transformer frequency dependency

[rob]

Hello all,
What determines the acceptable frequency for a given transformer?
I had believed that the higher voltages involved, you simply need to
use higher frequencies (ie the flyback transformer in a tv or
monitor).

But the power company routinely boosts 60Hz power up to several
thousand KV, and I am assuming they dont have to convert frequency of
the current to do it. What am I missing here? Thanks in advance...
Rob

 

[Roger Johansson]

rob <ngneer314@yahoo.com> wrote:

What determines the acceptable frequency for a given transformer?

The inductance, you cannot use a transformer effectively if the inductance
causes big losses at that frequency.
So the inductance needs to be fairly low in relation to the frequency.

Low inductance transformers, with a low number of turns, can use higher
frequencies.

I had believed that the higher voltages involved, you simply need to
use higher frequencies (ie the flyback transformer in a tv or
monitor).

The voltage is not related to the frequency, as you can see from the
replies you have been given already.

We use low inductance transformers which are physically small, at higher
frequencies, to save on the size and weight of the transformer.

The size cannot be smaller than what is needed by the power you want to
transform, there are limits to how much power can be sent through magnetism
through a transformer.
If you need more power you have to make it bigger.

A good example is the transformer in a microwave oven.
It was designed for a minimum of weight and a maximum of power.
So it had to be low inductance to use a high frequency, so the size could
be kept to a minimum.
It still had to be big enough though, to transform the high power needed.


--
Roger J. (No Emails)

 

[Jan Panteltje]

On Tue, 23 Mar 2004 04:40:51 GMT, rob <ngneer314@yahoo.com> wrote:

Hello all,
What determines the acceptable frequency for a given transformer?
Mainly the losses in its core (if there is one).

I had believed that the higher voltages involved, you simply need to
use higher frequencies (ie the flyback transformer in a tv or
monitor).
Yo uare confusing 2 effects.

The high voltage is generated by connecting a coild (inductance)
to a DC volatge, and then disconnecting it (llike in a car, and here
you can have REALLy high voltage with low engine rpm), a circuit
creaker (contacts) or a tcvacumm tube or a transistor can be used
as circyuit breaker.
Anyways, when you break the circuit, the energy stiored in the
magnetic field will try to maintain current, and a large voltage spike
will appear.

Nothing to do with transformers.
But of cause yo ucan put a seciond number of turns around the coil,
and transform that voltage peak even higher.
(Like in a car or TV).

But the power company routinely boosts 60Hz power up to several
thousand KV, and I am assuming they dont have to convert frequency of
the current to do it. What am I missing here? Thanks in advance...
Rob
Ratio number of turns sets the voltage,

if 100 turns primary and 100 V, 1000 turns secondary = 1000 V
The MINIMUM number of turns you can use is set by the frequency
used.
This is because in an inductor, if you apply a DC voltage, the current
will rise linear with time i = t /L (from l di/dt)
So if the period is long (low frequency) the current will rize and
rize and rize, and with a bit of bad luck the core metal will be 100
% magnetized, and no more energy can be stored in it.. and the
inductance will drop... and the current i will go up even more, but no
longer linear, but only limited by the resistance in Ohms, and the
thing will pop....
So as a rule, the lower the frequency the more turns per volt
you will need, that is why 50 or 60 Hz mains transformers have many
many turns per volt, and the same transformers for a high frequency
have only perhaps 1 turn per volt.
Hope this simple explanation gets some basics across.
JP

 

[Nick Kennedy]

You need to weigh frequency against the amount of magnetizing
inductance in the transformer.

Consider an ideal transformer. When there's no load on the secondary,
no current flows in the primary.

Now go to a more realistic but still fairly simple model. Draw the
ideal transformer with some inductance in series with the primary
(leakage inductance) and also a shunt inductor across the primary
(magnetizing inductance).

Obviously, the reactance of the magnetizing inductance determines how
much current the xfmr is drawing when unloaded. If this reactance is
too small (or the frequency is too low), that current can be
prohibitively high. You'd like it to be negligible compared with
rated current (maybe 10% or so?).

Big power transformers at switchyards and power plants have volts per
hertz relays to protect against low frequency operation.

Of course, I think you were concerned about limits on the other end.
That's not really that big a deal, although when you get up out of the
power frequency range, things like inter-winding capacitance and
frequency dependent core losses come into play.

In the RF range, you can cure a problem with too low magnetizing
inductance by resonating it with a capacitor and you gain selectivity
to boot.

Oh yeah, you said power transformers handle voltages in thousands of
kV. That's a bit of a stretch, since it equates to millions of volts.
Typically, 500 kV is pretty high, although you will see some at 750
kV. At the million volt transmission level, DC is usually employed.

There you go ...

