Transfrm and Mains Freq

[trem]

So what accounts for the extra stress on a PT when the mains frequency changes from 60-Hz to 50-Hz.?
What happens exactly.?
Thank You

[R.G.]

Hold tight, Robin: Bat-Turn...



If you have a transformer with a fixed core size (and that implies a fixed amount of magnetic energy the iron can hold without saturating) and a fixed number of turns, then the amount of voltage-times-time it can stand is fixed, and the practical result is that if it works at 60Hz, then lowering the frequency to 50Hz feeds it the same voltage profile for a 60/50 longer time.

It pushes the iron into saturation, and when that happens, the core no longer limits the amount of current that the primary feeds the energy in the core. The primary current skyrockets, limited only by the wire resistance, so it heats up.

To make a transformer design that's "on the edge" at 60 Hz work at 50Hz, you have to add more iron to withstand the bigger volt-time at the lower frequency, or add more turns (to raise the inductance, which amounts to lowering the amount of energy funneled into the iron) or some combination of both.

Note that a design that's "on the edge" at 50Hz works perfectly well at all high frequencies, including 60Hz. Designs that say "50/60Hz" are simply designed to be 50Hz, and they work at 60.

Er... I rewrote this three times to simplify it down. Never ask me about transformers if you're easily bored.

[trem]

R.G. .....I am never bored by Anything you write.
All you guys have been a pleasure to correspond with.
I wish there was something I could tell YOU for a change. :-)

So.....I hope I am exhibiting some kind of IQ here.....is that (one reason) why a transformer in aircraft are relatively small...because they operate at a higher frequency.? (i think 400-Hz is typical, but i could be wrong)
Thank You

[R.G.]

You're exhibiting an admirable IQ. That is exactly what is going on.

400Hz (and that is very common on aircraft; it's the whine you hear before they wind up the engines) is high enough to make the transformers 60/400 = 0.15 as big for the same throughput power, and not so high that leakage inductance and self capacitance losses eat you up, nor so high that eddy current losses in the iron itself make you go to fanciful materials.

It's also the reason that isolation transformers in switching power supplies get so very small when they're running at upwards of 100kHz to 1MHz. At those frequencies, the side effects and imperfections have a bigger say in the transformer size than the simple transforming does.

Right on!

[Enzo]

And those of us who used to play with world war 2 surplus military electronics also found them operating at 400 Hz.

[trem]

R.G.
Enzo
Once again.....Thank You Both.
Not sure how long I will remember all of this...but I am sure SOME of it will stick for the duration.
thanks gain

[Dave H]

Er... I rewrote this three times to simplify it down.

Don't sugar coat it. You dumbed it down for us

[trobbins]

Luckily, mains frequency doesn't vary to the order of 20% unless you move country.

Mains voltage variation to the order of 20% is much more likely in the same location. For my situation in Australia, we are moving from nominal 240V to 230V, which has benefit for transformers that were designed for keeping out of the region of rapidly increasing saturation at the peak voltage occurring with a 240Vrms +10% waveform (+10% is a nominal tolerance used for voltage spec range of parts). As I understand it, NE voltage has typically trended in the opposite direction over time (?)

Another complication locally in Australia is the much larger variation that is being observed in suburbs due to all the solar photovoltaic power systems that have been put on house roofs, and export power in to the mains - so the house mains voltage can vary by falling under nominal voltage due to street and house wiring voltage drop with load, just as much as rise above nominal voltage due to local generating systems and with minimal street and house loading.

[R.G.]

Luckily, mains frequency doesn't vary to the order of 20% unless you move country.

Except in Japan, where both 100V 50Hz and 100V 60Hz are used. There are some smaller countries/islands with isolated chunks at both 50 and 60. I don't count Japan as a "smaller island country".

Mains voltage variation to the order of 20% is much more likely in the same location.

Yep. In addition to the variation, it's common to have both 1xx and 2xx voltages in the country, presumably at different places/buildings, and/or odd voltages.
The ones that come up quickly in wikipedia are
Brazil
Guyana
Indonesia
Lebanon
Libya
Monaco
Mont Serrat
Morocco
Okinawa
Oman
Panama
Peru
Saudi Arabia
Somalia
South Africa (Grahamstad, Port Elizabeth and parts of King Williams 250V 8-[ )
St. Martin
Togo
Vietnam

[trem]

So.....just as an example...the USA might have wall a voltage of 120 in building "A"...and 240 in building "B".....is that what you guys are saying.?
Thanks

[Enzo]

120v is the standard house current in the USA, but most homes do have 240 coming in. The 120v is one side of the 240. Your breaker box alternates one phase or the other. Half your breakers are off one phase and half off the other. SOme heavy current users use the 240 - that is why those often have a dual breaker or just two breakers next to each other. Special outlets or wiring are in the home for the 240. A clothes drier might need 240. My electric stove in the kitchen runs on 240. My electric water heater, and my well pump run on 240v.

As far as I know there are no homes running on 240 right out of the wall.


There are other standards. heavey industrial users might have a three phase power system. Those tall poles with overhead lighting for highways might run on 480v. But none of those are residential.

[trem]

Hey Enzo -
Yeah...I understand that.
Perhaps I DO NOT understand what R.G. was saying.
I gather he was commenting on some countries having 2 different wall supply voltage as their "standard".
I was just using the USA as a fictitious example of what may be going on in other countries.?
Thanks

[J M Fahey]

To repeat the concept in simpler words:
you apply line voltage to transformer primary.
DC resistance is very low (a few ohms) so BIG current should flow , burning the winding, BUT it being a lot of wire wound around big iron, it's an inductor, which limits AC current to the proper level.
Now, if you decrease frequency by 20% (60>50 Hz), 2 things happen:
1) 20% more AC current pases through it, heating copper 44% more than before (power dissipated is a square function).
It does not mean you will BURN it, but it will be appreciably warmer and will be less comfortable at higher temperatures, such as inside a tube amp chassis.
Probably will have a shorter life too.
2) iron will be closer to saturation, which will lower inductance (which to begin with is not enough ) so it's also worse than straight 20%, it adds to the problem described in (1).
3) that said, living in Argentina I have for decades seen and worked on Fender tube amps brought from 60Hz USA into our Europan 50Hz mains with no problems *at all* , but then we are talking over engineered Fender stuff here.
A marginal cost cutting amp might have big trouble with that.

[Enzo]

My country is Enzonia. We are not all that developed here, but we do have to power our amps. So in Eastern Enzonia, we get 120v 60Hz power in from neighboring Amplandia. Over in western Enzonia we get power from the grid of Jimichonga. That happens to be 240v 50Hz. We don't have a universal standard here, it all depends on which end of the place you visit. OH, and most of the middle of Enzonia is a large desert, and in the one oasis we have, we still use the old generating system left by the old colonial power that ruled us, and that is a 100v 25Hz plant.

[loudthud]

I have a 1948 Hammond catalog that lists some transformers as designed for 25 "Cycles". The 25 Cy transformers weigh quite a bit more, like 15 pounds vs 9 pounds for the 50/60 Cy unit.

Many times a transformer that has only a 120V primary will be 60Hz only but one with dual primaries will specify 50/60Hz. This is a cost cutting move by the manufacturer to maximize profit in North America.