electrobuzz
Member
- Location
- Portland, OR
Does anyone know if there is a requirement to how much percentage you can overdrive a transformer?
Overdriving Transformers
- Thread starter electrobuzz
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iMuse97
Senior Member
- Location
- Chicagoland
You mean running a 75 KVA @ 82.5 for a 10% overdrive? Can't do it.
As far as I know, the only thing you can do is under-drive them.
broadgage
Senior Member
- Location
- London, England
A very brief overload, such as whilst starting a motor is normally fine.
Regular overloading would normally be a violation, "use and install equipment according to manufactuerers instructions" etc. etc.
There may may however be get outs re this.
Consult the supplier, the transformer may in fact be capable of far greater output than nominal in cold conditions, or for short term use.
A 100 KVA transformer continually loaded to 120 KVA in warm conditions wont last long.
OTOH, a 100 KVA transformer listed for continual operation in hot conditions, might last for decades at below freezing temperatures, even if loaded to 120 KVA
Likewise, a transformer listed for continous operation at 100 KVA might last for decades if loaded to 120 KVA for 2 hours a day, for 200 days a year.
Remember that it is heat that kills transformers, not load as such.
POCO or utilty transformers are not covered by the NEC and tend to be sized by old "rules of thumb"
Here in the UK, many rural houses are served by 5KVA transformers, despite the service being 60 amps at 240 volts, or about 15 KVA.
The transformers last for 50 years and more, since in practice almost no consumer would use more than 20 amps long term.
Heavy loading would tend to occur in very cold weather, and 40 amps on a 20 amp transformer would be fine in sub-zero conditions.
In warmer climates peak load may occur in the very hottest weather, and larger transformers will be required for a given sized service than in cold climates.
Regular overloading would normally be a violation, "use and install equipment according to manufactuerers instructions" etc. etc.
There may may however be get outs re this.
Consult the supplier, the transformer may in fact be capable of far greater output than nominal in cold conditions, or for short term use.
A 100 KVA transformer continually loaded to 120 KVA in warm conditions wont last long.
OTOH, a 100 KVA transformer listed for continual operation in hot conditions, might last for decades at below freezing temperatures, even if loaded to 120 KVA
Likewise, a transformer listed for continous operation at 100 KVA might last for decades if loaded to 120 KVA for 2 hours a day, for 200 days a year.
Remember that it is heat that kills transformers, not load as such.
POCO or utilty transformers are not covered by the NEC and tend to be sized by old "rules of thumb"
Here in the UK, many rural houses are served by 5KVA transformers, despite the service being 60 amps at 240 volts, or about 15 KVA.
The transformers last for 50 years and more, since in practice almost no consumer would use more than 20 amps long term.
Heavy loading would tend to occur in very cold weather, and 40 amps on a 20 amp transformer would be fine in sub-zero conditions.
In warmer climates peak load may occur in the very hottest weather, and larger transformers will be required for a given sized service than in cold climates.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
111014-0835 EDT
Overdrive is the wrong term for what the discussion seems to imply what the original question was. Overload would seem to be the correct term.
The current or power into a transformer when at its nominal voltage is determined by the load on the transformer, and not by something at the input to the transformer.
If you were to increase the input voltage to the transformer and force it into heavy magnetic saturation, then I would call that over-driving the transformer.
Most electromagnetic devices ( motors, transformers, relays, solenoids, etc. ) can be severely overloaded for short times because of their large thermal mass. This is in great contrast to semiconductor devices. Charles Kettering's invention of the electric cash-register and the automotive starter was based on this overload capability.
The important point is that during the overload time the breakdown points of the materials present are not exceeded. This is not a sharp threshold type of event, but more an overall stress situation.
.
Overdrive is the wrong term for what the discussion seems to imply what the original question was. Overload would seem to be the correct term.
The current or power into a transformer when at its nominal voltage is determined by the load on the transformer, and not by something at the input to the transformer.
