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Kontactors for star/delta


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#1 smiki

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Posted 12 November 2005 - 11:54 PM

Hello,
How can I define size of the contactor for star delta switch.
I have motor on the sit for 250 Kw I=465A.
I there any different in star and in delta.

#2 jraef

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Posted 13 November 2005 - 05:05 AM

The main run contactor must be sized for a minimum of 58% of the motor power, so in your case, 145kW or more. The other motor contactor needs only be rated for the remaining 42%, so in this case 105kW, but check with your preferred vendor to see if a mechanical interlock works with disimilar contactors, not all manufacturers do. If not, use the same size as the main. The star contactor needs to only be rated for 33%, so 83kW.
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#3 GGOSS

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Posted 15 November 2005 - 11:29 PM

Hello Smiki,

I am firm believer that all electical equipment should be selected on the basis of motor nameplate Full Load Current and not just kW.

Based on that I would suggest the following.

Line Contactor: 270 Amps AC3
Delta Contactor: 270 Amps AC3
Star Point Contactor: 154 Amps AC3

Make sure that you use a dedicated star/delta timer ie a dedicated star/delta timer includes a inbuilt delay to ensure that the Star Point Contactor has de-energised and the arc has been fully extinguished before the Delta Contactor is energised. A standard timer does not provide this functionality and if used can result in the Delta contactor closing on a short and welding.

jraef, I am interested in your ptroposal of having a motor contactor with a 42% rating. Could you please explain that for us.

Regards,
GGOSS

#4 jraef

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Posted 16 November 2005 - 03:27 AM

58% through the main "delta" contactor, 42% through the other one = 100%
Take a look at this drawing. In that configuration (sorry, US nomenclature), the 1M contactor is sized for 58%, but the 2M contactor needs only be sized for the remaining 42% technically.

http://www.ssusa.cc/reference/wiring_diagrams/standard_series_drawings/series_107/107_21.pdf

I was incorrect about the mechanical interlock however, that needs to go between 2M and S, not 1M and 2M, so it will almost always involve dissimilar contactor frame sizes. I was trying to o from memory, which is frequently faulty.



OK, I have edited this several times and can't get the BB code to ork right, so cut and paste into a browser window. Sorry.
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#5 marke

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Posted 16 November 2005 - 10:01 AM

Hello jraef

Surely the current through the main contactor and the delta contactor are equal? both are carrying winding current so should be rated at 58% of the line current?
Star delta starters are foreign to the USA so we have to make allowances!!

Best regards,

#6 Guest__*

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Posted 16 November 2005 - 10:26 AM

Let me put these in formulae and be more specific.

With reference to Jraef's circuit:

1. Isolator
must be at least rated for motor line current In=465A.

2. Star contactor S (first to energize)
must be at least rated for motor line current divided by 3

or In/3 = 465/3 =155A.

3. Main contactor 1M (energizes after S)
rated for motor line current divided by square root of 3
i.e. In/1.73=270A

4. Delta contactor 2M (third to energize, S off)
rated for motor line current divided by square root of 3
i.e. In/1.73=270A

5. Overload OL
rated for motor line current divided by square root of 3
i.e. In/1.73=270A

6. type of contactor

AC3: applicable for starting and stopping squirrel-cage motors

#7 msdaif

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Posted 16 November 2005 - 10:27 AM

QUOTE
Originally posted by Anonymous
Let me put these in formulae and be more specific.

With reference to Jraef's circuit:

1. Isolator
must be at least rated for motor line current In=465A.

2. Star contactor S (first to energize)
must be at least rated for motor line current divided by 3

or In/3 = 465/3 =155A.

3. Main contactor 1M (energizes after S)
rated for motor line current divided by square root of 3  
i.e. In/1.73=270A

4. Delta contactor 2M (third to energize, S off)
rated for motor line current divided by square root of 3  
i.e. In/1.73=270A

5. Overload OL
rated for motor line current divided by square root of 3  
i.e. In/1.73=270A

6. type of contactor

AC3: applicable for starting and stopping squirrel-cage motors





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