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Join Date: Jun 2011
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### Transformer Rating for Unit Motor

10/11/2012 6:19 AM

Hi Friends, What will be the MVA rating of the 22/6.6 kV transformer to feed a 4.65 MW unit motor 6.6kV, 3PH, 50Hz, FLC=625A and starting current (Is/Ir) = 4.67, PF@ full load=0.93, PF@ starting=0.15. Infact can anyone provide me a guideline how to do the calculation or is there any thumb rule. Thanks in advance for your valued advice.

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Guru

Join Date: Jul 2005
Location: Stoke-on-Trent, UK
Posts: 2053
#1

### Re: Transformer rating for unit motor

10/11/2012 6:28 AM

What's a unit motor?

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Active Contributor

Join Date: Jun 2011
Posts: 20
#3

### Re: Transformer rating for unit motor

10/11/2012 11:27 AM

Unit motor means one transformer to feed one motor

Commentator

Join Date: Mar 2010
Location: Islamic Republic of Pakistan
Posts: 78
#2

### Re: Transformer rating for unit motor

10/11/2012 7:14 AM

I have some confusion related to the given motor name plate 1. Confirm P.F is 0.93 at Full Load. 2. Confirm nameplate is correctly stated. Repeating the given Nameplate : 4650 KW, 6600 V, 50 Hz, 625 A, P.F 0.93 , (may be 994 RPM) Electrical Power = 1.732 * V * I * P.F Electrical Power = 1.732 * 6600 * 625 * 0.93 = 6644.38 KW Assume motor is 95 % efficient Mechanical Power = Electrical Power * Efficiency = 6644.38 * 0.95 = 6312 KW But according to given nameplate data it is 6312 KW instead it should be in the range between 4600 to 4800 KW. As per my understanding the nameplate may be: 4650 KW, 6600 V, 50 Hz, 487 A, P.F 0.87 Please confirm the exact name plate so that we proceed further toward the main problem. Thanks in advance

Guru

Join Date: Oct 2007
Location: India
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#4

### Re: Transformer Rating for Unit Motor

10/11/2012 1:57 PM

What is the starting method, DOL, soft starting etc.? What will be the starting torque?

Commentator

Join Date: Mar 2010
Location: Islamic Republic of Pakistan
Posts: 78
#5

### Re: Transformer Rating for Unit Motor

10/12/2012 12:52 AM

I have some confusion related to the given motor name plate

1. Confirm P.F is 0.93 at Full Load.

2. Confirm nameplate is correctly stated.

3. Is it a Wound Induction motor (having Liquid Resistance Starter) ?

Repeating the given Nameplate : 4650 KW, 6600 V, 50 Hz, 625 A, P.F 0.93 , (may be 994 RPM)

Electrical Power = 1.732 * V * I * P.F

Electrical Power = 1.732 * 6600 * 625 * 0.93 = 6644.38 KW

Assume motor is 95 % efficient

Mechanical Power = Electrical Power * Efficiency = 6644.38 * 0.95 = 6312 KW

But according to given nameplate data it is 6312 KW instead it should be in the range between 4600 to 4800 KW.

As per my understanding the nameplate may be: 4650 KW, 6600 V, 50 Hz, 487 A, P.F 0.87

Please confirm the exact name plate so that we proceed further toward the main problem.

Aghvel Niazi

Active Contributor

Join Date: Jun 2011
Posts: 20
#9

### Re: Transformer Rating for Unit Motor

10/13/2012 12:28 AM

The starting method is DOL. The starting torque is 19162 Nm.

Commentator

Join Date: Jun 2011
Posts: 66
#6

### Re: Transformer Rating for Unit Motor

10/12/2012 12:57 AM

http://mcglynnconsultancy.co.uk/html/tx_sizing.html

This will give you complete guidance on your question and the solved example will further help you.

Active Contributor

Join Date: Jun 2011
Posts: 20
#7

### Re: Transformer Rating for Unit Motor

10/12/2012 8:43 AM

Thank You my Friend, it was helpful

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#8

### Re: Transformer Rating for Unit Motor

10/12/2012 10:22 AM

Rule of thumb: 1 KVA per HP.

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