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Join Date: Feb 2017
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3 Phase Contactor Question

02/27/2017 4:53 PM

Hey guys quick question

Using a Siemens 3 phase contactor and im unfamiliar with this config

http://www.acdc.pl/pliki/3RT10251AP00.pdf

I was told there was a 4th switch inside that can turn on as a feedback to when the coil is energized

The diagram does not look like that is the case

From the picture im making the following assumption and looking to see if someone can confirm my thinking.

So obviously the control is A1 and A2

The two funny loooking half circle symbols before and after the square block (assuming the coil) in between A1 and A2 look like addtional terminals. Therfore if the coil from A1 - A2 is energized somehow those terminals allow current to flow from the two little half circle symbols. This could be used to drive an inpput into a PLC or something

Can someone confirm my thinking is correct? if its not perhaps correct me?

Thanks a bunch!

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#1

Re: 3 phase contactor question

02/27/2017 5:12 PM

1. The dash-dot line surrounding the diagram is very confusing.

2. If the coil is burned out, the presence of voltage across it will not confirm that the contacts are closed.

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#2

Re: 3 phase contactor question

02/27/2017 6:06 PM

This is a simple 3 phase contactor with 230V AC control coil and is not designed for PLC (low drive current and voltage) control. There is no 4th (normally open) contact included for the purpose of feedback (commonly used as a latching contact to latch the contact on when a momentary 3 wire start control is used).

As with most contactors of this size a 4th contact is available for this model that snaps on to the side (or top) for this purpose but it must be brought separately.

Smaller contactors are available with 4 poles built in rather than three as are contactors designed for low current PLC drive, but if you require a 7.5kW contactor your available options are likely to be limited.

The half circles just appear to indicate multiple connection points (top and bottom of the contactor), nothing more.

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#3

Re: 3 phase contactor question

02/27/2017 6:54 PM

Thanks for the input very much

That makes much more sense now

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#4

Re: 3 phase contactor question

02/27/2017 7:13 PM

That contactor has no auxiliary contacts - the specs on the link you provided indicate that there are no included auxiliary contacts on that particular device and is 3 pole only, so I would take that as being correct.

If you need auxiliaries, then you would fit an auxiliary contact block to achieve that. You can get contact blocks for these to fit either on the side (part# 3RH1921-1DA11 with 1 x NO contact) or on the front (part# 3RH1921-1HA11, 13, 22 or 31 with various NO & NC contact configurations) as desired.

Or you could order a 3RT1326-1AL20 which is basically the same device but with 4 poles, and use one of the poles as an auxiliary.

The SRT1025-1AP04 is also a similar rated contactor, is 3 pole with the 3RH1921-1HA22 front contact block pre-installed for an additional set of 2 NO and 2 NC contacts.

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#5

Re: 3 phase contactor question

02/27/2017 11:17 PM

Do NOT order the 4 pole version just to get a feedback signal. This is rated 40A, so you will pay for a 40A contact when a 5A rated auxiliary contact would suffice.

But more importantly if using low voltage low current circuits such as what you would have with a PLC, the big 40A contact will actually work against you. Films build up on contact surfaces from airborne contamination and ozone that forms from the arcing of contacts. That film will build up to the point of being resistive and interfere with your low power signals. Under normal power switching conditions, the arc from switching those bigger contacts burns the film away. But with a low power circuit, there is insufficient energy to do that and it gets progressively worse and worse until it fails.

The small auxiliary contacts that you add on, mentioned earlier, are specifically designed for use on low energy circuits such as found in PLCs. Use those.

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#6

Re: 3 phase contactor question

02/28/2017 12:01 AM

Already covered but the link provided by you showed this;-

As you can see it mentions an added auxiliary switch.

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#7

Re: 3 phase contactor question

02/28/2017 1:04 AM

In order to clarify the diagram, I have to say that the outer line is just a box containing the contactor. Therefore, it does not imply any electrical connection.

On the other hand, what you see drawn on each side of the coil are female connectors, according to the IEC standard, which means that you will need a compatible male contact for the wiring.

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#10
In reply to #7

Re: 3 phase contactor question

02/28/2017 1:59 PM

what you see drawn on each side of the coil are female connectors, according to the IEC standard

However, that's not actually what you get with the contactor. What you get are two linked sets of standard screw terminals.

Symbols can be quite irritating sometimes; it's almost like certain companies just use different symbol styles and standards or make them up rather than following a single current standard. The only way I found this out in the past was to actually examine a contactor because (like this example) the datasheet and drawings are vague and I didn't have the 3D CAD model viewing software to allow me to view the hidden bottom terminals or internal connections.

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#8

Re: 3 Phase Contactor Question

02/28/2017 10:07 AM

About 60 years ago, 3 phase contactors I had worked with some comes with a ganged micro-switch on the side, that can be used for latching the coil power... If this is your contactor type, I believe it can be used as an indicator for the 3 pole contacts position, if left unused?

just a thouht..

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#9

Re: 3 Phase Contactor Question

02/28/2017 10:51 AM

You will need to add something like this. It mounts on the side of the contactor and the tab reaches into the side of the contactor.

When the contactor actuates, the tab is moved and the aux contact actuates.

https://www.grainger.com/product/13A144?cm_mmc=PPC:BingPLA-_-Electrical-_-Starters%20and%20Contactors-_-13A144&s_kwcid=AL!2966!10!14921217564!1102900081492&ef_id=VRNLHgAABMs44jXT:20170228154840:s

Siemens also sells add on thermal protection blocks as well.

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#11

Re: 3 Phase Contactor Question

05/30/2017 11:52 PM

Most three phase contactors provide for additional Aux. set of contacts that snap or screw on so they are activated at the same time the contacts are pulled in and is some cases multiple aux. contacts can be added to turn on a light, feed back to the controller or interfacing other components dependent on the contactor being activated. Check the contactor accessories for the one you are using or just go buy a GE Fanuc.

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