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S

sparkz

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right well i've stumbled across a question and i can't figure out what i'm doing wrong.

Q.
At full load a 2Kw, three phase, 4 pole induction motor takes 5Amps at a power factor of 0.7 lagging from a 415V three phase 50Hz source and revs at 1425rpm.
Determine at full load:
i. The power supplied to the motor
ii. The motor effeciency

i have the formula for 1i. to be:
(square route of 3) x Vl x Il x Cos (pheta)

and filling in the formula with figures gives.

(square route of 3) x 415 x 5 x cos 0.7

which = 3593.7W (3.6Kw)

now i have a red circle around the 'cos' which is making the answer wrong meaning Q1ii can't be answered correctly either. so what is meant to go there?
i'm just confused now



any serious help needed.
 
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R

raylewis

  • Thread Starter Thread Starter
  • #2
Re: Three thase motor Q

Am a bit rusty here, isn't power factor 0.7 = cos (pheta)
I think you need to replace cos(pheta) with 0.7 not
cos 0.7

Ray
 
Last edited by a moderator:
G

Guest123

  • Thread Starter Thread Starter
  • #3
Re: Three thase motor Q

Hi.

Ditch the (cos) in front of the 0.7 value for the power factor, it shouldn't be there.;)
 
W

Wotsit

  • Thread Starter Thread Starter
  • #4
Re: Three thase motor Q

right well i've stumbled across a question and i can't figure out what i'm doing wrong.

Q.
At full load a 2Kw, three phase, 4 pole induction motor takes 5Amps at a power factor of 0.7 lagging from a 415V three phase 50Hz source and revs at 1425rpm.
Determine at full load:
i. The power supplied to the motor
ii. The motor effeciency

i have the formula for 1i. to be:
(square route of 3) x Vl x Il x Cos (pheta)

and filling in the formula with figures gives.

(square route of 3) x 415 x 5 x cos 0.7

which = 3593.7W (3.6Kw)

now i have a red circle around the 'cos' which is making the answer wrong meaning Q1ii can't be answered correctly either. so what is meant to go there?
i'm just confused now



any serious help needed.
Use this formula

P=(V*I*1.732)*motor efficency*powerfactor

where 1.732 is Root 3 (roughly) of the voltage in a 3 phase power supply.
 
Last edited by a moderator:

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