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9 comments
Anonymous Poster

Sizing the Cable for Delta Star

04/18/2006 8:51 AM

Anonymous Coward writes:
- If I have a 250kw, 3 phase, 380v motor connected on a delta star.
- Two runs of cables required to make such connection.
- The two cables shall be sized on full load or can we use two sizes of cables , i.e.,
1) Tow runs of 4Cx50mm2 XLPE/swa/pvc cable or,
2) One run 4Cx50mm2 XLPE/swa/pvc cable + one run of 4Cx25mm2 XLPE/swa/pvc cable as in star connection the current will be less.

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Pathfinder Tags: cable sizing delta star
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Anonymous Poster
#1

Why 2 cables

04/18/2006 10:32 PM

If you put the Star Delta controller close to the motor, you only need 1 cable run, and some control wires to the remote stop start buttons, or other control device.

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Power-User
Engineering Fields - Electrical Engineering - New Member Engineering Fields - Power Engineering - New Member Technical Fields - Technical Writing - New Member Engineering Fields - Construction Engineering - New Member India - Member - New Member Engineering Fields - Nanoengineering - New Member Safety - Hazmat - New Member

Join Date: Mar 2006
Location: Chennai, India
Posts: 309
Good Answers: 3
#2
In reply to #1

Sizing cable to motor

04/19/2006 10:26 AM

Cable shall withstand fault currents(short circuit and earth fault). Hence sizing shall consider not only full load current but also withstand capacity of fault currents for the time duration of fault.

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nvmani
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Guru

Join Date: Feb 2006
Posts: 1775
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#3
In reply to #2

Re:Sizing cable to motor

04/19/2006 10:05 PM

Actually the motors are only started on Delta & it takes a few seconds to change-over to Star for continuous run. so you need only one size of cable.

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#4
In reply to #3

Re:Sizing cable to motor

04/20/2006 3:58 AM

Are you sure? Motor is started in "Star" and run in "Delta",not other way? Please reconfirm and advise.

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nvmani
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Associate

Join Date: Sep 2005
Posts: 40
#5
In reply to #4

Re:Sizing cable to motor

05/04/2006 4:03 AM

Yes,. Most of the motors is design to run in Delta point. If you have a motor that design to run in Star point, then you cannot use this star/delta starter.

For Star-Delta starter, the motor is always start with star point before change to delta. This is because during the star point, the starting current will be 1/3 of the running current and as soon your motor pick up the speed and the torque, the delta will cut in. For star point, each phase voltage will be to supply to both motor winding that connected in series. R+Y, Y+B, B+R.

Once the motor has changes to delta point, each phase voltage will be supply to each winding only, and this will increase the torque of the motor to its norminal torque. The speed of the motor will remain to its design speed.

Of course, you can also reverse the method of starting with Delta point and change to Star point.

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Associate

Join Date: Sep 2005
Posts: 40
#6

Delta Star Starter

05/04/2006 4:07 AM

Are you sure that your motor start with Delta and Change to Star????

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Anonymous Poster
#7
In reply to #6

Re: Delta Star Starter

09/18/2008 8:42 PM

You're an idiot

John P

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#8

Re: Sizing the Cable for Delta Star

02/04/2011 7:12 AM

I ain't 100% sure but you may want to consider checking up on your cable size once again? :)

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Guru
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#9

Re: Sizing the Cable for Delta Star

02/04/2011 8:46 PM

Jeeze people, give him a break or don't post if you don't understand the question...

In a Star-Delta starter circuit, you have 6 cables going to the motor from the starter. Each set of cables can be sized at 58% of the motor nameplate FLA, plus whatever sizing rules apply in your neck of the woods.

Being that you used mm2, I can tell you are not in North America, but over here we must size motor cables at a minimum of 125% of the intended load. So 58% x 1.25 = 72.5% of the motor nameplate FLA, rounded up to the nearest standard size.

Your sizing rules may be different, I am not a mind reader of anonymous cowards (your words, not mine) so I have no idea where you are.

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Users who posted comments:

abasins (1); Anonymous Poster (2); Haajee (1); JRaef (1); nvmani (2); Simon (2)

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