# magnetizing current for transformer

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#### wireguru

##### Senior Member
How does one determine magnetizing current for a transformer? I need to figure out what the monthly cost is going to be for a 15kva 240D-208Y120 transformer to sit energized with little to no load.

#### cpal

##### Senior Member
How does one determine magnetizing current for a transformer? I need to figure out what the monthly cost is going to be for a 15kva 240D-208Y120 transformer to sit energized with little to no load.

I'd contact the mfg

#### weressl

##### Esteemed Member
How does one determine magnetizing current for a transformer? I need to figure out what the monthly cost is going to be for a 15kva 240D-208Y120 transformer to sit energized with little to no load.

You mean the no load losses?!
3% is a conservative estimate.

#### wireguru

##### Senior Member
You mean the no load losses?!
3% is a conservative estimate.

so I am getting around 324 kwh/mo is that right?

#### brian john

##### Senior Member
With the transformer in a no load condition measure the current. it'll get you in the ball park.

#### richxtlc

##### Senior Member
The amount of magnetizing inrush current of a transformer is usually 8 to 10 times full load current. The inrush current will change if the transformer has been recently deenergized as it still has residual magnetism left in the core. If the transformer has been deenergized a long time, the inrush current increases. Another factor, is the mass of the core, the greater the mass the greater the inrush.
The differential relay is designed to block for high inrush current and trip for that same value of fault current. Inrush current is high in second harmonics and the differential relay blocks when it detects it. Fault current has only 60 Hz and the relay is allowed to trip.
The next time you energize a transformer watch the ammeter, if it has been off for a while it may peg the ammeter. If it was deenergized for a short time it may barely move the needle. I have seen this occur and have measured the inrush with a recording oscillograph.

#### rcwilson

##### Senior Member
Eaton's Consulting Application guide has the following information for 15 kVA transformers, 600V class Dry Types:

NEMA TP-1

The no load loss is the magnetizing current energy loss. Note that there wil also be losses in the power system feeding the transformer.

I'd use the 150 W for an existing dry type, and 80 -100 W for a new unit since newer transformers are supposed to be energy efficient. Throw in 10W for other losses.

#### rcwilson

##### Senior Member
Eaton's Consulting Application guide has the following information for 15 kVA transformers, 600V class Dry Types:

The no load loss is the magnetizing current energy loss. Note that there will also be losses in the power system feeding the transformer.

I'd use the 150 W for an existing dry type, and 80 -100 W for a new unit since new transformers are supposed to be energy efficient. Throw in 10W for other losses.

(Corrected- couldn't get edit feature to work.)
__________________
Bob Wilson P.E.

#### Besoeker

##### Senior Member
With the transformer in a no load condition measure the current. it'll get you in the ball park.
That would get you VA rather than W and, on no load, the transformer pf is generally quite low.

#### zog

##### Senior Member
How does one determine magnetizing current for a transformer? I need to figure out what the monthly cost is going to be for a 15kva 240D-208Y120 transformer to sit energized with little to no load.

This sounds like one of those energy cost savings ideas. Keep in mind that if you idle a dry type transformer for any period of time you need to put it in layup. If it justs sits there de-energized the windings and insulation will absorb moisture and may fail when you energize it, so much for cost savings. You also need to factor the cost of labor to put it into a layup condition and to prepare it for re-energizing later and testing prior to re-energizing.

#### mivey

##### Senior Member
This sounds like one of those energy cost savings ideas. Keep in mind that if you idle a dry type transformer for any period of time you need to put it in layup. If it justs sits there de-energized the windings and insulation will absorb moisture and may fail when you energize it, so much for cost savings. You also need to factor the cost of labor to put it into a layup condition and to prepare it for re-energizing later and testing prior to re-energizing.
What is the ball-park for a testing on this size unit as a comparison to the no-load loss?

#### zog

##### Senior Member
What is the ball-park for a testing on this size unit as a comparison to the no-load loss?

The only time I would do full testing on a transformer this small is after installation so hard to estimate because that usually involves testing everything at a new facility.

After being in layup I would just do a megger test (>100M) and make sure the DAR and PI are >1.0, so that should be able to be done in house. Nothing you need to call a testing company for.

So cost wise you are looking at internal labor only, say 30 minutes to put it in layup and 30 minutes to test it before energizing. 1 hour internal labor?

But from my experience most places dont do anything, they just let them sit there for a year or more then when they re-energize them, poof! Time for a new transformer.

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#### wireguru

##### Senior Member
This sounds like one of those energy cost savings ideas. Keep in mind that if you idle a dry type transformer for any period of time you need to put it in layup. If it justs sits there de-energized the windings and insulation will absorb moisture and may fail when you energize it, so much for cost savings. You also need to factor the cost of labor to put it into a layup condition and to prepare it for re-energizing later and testing prior to re-energizing.

not an energy savings thing. The issue is our building is supplied with 120/240v single phase and 240 delta. Out of capacity on the 120/240 service which cannot readily be upgraded. The 240 delta service is not in use. The loads supplied by the transformer would be small and mostly intermittent. Trying to determine the monthly cost for the transformer being there.

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