Nick

--in Arkansas

rob <ngneer314@yahoo.com> wrote in message news:<4tfv50td9kv2j3jkje0tt82kg220dc6ctt@4ax.com>...

Hello all,
What determines the acceptable frequency for a given transformer?
I had believed that the higher voltages involved, you simply need to
use higher frequencies (ie the flyback transformer in a tv or
monitor).

But the power company routinely boosts 60Hz power up to several
thousand KV, and I am assuming they dont have to convert frequency of
the current to do it. What am I missing here? Thanks in advance...
Rob

 

[Winfield Hill]

Nick Kennedy wrote...

You need to weigh frequency against the amount of magnetizing
inductance in the transformer.

Consider an ideal transformer. When there's no load on the
secondary, no current flows in the primary.

Is there some reason you're choosing to ignore the primary
magnetizing current that even perfect transformers must have?
This current becomes very high at low frequencies and along
with the volt-second core-saturation issue is the reason for
a low-frequency limit. But you know that.

Thanks,
- Win

whill_at_picovolt-dot-com

 

[John Woodgate]

I read in sci.electronics.design that Winfield Hill
<Winfield_member@newsguy.com> wrote (in <c3sonm08tb@drn.newsguy.com&gt
about 'transformer frequency dependency', on Wed, 24 Mar 2004:

Is there some reason you're choosing to ignore the primary
magnetizing current that even perfect transformers must have?

The core of an ideal transformer has infinite permeability.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Paul Hovnanian P.E.]

Nick Kennedy wrote:

You need to weigh frequency against the amount of magnetizing
inductance in the transformer.

Consider an ideal transformer. When there's no load on the secondary,
no current flows in the primary.

Now go to a more realistic but still fairly simple model. Draw the
ideal transformer with some inductance in series with the primary
(leakage inductance) and also a shunt inductor across the primary
(magnetizing inductance).

Obviously, the reactance of the magnetizing inductance determines how
much current the xfmr is drawing when unloaded. If this reactance is
too small (or the frequency is too low), that current can be
prohibitively high. You'd like it to be negligible compared with
rated current (maybe 10% or so?).

Big power transformers at switchyards and power plants have volts per
hertz relays to protect against low frequency operation.

But not for the reasons you have given. Typical underfrequecy settings
might be a few tenths of a Hz below nominal. There is no problem
operating a 60 Hz transformer at 59.5 Hz for extended periods. The UF
trip is there for load shedding purposes. If the system frequency sags
for a certain period of time, the available generation isn't sufficient
to supply loads. Stations or circuits feeding lower priority loads are
set to be shed when the UF threshold has been reached for a given period
of time.


--
Paul Hovnanian mailtoaul@Hovnanian.com
note to spammers: a Washington State resident
------------------------------------------------------------------
Rube Goldberg is alive and working for Microsoft.

 

[Winfield Hill]

John Woodgate wrote...

The core of an ideal transformer has infinite permeability.

Sheesh, that's a mind-boggling concept!

Thanks,
- Win

whill_at_picovolt-dot-com

 

[John Woodgate]

I read in sci.electronics.design that Winfield Hill
<Winfield_member@newsguy.com> wrote (in <c3t2jv01873@drn.newsguy.com&gt
about 'transformer frequency dependency', on Wed, 24 Mar 2004:

John Woodgate wrote...

The core of an ideal transformer has infinite permeability.

Sheesh, that's a mind-boggling concept!

Well, if you make a transformer core from tin cans, and then change to

silicon iron, the magnetizing current goes down. If you change to
nickel-iron (neglecting any saturation effect), it goes down further. So
higher permeability is a better approximation to 'ideal'. It follows
that an ideal transformer has zero magnetizing current. But the
induction B = [mu]H = oo x 0, which can be any finite value.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Animesh Maurya]

Roger Johansson <no-email@home.se> wrote in message news:<nof63d7ont5o$.4hlmkkuihb05.dlg@40tude.net>...

rob <ngneer314@yahoo.com> wrote:

What determines the acceptable frequency for a given transformer?

The inductance, you cannot use a transformer effectively if the inductance
causes big losses at that frequency.
So the inductance needs to be fairly low in relation to the frequency.

Low inductance transformers, with a low number of turns, can use higher
frequencies.

I had believed that the higher voltages involved, you simply need to
use higher frequencies (ie the flyback transformer in a tv or
monitor).

The voltage is not related to the frequency, as you can see from the
replies you have been given already.

We use low inductance transformers which are physically small, at higher
frequencies, to save on the size and weight of the transformer.

The size cannot be smaller than what is needed by the power you want to
transform, there are limits to how much power can be sent through magnetism
through a transformer.
If you need more power you have to make it bigger.

A good example is the transformer in a microwave oven.
It was designed for a minimum of weight and a maximum of power.
So it had to be low inductance to use a high frequency, so the size could
be kept to a minimum.
It still had to be big enough though, to transform the high power needed.

