Understanding step down transformers

[Mint]

I could use some help in understanding why some transformers heat up
even when nothing is plugged into them.

Ex. Heavy duty shredder - heats up big time when shredder isn't
attached (15V and 20 watts)
- so I unplug it after each use

Have a great day,
Andy

http://intouch.org/magazine/daily-devotional
http://www.happynews.com

 

[amdx]

"Mint" <chocolatemint77581@yahoo.com> wrote in message
news:b6102f92-d6b1-4a87-9407-a80cc0fcafdd@u9g2000pra.googlegroups.com...

I could use some help in understanding why some transformers heat up
even when nothing is plugged into them.

Ex. Heavy duty shredder - heats up big time when shredder isn't
attached (15V and 20 watts)
- so I unplug it after each use

Have a great day,
Andy

My guess;
A cheap transformer can be made using less copper wire, that
causes an increase in the magnetizing current, producing extra heat,
even though it has no load.

 

[Phil Allison]

"Mint"

I could use some help in understanding why some transformers heat up
even when nothing is plugged into them.

Ex. Heavy duty shredder - heats up big time when shredder isn't
attached (15V and 20 watts)
- so I unplug it after each use

** The answer is the copper and iron losses due to magnetising current.

There is always some current flow in the primary winding of a transformer,
even with no load. This is called the "magnetising current" - naturally
there is also an amount power lost as heat due to this current aka " I
squared R " loss.

The designer of a transformer can allow the magnetising current level to be
any value he or she choses - including one that makes the unit get quite
hot.

The advantage is that the design will be smaller and cheaper if this is
done.

Usually, you only find it done when the unit will be under full load at all
times - eg the transformer inside a microwave oven.



..... Phil

 

[Sjouke Burry]

Mint wrote:

I could use some help in understanding why some transformers heat up
even when nothing is plugged into them.

Ex. Heavy duty shredder - heats up big time when shredder isn't
attached (15V and 20 watts)
- so I unplug it after each use

Have a great day,
Andy

http://intouch.org/magazine/daily-devotional
http://www.happynews.com


Bad quality, or when there are some shorted windings.

 

[whit3rd]

On Nov 23, 6:30 pm, Mint <chocolatemint77...@yahoo.com> wrote:

I could use some help in understanding why some transformers heat up
even when nothing is plugged into them.

Because the magnetization that occurs in a transformer,
and which changes sign with each AC half-cycle, is
MAXIMUM when there is no current drawn from the
secondary winding. Even in the absence of resistive
power loss (current in the windings) that changing
magnetic field causes heat generation in the iron core.

There's also, of course, at least a little bit of current
in the windings; the magnetization does draw power
in and of itself, though the major part of the energy
of magnetization just comes back in the next phase
of the AC cycle.

 

[Mint]

On Nov 23, 8:47 pm, "amdx" <a...@knology.net> wrote:

"Mint" <chocolatemint77...@yahoo.com> wrote in message

news:b6102f92-d6b1-4a87-9407-a80cc0fcafdd@u9g2000pra.googlegroups.com...

I could use some help in understanding why some transformers heat up
even when nothing is plugged into them.

Ex. Heavy duty shredder - heats up big time when shredder isn't
attached (15V and 20 watts)
                                    - so I unplug it after each use

Have a great day,
                         Andy

  My guess;
A cheap transformer can be made using less copper wire,  that
causes an increase in the magnetizing current, producing extra heat,
even though it has no load.

Interesting.

I will just put the transformer on a power strip.

Andy

 

[Winston]

Mint wrote:

On Nov 23, 8:47 pm, "amdx"<a...@knology.net> wrote:

(...)

My guess;
A cheap transformer can be made using less copper wire, that
causes an increase in the magnetizing current, producing extra heat,
even though it has no load.

Interesting.

I will just put the transformer on a power strip.

I wonder if this circuit or something similar
would limit power to your
shredder until it is turned on...?

Otherwise it dissipates a small amount of
'pilot power' into your transformer to sense
when the your switch is turned on.


