Kontactors for star/delta

[smiki]

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.

[jraef]

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.

[GGOSS]

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

[jraef]

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.

[marke]

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,

[Guest]

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

[msdaif]

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