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I dislike ordering stuff that is not in stock



 
 
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  #21  
Old November 30th 16, 04:05 PM posted to uk.tech.digital-tv
Ian Jackson[_5_]
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Default I dislike ordering stuff that is not in stock

In message , d writes
On Wed, 30 Nov 2016 13:54:57 +0000
Ian Jackson wrote:
A more accurate explanation is that a practical transformer can be
considered as having a primary which consists of two windings in
parallel. Winding 1 carries the current which transfers its power to the
secondary winding. Winding 2 carries the current required to generate
the magnetic field which facilitates the transfer - but otherwise any
current flowing in Winding 2 is wasted.


Eh? Without current in the primary winding there is no magnetic field or
current in the secondary. The secondary is purely passive surely?


Like I said, a practical transformer can be considered as having a
primary which consists of two windings in parallel (Windings 1 and 2).
[Note that I'm not saying is that there are two physical primary
windings.] I have made no reference to the secondary.

When you're analysing what a real-world transformer does, it's usual to
think of it as having two parts - one being a perfectly lossless
transformer (a primary - which I called Winding 1, which is magnetically
coupled perfectly to what's on the secondary side). The inductive
reactances of primary Winding 1 and the secondary are considered to be
infinite (so nothing shunts either the input of the output) and they
have no resistance.

The imperfections of the transformer (lack of infinite inductive
reactance and presence of loss resistance) are represented by a second
winding in parallel with the input (which I called Winding 2).

The important thing is that the inductive reactance of Winding 2 is
shunting the mains input, and at the frequency of the AC mains, the
reactance has to be high enough not to take too much current.

At frequency F Hz, the reactance of an inductor of L henries is 2 x pi x
F x L This means that the reactance increases with frequency (and vice
versa), so a reactance which is sufficient 60Hz might not be sufficient
for 50Hz. As a result, Winding 2 takes more current. This causes it (and
the whole transformer) to overheat, and is also likely to blow the mains
fuse.


So you're basically saying for a lower frequency you need more wire in each
winding?


Quite. You need more shunt inductance. More wire can be either using
more turns or a larger core (so either way, you'll need a longer length
of wire) - or both - or maybe a core with a higher mu material (but
you're not likely find it).

Please note that I'm no expert on this sort of thing. It's about 55
years since I did this sort of stuff - and even then, I was no expert.
You might get a better idea of what I'm on about by looking he
http://bit.ly/2fLP3xX
At least you will see that I'm not talking total ********!
--
Ian
  #23  
Old December 1st 16, 04:42 AM posted to uk.tech.digital-tv
Bill Wright[_3_]
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Default I dislike ordering stuff that is not in stock

On 30/11/2016 07:56, Mark Carver wrote:

Martin still seems to be grumpy after his eye op,


In fact what made him grumpy was the nurse smacking his bum to start him
breathing.

Bill

  #24  
Old December 1st 16, 08:38 AM posted to uk.tech.digital-tv
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Default I dislike ordering stuff that is not in stock

On Wed, 30 Nov 2016 22:14:25 +0000
David Woolley wrote:
On 30/11/16 12:34, d wrote:
Surely it would be the other
way around since a higher frequency contains more energy for a given max
amplitude (in this case voltage)?


That's not true. The energy depends on all of time, current and
voltage. For the same voltage and same time period, and as long as the
current (and its phase relationship to the voltage) is the same, the
energy consumption is the same.


I think I was getting confused with audio where power scales with frequency
presumably because the faster you have to shove air molecules backwards and
forwards for a given amplitude the more energy is required to do it and with a
DC signal obviously there is no sound.

The only sense in which the energy goes up with frequency is at the
quantum level, but the energy in one photon at 50Hz is miniscule, about
3.313E-32 Joules, or 0.0000003313 yotto Joules.


See above.

--
Spud

  #25  
Old December 1st 16, 08:41 AM posted to uk.tech.digital-tv
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Default I dislike ordering stuff that is not in stock

On Wed, 30 Nov 2016 17:05:22 +0000
Ian Jackson wrote:
In message , d writes
On Wed, 30 Nov 2016 13:54:57 +0000
Ian Jackson wrote:
A more accurate explanation is that a practical transformer can be
considered as having a primary which consists of two windings in
parallel. Winding 1 carries the current which transfers its power to the
secondary winding. Winding 2 carries the current required to generate
the magnetic field which facilitates the transfer - but otherwise any
current flowing in Winding 2 is wasted.


Eh? Without current in the primary winding there is no magnetic field or
current in the secondary. The secondary is purely passive surely?


Like I said, a practical transformer can be considered as having a
primary which consists of two windings in parallel (Windings 1 and 2).
[Note that I'm not saying is that there are two physical primary
windings.] I have made no reference to the secondary.