By the basic formula of transformer V2/V1 = N2/N1,
we can build a low frequency transformer with minimum number of coil
turns.
If this is the case we may soon burn it out.

Reactance offered by the coil depends on either inductance or
frequency.

So at low frequency we need to have large numbers of coils turns in
order to increase the inductance.

Iam not sure about the exact relation between coil turns and
inductance, hope it is something like L = (no. of turns * magnetic
flux)/current

Contrary to this at higher frequency we need less coil turns (small
inductance) to maintain the reactance.

When I hammered down tv flyback transformer, I saw secondary coil
quite thin as the primary one. So I can make a guess that current
carrying capacity is quite less.

You just mentioned about MOT which can handle large power, so is the
thickness of the coil consideration here.

Now suppose if I try to make a step down, low frequency transformer
with turns ratio quite small, but the thickness of coil is sufficient
to carry very large current without burning off.
Will the transformer work?

Speaking the other way can a high frequency transformer work at low
frequency?

Can you please suggest some link, that tells maximum current to be
carried out for a given thickness of coil.

Finally is there any relation between inductance and coil's thickness?

Thanks for reading

Animesh Maurya

mail id invalid

 

[Nick Kennedy]

"Paul Hovnanian P.E." <Paul@Hovnanian.com> wrote in message news:<4061FDF4.B218163A@Hovnanian.com>...

Nick Kennedy wrote:

Big power transformers at switchyards and power plants have volts per
hertz relays to protect against low frequency operation.

But not for the reasons you have given. Typical underfrequecy settings
might be a few tenths of a Hz below nominal. There is no problem
operating a 60 Hz transformer at 59.5 Hz for extended periods. The UF
trip is there for load shedding purposes. If the system frequency sags
for a certain period of time, the available generation isn't sufficient
to supply loads. Stations or circuits feeding lower priority loads are
set to be shed when the UF threshold has been reached for a given period
of time.

____________

That's not correct. The V/Hz relays protect the transformers (and
also generators and connected motor loads, if any) against the effects
of overexcitation, which can be overvoltage or underfrequency, but
underfrequency is the prime concern.

I'm on the power plant side and not the system side, but in general,
tripping does not occur automatically on simple overload. This is one
reason blackouts sometimes occur. No one wants to cut off a region
until it's almost too late. (Sometimes beyond almost.)

Tripping a generating station wouldn't make much sense at such a time.
You need more load, not less. But generators and lines (loads) are
typically automatically tripped for protection (faults) rather than
for overload. Loads may be tripped off manually in times of high
overload to keep the entire system from crashing down.

Volts per hertz protection is usually not a big concern for generators
connected to the grid. It is mainly useful to protect against
exciting (energizing) the field prior to bringing the generator up to
full speed, or against failure to trip the field coincident with
tripping the generator and its driver. Often there's no breaker
between the generator and its step-up transformers, so you must
protect both even when off the grid.

I know this is off-topic for this list but it seems that sometimes
discussion of power generator and transmission are of some interest to
group participants.

BTW--rather than talk off the top of my head, I did go back and review
the V/Hz protection bulletins of a Westinghouse and a General Electric
large generator excitation system, a V/Hz relay bulletin, EPRI
guidance, protective relaying texts, and so on. Yep--V/Hz protection
is to protect the equipment, not to deal with system overloads.

No doubt you are correct though, that inability to maintain grid
frequency is a major factor in the decision to selectively shed loads.
And maybe some of this does happen automatically.

Regards,

Nick
in Arkansas

 

[Nick Kennedy]

Yes there was a reason, maybe not a good one.

I thought for illustrative purposes I would first describe how we see
the ideal transformer, then discuss how that can be inaccurate if the
reactance of the magnetizing inductance is too small.

Guess in this case it simply confused things.


Regards,

Nick

(I hope this isn't a double post. Google went weird on the first
attempt.)



Winfield Hill <Winfield_member@newsguy.com> wrote in message news:<c3sonm08tb@drn.newsguy.com>...

Nick Kennedy wrote...

You need to weigh frequency against the amount of magnetizing
inductance in the transformer.

Consider an ideal transformer. When there's no load on the
secondary, no current flows in the primary.

Is there some reason you're choosing to ignore the primary
magnetizing current that even perfect transformers must have?
This current becomes very high at low frequencies and along
with the volt-second core-saturation issue is the reason for
a low-frequency limit. But you know that.

Thanks,
- Win

whill_at_picovolt-dot-com

 

[Guy Macon]

Nick Kennedy <wa5bdu@yahoo.com> says...

I know this is off-topic for this list but it seems that sometimes
discussion of power generator and transmission are of some interest to
group participants.

It's a lot better than the posts from the rude people who think that
it's OK to fill every newsgroup with US politics talk...