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allyrat
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Join Date: Feb 2010
Posts: 14

Calculating Starting Torque

03/01/2010 1:00 AM

When stared direct on line a three phase squirrel cage induction motor develops a startring torque of 120Nm, When started by an auto-transformer starter, the starter draws 48 amps from the supply and the motor draws 8- amps from the starter. Calculate the value of the starting torque developed by the induction motor when started by the auto-transformer starter.

I am looking for help to better understand my math in my electrical course. The tafe lecturere is no good and I'm having trouble with some of these questions. I am hoping it will be explined in simple terms which will allow me to then practise on the many other questions I have to fill out. This is just one example of them.
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Anonymous Poster
#1

Re: calculating starting torque

03/01/2010 2:19 AM

the starter draws 48 amps from the supply and the motor draws 8- amps from the starter ?
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allyrat
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Join Date: Feb 2010
Posts: 14
#2
In reply to #1

Re: calculating starting torque

03/01/2010 2:39 AM

sorry the 8- should be 80 amps
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PWSlack
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United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

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#3
In reply to #2

Re: calculating starting torque

03/01/2010 10:09 AM

Eh? Where did those other 32 amps come from?

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allyrat
Active Contributor

Join Date: Feb 2010
Posts: 14
#6
In reply to #3

Re: calculating starting torque

03/02/2010 4:16 PM

I don't know.. that is how one of the questions is written... I'm only asking for help on one question.. I have another 10 to do myself...
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jhonson
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Join Date: Nov 2009
Location: PA
Posts: 83
Good Answers: 1
#4

Re: Calculating Starting Torque

03/01/2010 10:41 AM

You got to learn form text books . This forum cant replace test books or school teaching.

http://books.google.com/books?id=P9p1jE9Do3YC&pg=PA287&lpg=PA287&dq=induction+motor+starting+torque+equation&source=bl&ots=UECSK-BjUJ&sig=w011CHRJUIiXS3rML02b9VyMIAY&hl=en&ei=c9-LS-OiO9r28QbjkZW3Dw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CAcQ6AEwADgK#v=onepage&q=induction%20motor%20starting%20torque%20equation&f=false

The current has to be same.
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rudy.leurs
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Join Date: May 2007
Location: Flanders (Belgium)
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#5

Re: Calculating Starting Torque

03/02/2010 5:31 AM

New starting torque is 43.2 Nm ????
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Anonymous Poster
#7

Re: Calculating Starting Torque

03/03/2010 4:57 AM

In a reduced voltage starting (eg with auto transformer)

starting torque is reduced by the square of voltage eg you reduce the voltage to half, the torque reduces to 1/4th.

In case of auto transformer (or others too) you vae the equation,

V1I1 = V2I2

Even if you dont knw the voltages, you know the voltage reductions and hence torque reductions?
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Users who posted comments:

allyrat (2); Anonymous Poster (2); jhonson (1); PWSlack (1); rudy.leurs (1)

Previous in Forum: Determining Input and Output Power   Next in Forum: Calculating the Number of Poles and Motor Terminal Voltage

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