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Transformer Rating for Motor

06/25/2009 11:51 PM

my question is very simple that is :

i want to install 30hp 3 phase motor , what would be the transformer rating for this motor.

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#1

Re: supply

06/26/2009 12:32 AM

my question is very simple that is

Unfortunately the answer is not.

http://cr4.globalspec.com/thread/39570/Supply-Transformer-Ratings

by supply transformer, i mean that there is an isolated/seperate transformer is required for 30 hp motor and what should be the exact rating of this transformer when

1)using DOL and

2)using soft starter expain pls.

As others have said, you should probably talk with either a consultant or electrician as the transformer size depends on a number of electrical and motor variables you have not mentioned, and as such there can be no exact or guaranteed right answer with the limited information given.

The closest figure I can give you is a transformer of at least 25kW when using a soft starter and say 100kW if using a DOL starter. Note that this doesn't guarantee your motor will work if your primary supply to the new transformer is too "soft" and sags excessively due to the additional load.

If what I am saying is still confusing, refer back to my original comment above and get a professional to look at your system.

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#6
In reply to #1

Re: supply

06/27/2009 12:41 AM

detail of my question:

induction motor specifications:

rated voltage=400

rated power =7.5KVA

rated current=14amp

transmission line=11kva

distribution line =220v single phase

now how to calculate the KVA of the required transformer for the motor :

1)drive by soft starter

2) direct on line

explain pls

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#7
In reply to #6

Re: supply

06/27/2009 1:00 AM

rated voltage=400

rated current=14amp

distribution line =220v single phase

Sounds more like a 3hp motor, not 30hp.

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#2

Re: supply

06/26/2009 3:53 AM

Whats the overload capability of the transformer?

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#3

Re: supply

06/26/2009 9:25 AM

Why is it being fed from its own transformer?

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#4

Re: supply

06/26/2009 9:44 AM

Are you talking about the transformer rating or relay settings?

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#5

Re: Transformer Rating for Motor

06/26/2009 11:23 PM

30hp = 22.4 kW

Add in some heat losses, etc.

Would a 30kVA transformer suffice? I don't know, it just seems rational, so I'm throwing it out there.

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Anonymous Poster
#8

Re: Transformer Rating for Motor

06/27/2009 2:43 AM

Considering :

1] You have no base load

2] Power factor of the motor : 0.8o lag

3]Type of starting : Star & delta

so 30 hp , works out to 22.38 kw

with 0.8 pf ---28 kva ratingkeeping 50 % factor of safety----42 Kva rating transformer

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#9

Re: Transformer Rating for Motor

06/28/2009 6:29 AM

Here's an example of why your question is not easy to answer. Also, engineering does not include "guessing", as that usually costs more money.

The below example is quite basic but will nonetheless give you enough information to know whether what you're doing is correct or incorrect. Please note all the assumptions I have made. My assumptions may not apply to your installation.

If you want your question answered correctly, hire a consultant...OR you could visit Yahoo Answers, where all manner of buffoons spout all manner of rubbish.

Why buy something that is oversized when the correct size can be calculated (& possibly save money)?

In order to correctly size a transformer for a single DOL motor, the following things must be known;

1] The FLA of the motor.

2] The rated voltage of the motor.

3] The Secondary phase-to-phase voltage of the transformer.

4] The impedance of the transformer.

5] The KVA rating of the transformer.

6] The Short Circuit Current capability of the transformer.

The following things will be assumed;

1] The supply to the transformer Primary has an unlimited Fault Current capability.

2] The motor is NEMA rated i.e. the motor is capable of efficiently operating at + or - 10% of it's nameplate voltage.

3] The motor start current is 600% of the FLA (FLA means Full Load Current).

4] All supply cable impedances are ignored.

Example 1.

You have a 22kW, 3 phase motor. The nameplate ratings are;

1] Voltage - 380.

2] P.F. - 0.8.

You also have a 3 phase, 30 KVA, 380 volt transformer with an impedance of 4%. Can this transformer be used with the above motor?

Step 1.

Find the FLA of the transformer.

30 KVA / (380 x 1.732) = 45 amps.

Step 2.

Find the Short Circuit capability of the transformer.

45 amps / 0.04 = 1125 amps.

Step 3.

Find the voltage drop caused by the motor start current. Allowed minimum voltage at motor terminals;

380 volts minus 10% = 380 - (380 x 0.1) = 342 volts.

Find KVA of motor.

380 x 45 x 1.732 = 29.6 KVA.

Find start KVA of motor.

29.6 KVA x 6 = 177.6 KVA.

Since voltage drop is proportional to load current, it will be expressed as a percentage of motor start KVA to transformer short circuit KVA.

Tranformer short circuit KVA = 380 x 1125 x 1.732 = 740 KVA

Motor start KVA = 177.6 KVA.

The voltage at motor start will be;

177.6 KVA / 740 KVA = 0.24 or 24% of the supply voltage.

The transformer supply voltage at motor start will therefore be;

380 x .76 = 288.8 volts

Since the minimum allowable voltage at the motor is 342 volts, it can be seen that this transformer is too small.

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#10
In reply to #9

Re: Transformer Rating for Motor

06/28/2009 11:47 AM

In supporting elkangorito's calculation, there is a very good information can be found in this link

http://www.powerqualityanddrives.com/short_circuit_transformer

- MS

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#11
In reply to #10

Re: Transformer Rating for Motor

06/28/2009 12:01 PM

This site is cool, I used it before for transformer sizing !!

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#12
In reply to #10

Re: Transformer Rating for Motor

06/28/2009 12:10 PM

Thanks MS .

This method is old but proven. All it takes is a little "expansionary knowledge" to apply the concept to reality.

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#14
In reply to #9

Re: Transformer Rating for Motor

07/06/2009 12:30 AM

i want to offer thanks to the this answer writer as you have tried best and helped me in this regard , budle of thanks

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#15
In reply to #9

Re: Transformer Rating for Motor

07/06/2009 12:37 AM

reply to ans: 9

this is a very useful discussion due towhich i reached at the design stage. there is rasie tow question which i would like to be cleared.

1) how you calculated the 4% impeadance

2) as you described

The transformer supply voltage at motor start will therefore be;

380 x .76 = 288.8 volts

in this eq. what is the fig . 0.76

discuss pls.

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#16
In reply to #15

Re: Transformer Rating for Motor

07/06/2009 7:52 PM

Let me answer it:

1) 4% is the transformer percentage impedance (or voltage impedance). It can be found in transformer name plate.

2) 1.00 - 0.24 = 0.74.

0.24 is from 24%, the percentage voltage drop at motor start.

- MS

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Anonymous Poster
#13

Re: Transformer Rating for Motor

07/05/2009 3:12 PM

if you install a transformer or transformers you have to know voltage and full load amps of the motor, and if it is going to be a continuing running motor.

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Users who posted comments:

Aldego (1); Anonymous Poster (2); elkangorito (2); GULKHAN (3); jack of all trades (1); lasam (1); Mitsurati (2); msamad (2); PWSlack (1); sb (1)

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