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ads114
Active Contributor

Join Date: Nov 2010
Posts: 11

Motor Theory

11/11/2010 3:43 PM

guys can you get me started please? i have an assignment and i am not really sure how to start to be honest.

it is intended to supply a 3 phase 22kw motor using copper pvc cables.the motor has assisted starting in the form of star/delta starter mounted in an mcc, housed in a substation 50m away from the motor.the efficency of the motor is 90% and power factor is 0.77,find................

1. full load current so is this then?

I=Power in kW/(sqrt of 3 x voltage x power factor)

I= 22 x 10^3 / Ö 3 x 415 x 0.77 x 0.9

I = 45.34amps.

2. power input: a bit confused with this if its a 22kw rated motor is this not the power input?

3. current starting on star?

any advice appreciated.
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Anonymous Poster
#1

Re: Motor Theory

11/11/2010 4:19 PM

I=Power in kW/(sqrt of 3 x voltage x power factor)

you forgot efficiency here.

FLA = KW/1.732*V*pf*eff

Power Input in terms of KVA:-

KVA = 22/0.77 = 26KVA

Recall the formula of Current and Voltage in Star Connection.

VLine=VPhase-Phase

Iline = 1.732*Iphase-phase

Now you have star delta connection so your FLA will be 3-4 times, Hence divide FLA/3 and you get Current in Star.
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ads114
Active Contributor

Join Date: Nov 2010
Posts: 11
#2

Re: Motor Theory

11/11/2010 4:42 PM

got it many thanks its quite clear now.
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JRaef
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United States - Member - New Member Engineering Fields - Power Engineering - New Member

Join Date: Sep 2006
Location: California, USA, where the Godless live next door to God.
Posts: 4665
Good Answers: 804
#3

Re: Motor Theory

11/11/2010 4:46 PM

From the CR4 FAQs, entitled; "Are there specific rules for posting?"

  • Do your own homework. CR4 is not a homework cheat site. While some here might relish the opportunity to sharpen up old rusty skills by working the homework problem, consider this and consider it well. If you cheat on your homework by using someone else's answers, you are only cheating yourself, because the purpose of any homework or other college assignments is to help you learn - by practice, repetition, and self-discovery.
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 ** All I every really wanted to be, was... A LUMBERJACK!.**
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ads114
Active Contributor

Join Date: Nov 2010
Posts: 11
#4

Re: Motor Theory

11/11/2010 4:58 PM

many thanks for comments i do not cheat so dont suggest i am doing so, i was stuck with a problem i didnt know how to complete.

its been explained and its clear to me now. i think thats the purpose of this forum sure you agree, by practice, repetition, and self-discovery.
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