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Guru

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Sizing a Control Transformer for Various Loads

04/06/2009 9:39 AM

Can anyone help me to size the control transformer for the following loads?

Total sustained load = 8.1 A

Total inrush load = 41 A

It will be used for the closing control circuit of a LV breaker.

Is 2KVA transformer OK?

Thanks,

- MS

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#1

Re: Sizing a Control Transformer for Various Loads

04/06/2009 10:01 AM

What voltage?

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Guru

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#3
In reply to #1

Re: Sizing a Control Transformer for Various Loads

04/06/2009 1:01 PM

120 VAC

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#2

Re: Sizing a Control Transformer for Various Loads

04/06/2009 11:23 AM

8.1 amps x voltage = VA or watts. Use this figure to size the transformer.

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#4

Re: Sizing a Control Transformer for Various Loads

04/06/2009 10:41 PM

Yes it's more then enough

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#5

Re: Sizing a Control Transformer for Various Loads

04/07/2009 1:40 AM

1 What are the voltages on the primary and secondary side?

2 Will there be any other loads on the transformer at a later date?

3 Will the breaker be operated occassionally or rather frequently?

4 Any other relevant details?

SAMUEL CHELLIAH

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#6
In reply to #5

Re: Sizing a Control Transformer for Various Loads

04/07/2009 1:46 AM

1. 110kV/33kV

2. No

3. It's regular operating breaker

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#7
In reply to #6

Re: Sizing a Control Transformer for Various Loads

04/07/2009 2:15 AM

I have enclosed the clculations. Transformers have to be designed for the starting currents. If you need any further clarifications please let me know

SAMUEL CHELLIAH

Inrush current in amps41
Sustained current in amps8.1
Secondary voltage in kV33
Ratio of inrush to sustained current5.061728
Sustained kVA (8.1*33)267.3
Inrush kVA (41*33)1353
TRANSFORMER RATING
CRITERIA 1 - 15% margin on operating load in kVA1555.95
CRITERIA 2 - Recommended transformer rating with 1.41894.20
times over load capacity for 1 min as per IS : 6600
Hence, theoretically required transformer rating1894.20

From the above, I'll suggest you can go in for a 1750kVA transformer

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#8
In reply to #7

Re: Sizing a Control Transformer for Various Loads

04/07/2009 2:22 AM

Sorry!!

Mr. Samuel , I have given voltage parameters for sensitive earth fault calculations not for Transformer sizing

Pls. ignore this

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#9
In reply to #8

Re: Sizing a Control Transformer for Various Loads

04/07/2009 8:55 AM

Mr Krishna,

Can you give me the primary and secondary voltages of the transformer? this is important to fix the rating of the transformer

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#10
In reply to #9

Re: Sizing a Control Transformer for Various Loads

04/07/2009 5:30 PM

The primary is 480V and secondary is 120V

Secondary loads:

Total sustained load = 8.1 A

Total inrush load = 41 A

Control Transformer size ?

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#11
In reply to #10

Re: Sizing a Control Transformer for Various Loads

04/08/2009 12:56 AM

I presume the currents you have indicated are the secondary side currents

Inrush current in amps41
Sustained current in amps8.1
Secondary voltage in V120
Ratio of inrush to sustained current5.06
Sustained kVA (8.1*120/1000)0.97
Inrush kVA (41*120/1000)4.92
TRANSFORMER RATING
CRITERIA 1 - 15% margin on operating load in kVA5.658
CRITERIA 2 - Recommended transformer rating with 1.4 times overload for 1 min as per IS: 66007.9212
Hence, theoretically required transformer rating7.9212

From the above, I'll suggest you can go in for a 8kVA transformer

SAMUEL CHELLIAH

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#12
In reply to #10

Re: Sizing a Control Transformer for Various Loads

04/08/2009 7:52 AM

dEAR,

Ratio starting Amp / Normal Amp = 41/8.1 = 5

As any transformer can take up the starting current 6 - 8 times , thermal effect is neglected.

You require transformer to suit 8.1 * 120V = 972 VA.

Add 15% safety for ambient variations, I suggest to go for 1.2 KVA rated transformer.

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#13
In reply to #12

Re: Sizing a Control Transformer for Various Loads

04/09/2009 12:26 AM

I was dealing with lift irrigation schemes where the motors are DOL (direct on line) starting. The manufacturers recommended the transformers should be rated for the starting current and not the rated current. In my experience I have seen the transformer voltage dropping drasticallyduring starting if not designed for the starting current. Hence the transformer should be designed for the starting current

SAMUEL CHELLIAH

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harry potter (1); msamad (2); PWSlack (1); Samuel Chelliah (5); VC KRISHNA (3); wareagle (1)

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