pattern (often referred to as an "inside the delta" connection). That then means the soft starter power devices only see 58% of the motor line current, which is the product of dividing the line current by the square root of 3 (hence the likely mistranslation of it being called "3 phase root"). That translates to being able to use a starter that is much smaller than the motor FLA rating, or as you may be seeing it, the same soft starter shows a higher current rating if used this way.

If you did not understand this, I HIGHLY recommend that you not attempt it. The practice saves a little money on the up-front cost of the soft starter, but is doing so at an extreme risk of failure of the motor. Equipment manufacturers who want the lowest possible cost but are not concerned for functionality beyon their warranty are the only ones who benefit. End users will eventually pay MORE in the long run because the risk of failure is high, and the cost of that failure is not the cost of the motor (because you can use a shunt trip in the breaker protecting it), but the cost of the down time as you repair it.

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#4
In reply to #2

Re: 3phase root switching?

02/16/2017 11:08 AM

Thanks

Appreciate the explanation

And i was not being lazy, i simply did not realize it was not an active link

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#6
In reply to #4

Re: 3phase root switching?

02/16/2017 11:51 AM

First: Paste the link directly into where you are typing the body of the message.

Second: click on the chain link icon on the bar above the text window. There is a make link and a break link symbol there.

Third: Paste the link again to the top line when the object for creating active links opens.

Fourth: (Optional): overwrite the gibberish looking link with a catchy phrase you would rather have for a link title in the second box from the top.

That should help.

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#7
In reply to #4

Re: 3phase root switching?

02/16/2017 11:55 AM

I made it an active link for you.

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#16
In reply to #4

Re: 3phase root switching?

02/17/2017 9:14 AM

The link works fine, launches a pdf (now I see why)

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#5
In reply to #2

Re: 3phase root switching?

02/16/2017 11:48 AM

Thank you for the clear and beautiful lesson, always appreciated.

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#8
In reply to #5

Re: 3phase root switching?

02/16/2017 11:59 AM

And yes, thanks for the explanation. If a poster is new to CR4 and the TinyMCE editor I don't think s/he necessarily knows that the link doesn't go live automatically, the way it would do in Word.

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#9

Re: 3-phase Root Switching?

02/16/2017 3:45 PM

i will make sure i do that next time

I have never been a forum person really but now i know what an active link is

Thanks

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#10

Re: 3-phase Root Switching?

02/16/2017 4:25 PM

I think i found a video on you tube that talks bout the same thing

It talks about essentially switching the induction motor winding config after startup

The intial start draws 1/3 the current

really interesting stuff

Although above you said it was some drawbacks. If you have time id be curious to know more about the harshness to the motor caused by this

inside Delta

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#11

Re: 3-phase Root Switching?

02/16/2017 5:56 PM

I have the time now, I was on my iPhone before.

It's not that it makes the motor run differently, it is an increased failure risk. In the picture below, the "Line Start" connection is on the left, the "Inside the Delta" connection is on the right.

One risk of having any soft starter is that the SCRs, the power devices, are susceptible to damage from voltage and/or current spikes. But the damage is not always catastrophic in that they fail shorted (like a welded contact) so if it happens when the motor is running, you never know. You only find out when you go to turn it off, and the one device does not. It's easily detected by monitoring the voltage drop across the devices, so no voltage drop = not turning off. So if you consider the 3 wire connection, the starter then trips and prevents you from restarting until you replace it, but other than that, no harm because one shorted device does not create a path for power to flow, because it has to valid return path.

On the Inside the Delta (6 wire) version, follow the wires; 1/2 of all three phases is always powered, connected to the motor. So if even ONE of the SCRs shorts, there IS a valid current flow path through one winding of the motor, and no way to stop it. The motor winding just heats up and fails. In this particular drawing, they show a "Line Contactor", and that's fine, that is a solution. However the only possible PURPOSE of using the Inside the Delta configuration is to save money, and if you must buy a Line Contactor to make it work, you totally defeat the purpose of using that style in the first place! As a subtle form of legitimizing their design, they also show a Line Contactor in front of the 3 wire version, but I assure you, almost NOBODY bothers with that, it's basically unnecessary (unless you live somewhere that is susceptible to heavy lightning strikes). They have created a "false equivalency" here (this came from a manufacturer who promotes this heavily. The other problem is just in the idea of having 6 leads going to the motor instead of 3. With three leads, your connections always work, but sometimes in reverse, so you swap them. In the 6 wire design, one mis-wired terminal and a phase burns out the instant you turn it on. As I said earlier this is OK for OEM equipment mfrs, their technicians will always know exactly how to hook them up. But once the end user takes over, it often hap[pens that something goes wrong in the middle of the night and the technician on duty has no idea what's going on, hooks something up wrong, and blows out the starter. Any money saved in buying that cheaper start just evaporated, and then some.

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#12
In reply to #11

Re: 3-phase Root Switching?

02/16/2017 6:44 PM

Brilliant post.

I previously thought that I knew a fair bit about these things, but you continue to educate me every time I read your replies, and I thank you for that.

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#17
In reply to #11

Re: 3-phase Root Switching?

02/17/2017 9:59 AM

Really nice, thanks!

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#15

Re: 3-phase Root Switching?

02/17/2017 6:21 AM

These days, invitations to pick up the telephone and actually talk to the people at the original equipment manufacturers' technical helplines on matters of equipment detail usually attract "off topic" votes, despite the blindingly obvious efficacy of such an action.

  • There has never been a time when there have been so many telephones available to so many people in the world.
  • There has never been a time when the telephone has been so little used for its originally-intended purpose.

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#19
In reply to #15

Re: 3-phase Root Switching?

02/17/2017 10:39 AM

... and there has never been a time in which the people on the other end of that line have been so clueless...

Seriously, try calling Siemens to get tech support. They are so big and have so many different internal entities, it can take hours just to find the right number, then you leave a message and someone calls you back 3 days later. When I worked there a decade ago, I made sure everyone I dealt with knew exactly how to get hold of me directly for help. But if anyone called the general tech support line, they were almost violently angry by the time they got directed to me. After I left they never replaced me, so if someone wants the kind of pre/post sales engineering assistance I provided, it's done from a call center by some kid reading from a script who, in all likelihood has never actually even seen or touched the product. If that doesn't help, they want a purchase order to send out a field technician.

Thank goodness they no longer pay me to care...

PS: I'm not picking only on Siemens, virtually all of them are like that now. I have a similar role in another company now, anyone who finds out I am in this role is flabbergasted that my role (Technical Consultant) even exists any more and become almost instantly loyal in order to have access. I'm not bragging, I think it's sad that my role has become so rare. And when I retire, I'm relatively sure I will not be replaced. I was up for a promotion last year to go into management and although I didn't really relish it, the pay was a lot better. But before I could go, I had to find and recruit my replacement. 6 months of searching proved fruitless... So they just gave me the pay to keep me from leaving and I'm happier anyway. But when I want to retire, or die at my desk, I think the same thing will happen again. It seems nobody wants to think this hard any more...

Rant complete.

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#18

Re: 3-Phase Root Switching?

02/17/2017 10:28 AM

In my opinion, the meaning of "3-phase root switching" is at INSIDE-DELTA CIRCUIT .See:

http://w3app.siemens.com/mcms/infocenter/dokumentencenter/ce/Documentsu20Brochures/e20001-a1040-p302-v5-7600.pdf

page 7.

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