/------+-----------------\
| | |
---- o\ /-+-- |
Relay nnnn o | \-. |
| | | --- )|| |
| | | --- )|| |
Current uuuu | | /-' |
Sense ---- | \-+-- |
Transformer nnnn | | Resonant |
| | | | Tank |
| | | | |
Line ------/ \-+------/ |
/------+ Load
| | Transformer
.-. | on plug strip
Neon ( X ) \-. ,-
'-' )|(
| )|(
Neutral -----------------------------+--------' '-

Current sensor powers plug strip
only after load is turned on



AACircuit www.tech-chat.de


--Winston

 

[m II]

On 10-11-24 12:27 AM, whit3rd wrote:

There's also, of course, at least a little bit of current
in the windings; the magnetization does draw power
in and of itself, though the major part of the energy
of magnetization just comes back in the next phase
of the AC cycle.

I don't see that. How do you get to your statement from examining a
hysteresis loop? The current in the next cycle has to flip the magnetic
domains right around again. That would entail a net loss of energy, not
a gain and also cause heating of the core (adding to that caused by
induced eddy currents)

<i would imagine that the more unsuitable a chunk of metal is in regards
to hysteresis, the greater the heat build up. The poor design of
transformers can be directly attributed to some evil foreign power
wanting a higher profit margin. People like that will even keep putting
LEAD into the paint and plastic of children's toys. Repeatedly.



mike

 

[whit3rd]

On Nov 24, 8:37 pm, m II <C...@in.the.hat> wrote:

On 10-11-24 12:27 AM, whit3rd wrote:

There's also, of course, at least a little bit of current
in the windings; the magnetization does draw power
in and of itself, though the major part of the energy
of magnetization just comes back in the next phase
of the AC cycle.

I don't see that. How do you get to your statement from examining a
hysteresis loop?

It's easy; the area enclolsed by that loop is exactly
the quantity of energy lost to heat in each AC magnetization cycle.
The closure of the loop means that the cycle also periodically
removes the memory of the past, which requires that the
magnetization energy be removed from its storage as a
net magnetic field in the core. Then replaced, in a
different orientation... and so on.

The remagnetization is caused by current in the wiring,
which generates heat by Ohmic resistance. Even though
the magnetic energy (total energy of magnetization) isn't
really lost, there are resistive losses in the wires that
deliver that energy and remove it.

 

[John Fields]

On Thu, 25 Nov 2010 12:18:14 -0800 (PST), whit3rd <whit3rd@gmail.com>
wrote:

On Nov 24, 8:37 pm, m II <C...@in.the.hat> wrote:
On 10-11-24 12:27 AM, whit3rd wrote:

There's also, of course, at least a little bit of current
in the windings; the magnetization does draw power
in and of itself, though the major part of the energy
of magnetization just comes back in the next phase
of the AC cycle.

I don't see that. How do you get to your statement from examining a
hysteresis loop?

It's easy; the area enclolsed by that loop is exactly
the quantity of energy lost to heat in each AC magnetization cycle.
The closure of the loop means that the cycle also periodically
removes the memory of the past, which requires that the
magnetization energy be removed from its storage as a
net magnetic field in the core. Then replaced, in a
different orientation... and so on.

The remagnetization is caused by current in the wiring,
which generates heat by Ohmic resistance. Even though
the magnetic energy (total energy of magnetization) isn't
really lost, there are resistive losses in the wires that
deliver that energy and remove it.

---
There are also eddy currents in the core which contribute to the
heating of the transformer.

---
JF

 

[whit3rd]

On Nov 26, 4:10 am, John Fields <jfie...@austininstruments.com> wrote:

On Thu, 25 Nov 2010 12:18:14 -0800 (PST), whit3rd <whit...@gmail.com
wrote:
the magnetic energy (total energy of magnetization) isn't
really lost, there are resistive losses in the wires that
deliver that energy and remove it.

There are also eddy currents in the core which contribute to the
heating of the transformer.

True; the effect is also to add a second H-induction to the external
circuit, so that the B-H curve tracer (really just a voltage/current
measurement of the transformer core primary) will show a bit of
open area due to the external-H being phase shifted from the
sum of external-H and eddycurrent-H. The only way to be sure
the loop's eye is a hysteresis effect is to look at it in the
low-frequency excitation limit. For accurate measurements,
I've seen this done at circa 1 Hz.