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Join Date: Jan 2013
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Cable Sizing for a 15kW Motor With ASD Starter

04/26/2013 2:59 PM

Hi

I am facing a slight problem, i have basic understanding of star delta circuits but recently i saw someone at my workplace selecting a cable for a 15kW motor. The motor is run using an Auto Star Delta Circuit. The voltage available is 3~400V and the p.f and efficiency can be taken as 0.85 and 0.9. I get it that line current for the motor will be 28.3A. I[load]= 15k/(1.732*400*.9*.85). Now for that current the cable catalogue suggests using a 4mm^2 with temperature and Voltage drop considered.

But what that guy did is he divided the current by 2 and approximating a 15A current he selected a 2.5mm^2 cable. All he said to me is this is because of the starter circuit.

But i did not understand it, primarily due to almost no background in motors or electrical power. Can anyone explain this to me or refer me a link answering this question.

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Pathfinder Tags: Auto Star Delta cable sizing
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#1

Re: Cable Sizing for a 15kW motor with ASD starter

04/26/2013 3:20 PM

check this source

Motor Calculations

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#2

Re: Cable Sizing for a 15kW motor with ASD starter

04/26/2013 3:39 PM

You should read some text books so that YOU will UNDERSTAND the basics yourself, and not have to ask others to tutor you.

An Elementary Treatise on Electricity: Second Edition (Dover Books

Practical Electric Motor Handbook - Irving Gottlieb - Google Books

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#4
In reply to #2

Re: Cable Sizing for a 15kW motor with ASD starter

04/26/2013 4:11 PM

i know i will have to study these things in detail but if i could get my hands on the book at this moment i wouldnt be posting here and i did ask if someone can provide me a link to some web site where this thing is explained...so if you think i am here for spoon feeding you are wrong all i asked is for a bit of help and thats what forums are for..

just dont start judging people

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#5
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Re: Cable Sizing for a 15kW motor with ASD starter

04/26/2013 5:03 PM
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#3

Re: Cable Sizing for a 15kW motor with ASD starter

04/26/2013 3:43 PM

When using Star-Delta starting, you are running 6 conductors from the starter to the motor instead of 3, because you are running separate conductors to each end of each winding. So because the current through each is split between conductors, you can size the conductors at 58% of the motor current value as a result (plus whatever normal safety factors your code may require).

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#6
In reply to #3

Re: Cable Sizing for a 15kW motor with ASD starter

04/26/2013 5:07 PM

I'd say that pretty much explains it

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#7
In reply to #3

Re: Cable Sizing for a 15kW motor with ASD starter

04/26/2013 5:16 PM

Thanks JRaef after drawing a DOL circuit for delta connected motor and an ASD circuit for a motor with 6 terminals and keeping in view your explanation i think i get it now.

Please correct me if my understanding is wrong. In the ASD circuit each conductor acts as the leg of a motor winding so one pair of conductor constitutes one phase of that motor and it carries the phase current for that winding not the load current. As we know that Ip = Il/1.732. so the current in those conductors is less than the Line current.

This means unlike a simply delta configured motor we are providing phase currents and not the line current through the cables in the field.

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#9
In reply to #3

Re: Cable Sizing for a 15kW motor with ASD starter

04/27/2013 5:26 PM

That applies to the 6 wires from the starter to the motor, but I can't see how it applies to the 3 phases in the supply to the starter. The OP didn't distinguish, but he needs to get both cables right.

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#10
In reply to #9

Re: Cable Sizing for a 15kW motor with ASD starter

04/28/2013 1:00 PM

I think that's a good point, you are right that the OP did not specify which cables he was referring to, I just assumed.

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#11
In reply to #10

Re: Cable Sizing for a 15kW motor with ASD starter

04/28/2013 2:03 PM

I thought afterwards - if the starter is in an MCC likely to be a short run from the busbars to the starter. Still need to size correctly, but unlikely to be much of an issue. But if there's only 1 or a few motors and no MCC, might be quite a distance to the starter from the main supply board.

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#12
In reply to #11

Re: Cable Sizing for a 15kW motor with ASD starter

04/28/2013 2:31 PM

valid point..but i was talking of a case when we have an MCC with bus bars so the main concern was the six conductors from the MCC to the motor in the field.

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#8

Re: Cable Sizing for a 15kW motor with ASD starter

04/26/2013 11:30 PM

For cable selection,you should take into account the length,ambient temperature, grouping factor too. In some cases the cable from s/d starter to motor and from CB to starter will be the same.

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