Dry transformer no-load consumption

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mobo

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How much power will a 75 KVA dry transformer consume under a no-load condition on the secondary? Assume a primary voltage of single phase 480v and 240/120v secondary. I have heard anywhere from 3-6% of the rated current draw. Is this correct? My understanding is the secondary, of course, would draw no current but the primary would. Would appreciate any input on this.
 
Jraef

Jraef

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3-6% is pretty extreme. Most likely that’s someone’s guess based upon the impedance (%Z), falsely assuming that equates to losses.

I didn’t read all of that old thread, but in case it was missed, the magnetixing current you can read isn’t necessarily going to equate to a lot of power, because as magnetizing current it is going to be at an extremely low power factor, as in a .2PF in most cases. So although it may look like 2A at 480 V is 960VA, the actual Watts, which is what you PAY FOR, is only 960 x .2 = 192W. Over the course of a month of being energized 24/7, it’s only 140kWh, so about $15/ month for most people.
 
jumper

jumper

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3 Hr 2 Min from Winged Horses
Jraef said:
3-6% is pretty extreme. Most likely that’s someone’s guess based upon the impedance (%Z), falsely assuming that equates to losses.

I didn’t read all of that old thread, but in case it was missed, the magnetixing current you can read isn’t necessarily going to equate to a lot of power, because as magnetizing current it is going to be at an extremely low power factor, as in a .2PF in most cases. So although it may look like 2A at 480 V is 960VA, the actual Watts, which is what you PAY FOR, is only 960 x .2 = 192W. Over the course of a month of being energized 24/7, it’s only 140kWh, so about $15/ month for most people.
Click to expand...

Why is the PF of the tranny so low under no load condition?
 
electrofelon

electrofelon

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Cherry Valley NY, Seattle, WA
I think the no load losses can vary quite a bit depending on the design of the transformer. Very likely the newer the transformer, the lower the losses will be. .5% is the general figure given and I think in the ballpark for modern units. Pad mounts seem to have lower losses based on measurements I have taken. I have a 15 kva pad, 2400 to 120/240 and I measured it at 60 watts. I also have the same thing in a dry type and it is three times that. I have a real old control transformer, 10 kva same voltages, and that thing is a whopping 250-300 watts.
 
T

topgone

Senior Member
electrofelon said:
I think the no load losses can vary quite a bit depending on the design of the transformer. Very likely the newer the transformer, the lower the losses will be. .5% is the general figure given and I think in the ballpark for modern units. Pad mounts seem to have lower losses based on measurements I have taken. I have a 15 kva pad, 2400 to 120/240 and I measured it at 60 watts. I also have the same thing in a dry type and it is three times that. I have a real old control transformer, 10 kva same voltages, and that thing is a whopping 250-300 watts.
Click to expand...

IMO, DOE's figure on no-load loss of a 75kVA Low Voltage Dry Transformer is about 120W (0.16%).
 
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