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asdabh
Active Contributor

Join Date: Oct 2007
Posts: 13

Motor Current

02/14/2010 11:08 PM

In one of the phase connected to 5.5 KW motor shows 5% less current than other phases.Is this situation dangerous ? Why unbalnce current occurs in motor?
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wareagle
Guru
United States - Member - New Member

Join Date: Jun 2008
Location: Alabama
Posts: 1604
Good Answers: 63
#1

Re: motor current

02/14/2010 11:53 PM

Check the voltage and see if it is unbalanced.
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msamad
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Join Date: Feb 2009
Location: Houston, USA
Posts: 946
Good Answers: 244
#2
In reply to #1

Re: motor current

02/15/2010 4:20 PM

The main cause of the motor current unbalance is the supply voltage unbalance. So, measure the line to line voltage for all three phases and calculate the voltage unbalance % (see the links below for how to calculate it). If the voltage unbalance is more than 1%, find the root cause of the unbalance and rectify it. More than 1% voltage unbalance may cause motor damage. 5% current unbalance is ok for the motor but it should be considered an alarming situation.

There are some other reasons for current unbalance:

Loose terminal connection

Build up of carbon or dirt on contactor inside the starter

Inter-turn short of the motor stator winding

For addition help, see

http://progress-energy.com/custservice/carcig/resourcectr/presentations/MotorProtection_VoltageUnbalance.pdf

http://www.patchn.com/index.php?option=com_content&task=view&id=41&Itemid=74

http://www.elongo.com/pdfs/voltages.PDF

http://www.pge.com/includes/docs/pdfs/mybusiness/customerservice/energystatus/powerquality/voltage_unbalance_rev2.pdf

- MS

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brich
Guru

Join Date: Oct 2009
Location: sometimes Wales,UK.. was Libya, now Oman!
Posts: 1715
Good Answers: 117
#3

Re: Motor Current

02/15/2010 10:44 PM

you might also check the supply cable from the motor controller to the motor terminals, flat cable has a habit of causing an imbalance in the current, also the age of the cable.

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Anonymous Poster
#4

Re: Motor Current

02/15/2010 11:19 PM

You did not mention actual 3-phase line voltage and current.

A possibility could be that there is a 1 phase brake winding between two of the phases.
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Anonymous Poster
#5

Re: Motor Current

02/16/2010 2:37 AM

you also may got an defected winding motor, such fault may cause such imbalance in the current and shaft power

you can use an motor tester device to check the motor windings, although such tests are essential and required only for high power motors
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Anonymous Poster
#6

Re: Motor Current

02/16/2010 11:59 AM

Please Check the winding resistance, I must be un-balance.

Diagnosis.------ Motor winding fault.

regards.

Pervez Iqbal
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Tazman20
Associate

Join Date: Oct 2008
Location: Regina, Saskatchewan
Posts: 51
Good Answers: 1
#7

Re: Motor Current

02/16/2010 3:19 PM

As the previous people have said check your current and voltage. Some other reasons are you could have a bastard leg and check to see if it is a Delta or Y.
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DannyD
Associate

Join Date: Oct 2009
Location: kuruman NC. SOUTH AFRICA
Posts: 51
Good Answers: 1
#8

Re: Motor Current

02/19/2010 4:23 AM

5% unbalance is allowed and yes check you motor phases impedence and supply voltage per phase they are definetly not the same.

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