Transformer Question
 

Someone asked me a question the other day and gave me these specs about
a transformer and I did not know the answer.

They gave me the scenario of a 1000VA transformer with losses of 5%
core losses and 3% copper losses. I told them that the losses in the
core are due to voltage and frequency and that the losses in the copper
are due to amperage. I also told them that eddy currents cause core
losses, but that these are minimized by the the laminations of the
transformer.

Since these losses are related to the VA of the transformer I just took
8% of 1000 and assumed the total losses to be 80VA. I assume that this
is correct.

He then asked about what if the transformer was loaded to its maximum.
This is the part I was unsure of, but going back to my memories of AC
circuit analysis I told him that I thought that the VA losses remained
the same for the core and copper losses since the input to the primary
does not change. However, the voltages change depending on whether the
transformer is a step up or down and that has to do with the number of
turns. I also told him that the amperage will be different in each side
of the transformer depending on if it is a step up or down but that
this did not effect the core and copper losses.

Was I completely incorrect in this assumption?

Thanks, Steve

Transformer Question
 

wrote in message
oups.com...
Someone asked me a question the other day and gave me these specs about
a transformer and I did not know the answer.

They gave me the scenario of a 1000VA transformer with losses of 5%
core losses and 3% copper losses. I told them that the losses in the
core are due to voltage and frequency and that the losses in the copper
are due to amperage. I also told them that eddy currents cause core
losses, but that these are minimized by the the laminations of the
transformer.

Core losses are a function of core material, core design, and frequency.
With certain exceptions, core loss tends to be fairly constant regardless of the
load on the transformer. That is why an energized but unloaded transformer
still gets quite warm. Copper loss is simply a function of Ohm's law (I squared
R). Copper loss would be near zero on an unloaded transformer and at its
maximum with a fully loaded transformer.

A transformer designed for step-down will typically have heavier copper in
the secondary to efficiently accommodate the larger current, while a step-up
transformer would normally have the reverse design.

Vaughn

Transformer Question
 

Without cracking a book I will venture this opinion:

Transformers are speck'ed fully loaded. That is to say the stated losses
occur at rated load. Eddy currents and magnetic hysteresis account for the
5% core loss; 3% copper loss is the combined loss of both primary and
secondary.

Bob Swinney

wrote in message
oups.com...
Someone asked me a question the other day and gave me these specs about
a transformer and I did not know the answer.

They gave me the scenario of a 1000VA transformer with losses of 5%
core losses and 3% copper losses. I told them that the losses in the
core are due to voltage and frequency and that the losses in the copper
are due to amperage. I also told them that eddy currents cause core
losses, but that these are minimized by the the laminations of the
transformer.

Since these losses are related to the VA of the transformer I just took
8% of 1000 and assumed the total losses to be 80VA. I assume that this
is correct.

He then asked about what if the transformer was loaded to its maximum.
This is the part I was unsure of, but going back to my memories of AC
circuit analysis I told him that I thought that the VA losses remained
the same for the core and copper losses since the input to the primary
does not change. However, the voltages change depending on whether the
transformer is a step up or down and that has to do with the number of
turns. I also told him that the amperage will be different in each side
of the transformer depending on if it is a step up or down but that
this did not effect the core and copper losses.

Was I completely incorrect in this assumption?

Thanks, Steve

Transformer Question
 

On 18 Mar 2006 22:57:51 -0800, "
wrote:

Someone asked me a question the other day and gave me these specs about
a transformer and I did not know the answer.

They gave me the scenario of a 1000VA transformer with losses of 5%
core losses and 3% copper losses. I told them that the losses in the
core are due to voltage and frequency and that the losses in the copper
are due to amperage. I also told them that eddy currents cause core
losses, but that these are minimized by the the laminations of the
transformer.

Since these losses are related to the VA of the transformer I just took
8% of 1000 and assumed the total losses to be 80VA. I assume that this
is correct.

He then asked about what if the transformer was loaded to its maximum.
This is the part I was unsure of, but going back to my memories of AC
circuit analysis I told him that I thought that the VA losses remained
the same for the core and copper losses since the input to the primary
does not change. However, the voltages change depending on whether the
transformer is a step up or down and that has to do with the number of
turns. I also told him that the amperage will be different in each side
of the transformer depending on if it is a step up or down but that
this did not effect the core and copper losses.

Was I completely incorrect in this assumption?

