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Join Date: Jan 2013
Posts: 41

Star and Delta connected Loads

11/09/2013 4:06 PM

Hi

i am trying to improve my understanding of 3 phase power systems. So while reading about them i came across a statement

"the Eqt P = 1.732 x I(line) x V(line-line) x pf gives the power of a balanced three-phase load in terms of line quantities regardless of the connection of the load"

so this should mean that if i have a balanced 3 phase load it will have the same power consumption regardless of the configuration in which i connect the load.

But further in the text in an example a load is connected to a star power source first in star and then in delta and the author comes up with completely different results for the consumed power. This has completely baffled me, may be i have misunderstood the statement quoted above. I can share the calculation performed by the author if required.

Any thoughts on the subject? Oh and i am reading Elec Machinery Fundamentals 5th ed by Chapman

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#1

Re: Star and Delta connected loads

11/09/2013 8:17 PM
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#2

Re: Star and Delta connected loads

11/09/2013 8:41 PM

Dear friend,

Please note that three phase A.C. power is 3 x (single phase A.C. power)

Single phase A.C. power is = Vph x Iph x p.f.

So, three phase A.C. power is 3 x (Vph x Iph x p.f.)

But, generally one can get to measure only line quantities; thus, the above formula has to be converted on line quantity terms.

Thus, if it is three phase Star connection, then:

three phase A.C. Power = 3 x (VL/Root 3) x IL x p.f. (as Vph = VL/ Root 3 in star)

(i.e.) three phase A.C. Power in star = Root 3 x VL x IL x p.f.

Whereas if the three phase connection is Delta, then:

three phase A.C. power = 3 x VL x (IL/Root 3 ) x p.f. (as Iph = IL/Root 3 in delta)

(i.e.) three phase A.C. power in delta = Root 3 x VL x IL x p.f.

As can be seen, irrespective of the connection, the three phase power formula remains same.

Hope it is clear!

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#8
In reply to #2

Re: Star and Delta connected loads

11/24/2013 6:20 AM

Sir,

formula is same but let me know that what will be power consumption ration for star to delta

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#9
In reply to #8

Re: Star and Delta connected loads

11/24/2013 6:46 AM

Power consumption does not change. To deliver the same power what will be the current loading in the winding is what differs between star & delta.

In star, to deliver the same power 1.732 times more current is drawn by the windings and thus the heat generated in 3 times more, when compared with delta.

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#3

Re: Star and Delta connected loads

11/10/2013 6:41 AM

If the Load, in terms of Watts and VA, is the same, then surely, the consumption will be the same (logically).

In your case, maybe the load is varying, due to the fact that the voltage is changed from a star to a delta connection: example; a 3 phase motor going from start to delta when starting.

you should that you should share the calculation performed by the author for a better contribution...

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#4

Re: Star and Delta connected loads

11/10/2013 7:46 AM

The first formula is OK.

You need to tell us what the load is and how the components are connected in the second case.

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#5

Re: Star and Delta connected Loads

11/10/2013 12:50 PM

I got it.. i was looking at it the wrong way. Initially i thought that the statement meant so this should mean that if i have a balanced 3 phase load it will have the same power consumption regardless of the configuration in which i connect the load.

It does not actually say that the power will be constant

Come to think of it, it was a stupid interpretation

thanks all for the feedback

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#6
In reply to #5

Re: Star and Delta connected Loads

11/11/2013 1:23 AM

I think you miss the point that the current will be less in star than in delta connection.

If you have a motor and you run it in delta it will consume more energy than in star there fore although the formula remains the same the values are different.

Simply seen, you may think of the motor having two windings in series between phases when in star and only one when in delta. The resistance of the two windings are more than that of the one therefor the current will be less and so the power consumption.

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#7
In reply to #6

Re: Star and Delta connected Loads

11/11/2013 7:58 AM

Not so fast.

If the Load (mechanical load on the motor shaft) is the same, then the power consumption will be theoretically the same provided that the motor is not overloaded when going from delta to star. The motor rating (nameplate) specifies the conditions for each configuration in terms of Voltage required and expected current for the same power output.

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