Ok, but from my laymans point of view a real world transformer has 2 coils,
whether you call them windings or primary/secondary is immaterial.

http://bit.ly/2fLP3xX

Bit over my head I'm afraid. I did nice tidy digital electronics (as part of a
CS course), analogue is all a bit voodoo to me

--
Spud


  #26  
Old December 1st 16, 10:03 AM posted to uk.tech.digital-tv
Ian Jackson[_5_]
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Default I dislike ordering stuff that is not in stock

In message , d writes
On Wed, 30 Nov 2016 17:05:22 +0000
Ian Jackson wrote:
In message ,
d writes
On Wed, 30 Nov 2016 13:54:57 +0000
Ian Jackson wrote:
A more accurate explanation is that a practical transformer can be
considered as having a primary which consists of two windings in
parallel. Winding 1 carries the current which transfers its power to the
secondary winding. Winding 2 carries the current required to generate
the magnetic field which facilitates the transfer - but otherwise any
current flowing in Winding 2 is wasted.

Eh? Without current in the primary winding there is no magnetic field or
current in the secondary. The secondary is purely passive surely?


Like I said, a practical transformer can be considered as having a
primary which consists of two windings in parallel (Windings 1 and 2).
[Note that I'm not saying is that there are two physical primary
windings.] I have made no reference to the secondary.


Ok, but from my laymans point of view a real world transformer has 2 coils,
whether you call them windings or primary/secondary is immaterial.


OK. Just try to believe that a transformer has to have sufficient
inductance at the frequency in question.

So when connected across the mains supply with no load connected to the
secondary. its impedance (inductive reactance, or action as a choke - in
inductive ohms) is high enough to prevent it taking too much current.

As the reactance is proportional to frequency, the higher the frequency,
the greater the ohms. [I have a tiny transformer which is 120V in, 6V
out. However, it is out of an aircraft, where the onboard 'mains' used
to be 600Hz. If I fed it with 120V 50Hz, there would be a big bang.]

Conversely, if you lower the frequency, there are fewer ohms - and you
might find that the transformer is taking too much current for its own
good. To prevent this, the transformer has to be designed to have more
inductance (ie more turns, larger core - or both).

http://bit.ly/2fLP3xX

Bit over my head I'm afraid. I did nice tidy digital electronics (as part of a
CS course), analogue is all a bit voodoo to me

I have to admit I only have a grasp of the simplistic versions.

However, if you don't learn anything else about analogue, at least learn
Ohm's Law.
--
Ian
  #27  
Old December 1st 16, 10:18 AM posted to uk.tech.digital-tv
Andy Burns[_12_]
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Default I dislike ordering stuff that is not in stock

Ian Jackson wrote:

I have a tiny transformer which is 120V in, 6V
out. However, it is out of an aircraft, where the onboard 'mains' used
to be 600Hz. If I fed it with 120V 50Hz, there would be a big bang.


Not 400Hz?

  #28  
Old December 1st 16, 10:25 AM posted to uk.tech.digital-tv
Mark Carver[_2_]
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Posts: 252
Default I dislike ordering stuff that is not in stock

On 01/12/2016 11:18, Andy Burns wrote:
Ian Jackson wrote:

I have a tiny transformer which is 120V in, 6V
out. However, it is out of an aircraft, where the onboard 'mains' used
to be 600Hz. If I fed it with 120V 50Hz, there would be a big bang.


Not 400Hz?


Should be, yes.

Yonks ago I used my mains powered shaver on a plane, it didn't sound
happy, though it still did the job.



--
Mark
Please replace invalid and invalid with gmx and net to reply.
  #29  
Old December 1st 16, 10:50 AM posted to uk.tech.digital-tv
NY
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Default I dislike ordering stuff that is not in stock

"Mark Carver" wrote in message
...
On 01/12/2016 11:18, Andy Burns wrote:
Ian Jackson wrote:

I have a tiny transformer which is 120V in, 6V
out. However, it is out of an aircraft, where the onboard 'mains' used
to be 600Hz. If I fed it with 120V 50Hz, there would be a big bang.


Not 400Hz?


Should be, yes.

Yonks ago I used my mains powered shaver on a plane, it didn't sound
happy, though it still did the job.


I'd have expected that sockets which were intended to be used by the public,
rather than for avionics etc, would be 60 Hz (or 50 Hz for 240 V) for that
very reason, even if it has to be generated by rotary converter or
rectifier/inverter from the plane's own 400 Hz supply.

  #30  
Old December 1st 16, 11:12 AM posted to uk.tech.digital-tv
Mark Carver[_2_]
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Posts: 252
Default I dislike ordering stuff that is not in stock

On 01/12/2016 11:50, NY wrote:
"Mark Carver" wrote in message

Yonks ago I used my mains powered shaver on a plane, it didn't sound
happy, though it still did the job.


I'd have expected that sockets which were intended to be used by the
public, rather than for avionics etc, would be 60 Hz (or 50 Hz for 240
V) for that very reason, even if it has to be generated by rotary
converter or rectifier/inverter from the plane's own 400 Hz supply.


Not having an oscilloscope to hand, it was hard to tell what the supply
was :-) I must admit I assumed it was 400 Hz, but you're right, it makes
more sense that it was something 50/60 Hz ish.

I was on the way to the US, and the shaver just ran with a slightly
higher 'tizz' once there (as you'd expect). Perhaps the plane's supply
was 60 Hz, but nothing resembling a sine wave !


--
Mark
Please replace invalid and invalid with gmx and net to reply.
 




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