Thanks, Steve

Core loss will remain constant (for given excitation voltage and
frequency) regardless of load.

Copper loss will increase as the square of load current.

Transformer Question
 

Transformer Question
 

"Richard J Kinch" wrote in message
.. .
writes:

He then asked about what if the transformer was loaded to its maximum.
This is the part I was unsure of, but going back to my memories of AC
circuit analysis I told him that I thought that the VA losses remained
the same for the core and copper losses since the input to the primary
does not change.

Of course it changes. More output current draws more input current.

Actually, core losses don't change much until the core undergoes saturation,
then they go sky high.

But copper loss increases more or less directly with current, but
exacerbated by temperature rise.

LLoyd

Transformer Question
 

In article ,
Don Foreman wrote:
:
:Copper loss will increase as the square of load current.

Faster than that, actually. As the transformer heats up, the winding
resistance rises, further increasing the losses. That can cause an
overloaded transformer to go "over the hill" pretty rapidly.

--
Bob Nichols AT comcast.net I am "RNichols42"

Transformer Question
 

On Fri, 24 Mar 2006 12:39:21 GMT, "Lloyd E. Sponenburgh"
wrote:


"Richard J Kinch" wrote in message
. ..
writes:

He then asked about what if the transformer was loaded to its maximum.
This is the part I was unsure of, but going back to my memories of AC
circuit analysis I told him that I thought that the VA losses remained
the same for the core and copper losses since the input to the primary
does not change.

Of course it changes. More output current draws more input current.

Actually, core losses don't change much until the core undergoes saturation,
then they go sky high.

But copper loss increases more or less directly with current, but
exacerbated by temperature rise.

LLoyd

Copper loss increases as square of current, further axacerbated by
increased resistance due to temp rise as noted.

Peak core flux density changes very little with load; it actually
goes down a bit with load due to IR drop in the primary. It is
determined by primary voltage and frequency. It it isn't saturated
under noload condx then it won't saturate under load. Core loss may
change slightly with core temperature, not sure how that works with
silicon steel.

Transformer Question
 

"Don Foreman" wrote in message
...
Copper loss increases as square of current, further axacerbated by
increased resistance due to temp rise as noted.
Peak core flux density changes very little with load; it actually
goes down a bit with load due to IR drop in the primary. It is
determined by primary voltage and frequency. It it isn't saturated
under noload condx then it won't saturate under load. Core loss may
change slightly with core temperature, not sure how that works with
silicon steel.

Don.

1) I know the I^2R law. I said "more or less directly" because there are
several things that can change it, among them being the frequency of AC
being transformed, and interwinding/inter-turn capacitance. Even though
it's usually lumped together as "copper loss", it's not usually _only_ the
DCR of the wire they're accounting for in power transformers. They break
all that stuff out when spec'ing audio and RF transformers, but seldom
50-400Hz stuff.

2) Many transformers operate in rectifier circuits, and may have some DC
component in the secondary, as well as AC. Increasing the DC can indeed
induce saturation of the core. You know that.

(Prototypeing lab supervisor, Florida Transformer corp., div of Transitron,
1968)

LLoyd

Transformer Question
 

On Fri, 24 Mar 2006 18:03:12 GMT, "Lloyd E. Sponenburgh"
wrote:


"Don Foreman" wrote in message
.. .
Copper loss increases as square of current, further axacerbated by
increased resistance due to temp rise as noted.
Peak core flux density changes very little with load; it actually
goes down a bit with load due to IR drop in the primary. It is
determined by primary voltage and frequency. It it isn't saturated
under noload condx then it won't saturate under load. Core loss may
change slightly with core temperature, not sure how that works with
silicon steel.

Don.

1) I know the I^2R law. I said "more or less directly" because there are
several things that can change it, among them being the frequency of AC
being transformed, and interwinding/inter-turn capacitance. Even though
it's usually lumped together as "copper loss", it's not usually _only_ the
DCR of the wire they're accounting for in power transformers. They break
all that stuff out when spec'ing audio and RF transformers, but seldom
50-400Hz stuff.

2) Many transformers operate in rectifier circuits, and may have some DC
component in the secondary, as well as AC. Increasing the DC can indeed
induce saturation of the core. You know that.

(Prototypeing lab supervisor, Florida Transformer corp., div of Transitron,
1968)

Good point about rectification. I did forget about that, and that
would indeed be load-dependent. Thanks.