Transformer draw

[Pullnwire]

Guys,
I have a client that has two different 300KVA Transformers that are just Idling. The mains in the secondary are off. I shut them down this morning, but I was curious as to how much current went up in heat since these were installed in '06. I looked for a table somewhere, but no luck. is it just as simple as the total efficiency and the size of the transformer?

Thanks guys

 

[220/221]

I once did an installation (with my objections noted) of a 200 amp transformer to step up voltage from 208 to 480.

Even though it was only used on Sunday, the thing ran mostly idle, 24/7 to power a huge AC unit on a church. I read amperage @ 30 amps. Seemed like a waste of resources.

 

[Pullnwire]

Holy cow!

Holy cow!

I once did an installation (with my objections noted) of a 200 amp transformer to step up voltage from 208 to 480.

Even though it was only used on Sunday, the thing ran mostly idle, 24/7 to power a huge AC unit on a church. I read amperage @ 30 amps. Seemed like a waste of resources.

Wow, I saw an article somewhere about a 75KVA trans using about 1kwh at idle. your situation is much worse!

 

[mivey]

I will estimate 0.7 kW of no-load loss per 300 kVA unit, so roughly:

2 * 0.7 * 8760 = 12,264 kWh per year.

add: for a dry-type, normal efficiency, I found some loss data for a 480-208 that shows around 1760 watts to get you 30,835 kWh per year

 

[winnie]

Even though it was only used on Sunday, the thing ran mostly idle, 24/7 to power a huge AC unit on a church. I read amperage @ 30 amps. Seemed like a waste of resources.

Keep in mind that a significant amount of this 30A is magnetizing current, and that you will have very poor power factor when the transformer is idle. If you are getting charged for power factor, this could be bad...but if you are only being charged for KW, then this will be far less than the 10.8 KVA calculated from 30A at 208V.

-Jon

 

[Besoeker]

I will estimate 0.7 kW of no-load loss per 300 kVA unit, so roughly:

2 * 0.7 * 8760 = 12,264 kWh per year.

add: for a dry-type, normal efficiency, I found some loss data for a 480-208 that shows around 1760 watts to get you 30,835 kWh per year

I think the 1760W is more probable. The standard model I made and use for calculating losses gives the Fe loss as 2100W. Since we have to demonstrate that we meet system efficiency figures (or face financial penalties) I have probably gone a bit conservative.

 

[Besoeker]

Keep in mind that a significant amount of this 30A is magnetizing current,
-Jon

Assuming that it was measured on the input side of the transformer, of course.

 

[charlie]

For what it is worth, we figure that on average we are running a hair dryer for every transformer that is energized in waste heat. :smile:

 

[ohm]

Keep in mind that a significant amount of this 30A is magnetizing current, and that you will have very poor power factor when the transformer is idle. If you are getting charged for power factor, this could be bad...but if you are only being charged for KW, then this will be far less than the 10.8 KVA calculated from 30A at 208V.

-Jon

Had a small 480V x 120V area lighting transformer in a plant. They controlled the load side so during the day the transformer case got so hot you couldn't touch it.

 

[jim dungar]

Idling transformers was one of the reasons for the latest round (last couple of years) of energy efficient transformers. The new TP-1 design is supposed to be 'optimized' for 30% loading.

 

[ohmhead]

Well question a 300kva trans has a iron loss of 457 watts and a copper loss of 137 watts at half load what would this iron or copper loss be at at no load or full load ?


If temperature rises with load increase in winding current ? winding resistance would rise ? meaning at what point is a transformer at its most efficient point in time ?

Is it a rest no load ?

Is it at half load ?

Is it at full load ?

Just wondering about the effects of heat and power transfer or efficiency at different levels of load or no load ? best to yas

 

[mivey]

Idling transformers was one of the reasons for the latest round (last couple of years) of energy efficient transformers. The new TP-1 design is supposed to be 'optimized' for 30% loading.

FWIW, the TP-1 will give you 500 watts for a 150 deg rise and 740 for an 80 & 115 deg rise...much better than the K-4 in post #4.

add: the TP-1 efficiency is:
25% load = 98.7%
35% load = 98.8%
50% load = 98.7%
75% load = 98.6%
100% load = 98.2%

the K-4 is:
25% load = 97.3%
50% load = 97.9%
75% load = 98.0%
100% load = 97.7%