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GULKHAN
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Join Date: Jun 2009
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Motor Rating Calculation

08/04/2009 3:31 AM

dears,

i have three phase induction motor and the name plate parameters are as under

1) horse power=10

2) power in va=7.5kva

3) rated current=14a

4) rated voltage =420v

now according to three phas eequation the power VA= rated voltage*current*1.732

= 420*14*1.732=10184VA=10.184KVA

THIS VALUE IS DIFFERENT FROM THE NAME PLATE VALUE i.e power 7.5kva

so explain this difference factor?
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PWSlack
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#1

Re: motor rating calculation

08/04/2009 4:01 AM

The equation is wrong.

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Anonymous Poster
#2
In reply to #1

Re: motor rating calculation

08/04/2009 4:14 AM

PW

Now that is a really friendly helpfull answer (albeit correct) - bad hair day?

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PWSlack
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#4
In reply to #2

Re: motor rating calculation

08/04/2009 4:54 AM

And the correct answer is what, Guest?

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Anonymous Poster
#5
In reply to #4

Re: motor rating calculation

08/04/2009 5:02 AM

Well you see, PW, the equation is not wrong. Probably the information is wrong as suggested by Rakesh.

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PWSlack
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#6
In reply to #5

Re: motor rating calculation

08/04/2009 5:51 AM

Well, 420V * 14A = 5880W. It's on the right cricket pitch.

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Anonymous Poster
#10
In reply to #6

Re: motor rating calculation

08/04/2009 8:40 PM

Ah - but 5880*1.732 = 10184. (Right church and parish)

Correct equation, correct calculation. Probably power on the nameplate should read kW as stated by others with Pf slightly less than .8
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rakesh_semwal
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#13
In reply to #6

Re: motor rating calculation

08/05/2009 12:38 AM

I think thats right pitch for a T-20 cricket, not for a test match.
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GULKHAN
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#15
In reply to #13

Re: motor rating calculation

08/05/2009 3:37 AM

yes that why selected shahid afridi as captain
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rakesh_semwal
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#3

Re: motor rating calculation

08/04/2009 4:20 AM

Please read Read your name plate once again I think it should be 10HP and 7.5KW,not 7.5KVA.
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GULKHAN
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#11
In reply to #3

Re: motor rating calculation

08/04/2009 11:42 PM

yes ,it is 7.5w, then what is the result pls
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rakesh_semwal
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#12
In reply to #11

Re: motor rating calculation

08/05/2009 12:33 AM

Correct Answer is you can not compare KVA with KW. both are different.
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Anonymous Poster
#16
In reply to #11

Re: motor rating calculation

08/05/2009 5:26 AM

Read #10. Your equation was correct to get VA, but the equation for Watts is 3^0.5*A*V*Pf

Where Pf is power factor. This should also be on the nameplate as Pf or cos phi.
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Sapper
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#7

Re: motor rating calculation

08/04/2009 6:47 AM

Maybe...just maybe...because the rated power is the Output Power and the electrical formula you are using is the Input Power.

There will be a difference.

Or you forgot a little thing like Power Factor...

Just a Thought...

Regards,
Sapper.

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Aldego
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#8

Re: motor rating calculation

08/04/2009 8:38 AM

Gulkhan,

a 10Hp motor equates to a 7.5kW motor. I'm sure your "7.5kva" should be 7.5kW.

1Hp = 0,7457kW (rounded off to 0,75kW)

The kVA you calculated in your equation is the, electrical input to the motor when operating at 100% load (by the way your equation is correct).

The 7.5kW or kva as you say is the mechanical output of the motor and you cannot calculate this from the equation you used.. To calculate the mechanical output (from the electrical input) you need to bring the power factor and motor efficiency into the equation..

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msamad
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#9
In reply to #8

Re: motor rating calculation

08/04/2009 9:58 AM

There is something wrong with this data.

KW = HP x 0.746 = 10 x 0.746 = 7.46

PF = KW/KVA = 7.46/7.5 = 0.995 which is not a real value for an induction motor

The motor power is never rated in VA, it should be either HP or KW. It seems the item #2 should be:

2) power in KW=7.5 KW

You need to talk to the vendor about this.

AND finally, of course you need to keep efficiency (as well as PF) in your ampere calculation from the motor power.

- MS

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Aldego
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#14
In reply to #9

Re: motor rating calculation

08/05/2009 1:00 AM

Do you know what "rounded off" means?

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ikhanuk
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#17
In reply to #14

Re: motor rating calculation

08/07/2009 4:05 PM

Gulkhan

just to add a little

HP OR KW is mech output if u add losses to it it will give input KVA which will give ur FLA

motor full load amp ..

N= output/input ----

N=efficiency of motor

SO

MECH POWER IN HP OR KW /1.732*V*I*PF =EFFICIENCY

0.85 TAKEN usally for pf

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Users who posted comments:

Aldego (2); Anonymous Poster (4); GULKHAN (2); ikhanuk (1); msamad (1); PWSlack (3); rakesh_semwal (3); Sapper (1)

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