Selection of Contactors for Star Delta Starter. 2

[durgeav]

Could someone please help me out with the current rating selection for star-, delta- and main contactor for a star-delta starter?

Regards,
durgeav

 

[itsmoked]

Are you sure you don't want a soft starter instead of a problematic star-delta starter?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Kieth is right, but if you insist on creating more problems than you want to solve:
The 2 main contactors can be sized at 58% of the motor current, the star (shorting) contactor can be 33%. But some manufacturers will require you to use identically sized contactors even for the Star because their mechanical interlock mechanism may not work on disparate sizes.

 

[waross]

I saw some serious problems with undersized contactors. The chief engineer (who flaunted his PHd) did every test he could think of before using a 250 HP contactor to control a 400 HP wound rotor motor. The test that he forgot was interrupting overload currents. After the smoke cleared, we had a motor shaft bent, a fan shaft bent, parts of the coupling in every part of the room, and a destroyed control panel.
I would suggest using the HP ratings of the contactors rather than the ampacity ratings. I would use full HP rating for the running contactor. I would consider rating the starting and shorting contactor based on the reduced HP on the star connection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

... can't believe I spelled Keith wrong. Spell checker must have it both ways. Sorry Keith.

 

[ScottyUK]

'I' before 'E' except after 'K'... no, that's not right...

LOL


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If we learn from our mistakes I'm getting a great education!

 

[itsmoked]

No problem Jeef...



LOL
Hey I didn't even notice. And to be honest I sometimes spell it wrong myself. Left to themselves everyone will miss spell it because "i before e except after c" (or k!!!)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[durgeav]

Thanks everyone for the replies...

If I want to size the contactors for a 10 kW, 415V,50 Hz 3-phase squirrel cage motor, according to Waross I'll be sizing the delta and main contactors @ 58% of the motor rating. This means the delta and main contactors shall be rated for {10,000/(1.732x415x0.8)}x .58 = 10 A and star contactor shall be sized for 17.4 A x 0.33= 5.7A.

Waross,
Have I understood it right?

Please also note that the ambient temperature is 45 deg C.

What are the other factors that I have to consider for the sizing of the contactors?

 

[jraef]

Those would be absolute minimum ratings. Larger is better. Open Transition Star-Delta is exceptionally nasty duty for contactors.

Also, it is always best to use motor nameplate FLA rather than calculated FLA if at all possible. Not all motors are created equal.

Great treatise on the subject:

http://www.lmphotonics.com/m_start.htm#StarDelta

http://www.lmphotonics.com/star_delta.htm

 

[ScottyUK]

Is this motor driving something unusual like a very high inertia load? It's a strange rating - 11kW is the nearest standard IEC motor size.

For a 10kW motor is a Y/D starter worth the trouble? An 11kW motor stared DOL is within the capacity of most industrial supplies, even in small premises. If the supply is weak enough to warrant reduced voltage starting then the solid state soft starter mentioned earlier is likely to be more effective in preventing disturbance to the supply.


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If we learn from our mistakes I'm getting a great education!

 

[electrolitic]

Just an adittional suggestion.
The overload relays ( 03 ) must be connected in each branch of the motor winding. (six leads brought out)
The adjustment selected for 0.577 motor full load current.
This gives the motor full overload protection on the starting and running steps.

 

[jraef]

No, you only need an OLR in the 3 leads off of the main contactor. It is in the circuit all the time.

 

[waross]

The point that I was trying to make:
For motor duty use the motor HP ratings on the contactors.
For non-motor duty use the ampacity ratings on the contactor.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electrolitic]

Jraef
Definitively yes.
OLR (3 ph type)to be connected after 1M contactor.
Contactor 2M simply complete the delta connection.
OLR adjustment: 0.577 motor full load current.
Please see "Magnetic Control of Industrial Motors"-page 64
Gerhart W. Heumann/ John Wiley & Sons