If you were to increase the input voltage to the transformer and force it into heavy magnetic saturation, then I would call that over-driving the transformer.
Most electromagnetic devices ( motors, transformers, relays, solenoids, etc. ) can be severely overloaded for short times because of their large thermal mass. This is in great contrast to semiconductor devices. Charles Kettering's invention of the electric cash-register and the automotive starter was based on this overload capability.
The important point is that during the overload time the breakdown points of the materials present are not exceeded. This is not a sharp threshold type of event, but more an overall stress situation.
.
Temperature Rise
Temperature Rise
What is the temperature rating of the transformer, standard transformers are built with 220 degree C insulation, you can select a transformer with either 80, 115, or 150 degree rise, then you add in the 40 degree c operating temperature and 30 degree c hot spot and on a 150 you get 220. So if you have an 80 or 115 degree transformer you may be able to "overload" the transformer based on the load, but like it was stated earlier heat is the killer, so the overall life of the transformer would be shortened, definately not a recommended practice, what size is the primary OCPD of the xfmr?
Temperature Rise
What is the temperature rating of the transformer, standard transformers are built with 220 degree C insulation, you can select a transformer with either 80, 115, or 150 degree rise, then you add in the 40 degree c operating temperature and 30 degree c hot spot and on a 150 you get 220. So if you have an 80 or 115 degree transformer you may be able to "overload" the transformer based on the load, but like it was stated earlier heat is the killer, so the overall life of the transformer would be shortened, definately not a recommended practice, what size is the primary OCPD of the xfmr?
zog
Senior Member
- Location
- Charlotte, NC
A common thumb rule for life expectancy of transformers is as follows:
-Class ?A? insulation - 8C above max temp rating= ? life expectancy
-Class ?B? insulation - 12C above max temp rating= ? life expectancy
-Class ?A? insulation - 8C above max temp rating= ? life expectancy
-Class ?B? insulation - 12C above max temp rating= ? life expectancy
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
You can get more out of a transformer by reducing the temperature. Hence the ability to get an additional 13% simply by going from 55 deg C rise to 65 deg C rise. However, this is for larger power transformers. Fans can add 25-33% percent increase depending on the design.
Something to watch for is voltage regulation. When you try to get more on the load side then rated, the voltage drop across the transformer will go up thereby possibly defeating the whole purpose of a temp overload, especially if it was to start a motor.
Something to watch for is voltage regulation. When you try to get more on the load side then rated, the voltage drop across the transformer will go up thereby possibly defeating the whole purpose of a temp overload, especially if it was to start a motor.
Overloading xfmrs is typically done in industry and utility for short durations (emergency ratings). Each electrical device has a thermal damage curve, which plots right on your TCC. ETAP or SKM usually plot this for you based on the rating of the xfmr. Set your OPD under this curve, and you allow for some short-term overload when required, but still don't change the life expectancy of the equipment.
Damage is also exponential...The greater you are overloading a transformer, the shorter the life span. Also, don't design a system where the transformer is constantly overloaded...
Rob Wolf, PE
Damage is also exponential...The greater you are overloading a transformer, the shorter the life span. Also, don't design a system where the transformer is constantly overloaded...
Rob Wolf, PE
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
Why not utilities do it all the time.............:lol:
Lady Engineer
Senior Member
- Location
- New Jersey
You know I'm a utility power consultant, and we do this sometimes. I know...yikes
It does shorten the life, and most of the transformers we allow the client to have are oil filled. From my experience oil filled MV Xfmrs have been very durable. BUT, it's not a good idea. However, some of the transformers are rated to be overdriven by up to 12%, but I do not recommend you to intentionally do it just to avoid the next size.
My two cents..
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Maybe not all of the time. but they definitely do it on occasion.
Don't conductor losses go up as the 'square of the increase', so a 115% load would yield 132% of losses, which they would have to eat.
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