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Re: Transformers: mind-boggling, mysterious beasts #60055 12/22/05 10:30 AM
Joined: Sep 2005
Posts: 202
WFO Offline
It's hard to top a response like winnie's.

Just one other comment on the size of wire to it's current carrying capacity. Think of it in terms of not how much current a wire can carry, but how hot it gets while carrying it.

You'll notice in the ampacity spec pages that there are various ampacities listed for different types of insulated wires, yet they have the same guage. It is a question of how hot that wire will get before its particular type of insulation melts. So while THHN insulated 12 AWG wire is limited to 20 amps, this same conductor can carry significantly more than that if it is asbestos covered.

When you get to your transformer, gauging the wire will not tell you how thick the insulating varnish is, or how well air can circulate around them. It's all about heat disipation.

Large power bank oil filled transformers often have several ratings, depending on how they are cooled. For instance, you might see a transformer rated at 12/16/20 MVA OA/FA/FA. This would translate to 12 MVA in open air (OA), 16 MVA with one bank of fans running (forced air or FA), or 20 MVA with both banks of fans running (FA?FA).

Work Gear for Electricians and the Trades
Re: Transformers: mind-boggling, mysterious beasts #60056 12/23/05 04:01 AM
Joined: Dec 2005
Posts: 859
How will a transformer fail anyway? I don't know, but would assume that it would be the wire burning in half and/or shorting out due to over-amping. SMF


Faillures in dry transformers:

Insulation breakdown: Primary to secondary windings, this is one of the main reasons that the sec. winding needs to be earthed at a low voltage TX. to avoid getting a higher mains voltage on the sec. (safe) side.

Shorted turns: Depending on how well the windings touch each other eg. low resistance contact between turns means that the secondary becomes a single turn transformer and the winding will burn out very quickly and a lot of damage will be done to the remainder of the windings and a rewind or write off is necessary.

Winding open circuit: Could be at a tapping or shorted turn gone o.c.

Faillures in oil filled distribution transformers.

Same as for dry TX's and a few more below

In oil filled TX's there are acidity levels in old oil , ingress of moisture, deteriation of paper insulation to name a few.

In New Zealand we usually take oil samples twice a year from substation TX's to determine the condition of the oil and TX.
Carbonlevels indicate some form of arcing at the windings or terminations. Acidity can cause leakage current to earth. lots of varnish can block radiator pipes and reduce circulation hence overheating etc.

I draw a line here now because I think the subject pertains more to dry TX's.

Merry X mas and a great 2006. Raymond

The product of rotation, excitation and flux produces electricty.
Re: Transformers: mind-boggling, mysterious beasts #60057 12/23/05 12:10 PM
Joined: Jul 2003
Posts: 141
Spark Master Flash Offline OP
Thanks, Rodalco...

As for winnie, jwhite and WFO, maybe you guys can answer the question I posed to Larry above. I read what you suggested, winnie, and didn't find the answer. Thanks for passing along all that book knowledge that didn't really answer the question. I did learn some things from that, so thank you!

It appears that if I get shocked on the secondary side, it would be the same VA as if I got shocked on the primary side...which appears that it would make no difference. People say to me, "it's the amps that kill you, not the volts." I reply, "OK then, how about if I grab ahold of 480 volts at 1 amp?" They reply, " higher voltage, the volts will kill you." Either the volts will get me, or the amps will get me, but in my example above, the VA is the same. What's the difference in how badly I get shocked on the secondary vs. the primary, and why?

Either you know the answer or you don't. Please don't act wise and tell me "go read the manual"'s not in there.


Re: Transformers: mind-boggling, mysterious beasts #60058 12/23/05 01:18 PM
Joined: Jul 2004
Posts: 625
SolarPowered Offline
It's not the amps that kill you, it's the milliamps.

And volts drive milliamps--Ohm's Law. 12 VAC doesn't typically drive enough current to kill someone. Although it can, under the right (wrong) conditions. 120 VAC often can drive enough current to kill. 480 VAC is a lot more likely to kill than 120.

I use the term "VAC" above because frequency is related to lethality. DC is much safer than AC. It turns out that Mr. Westinghouse's choice of 60 Hz puts our AC power system pretty close to the peak of the lethality curve.

The VA rating of a transformer is pretty much irrelevant here--I don't know what the power is that's needed to kill, but it's much less than the VA rating of most transformers.

You're barking up the wrong tree trying to understand transformers though Ohm's Law--they don't work that way. Resistance is just a parasitic that's incidental to the operation of the transformer; if you could make the resistance zero, the transformer would work better. Transformers work by magnetic reactance and by magnetic coupling.

Seriously, if you really want to understand this stuff, I'd suggest a couple semesters of Differential Equations, a semester each of Complex Variables and Linear Algebra, and then take a few courses in Circuit Analysis and Engineering Electromagnetics. That's what the rest of us have to do to really understand what's going on. When you can understand it as a system of interacting, time-varying tensor fields in a complex Hilbert space, a transformer is no longer such a "mind-boggling, mysterious beast."

[This message has been edited by SolarPowered (edited 12-23-2005).]

Re: Transformers: mind-boggling, mysterious beasts #60059 12/23/05 03:11 PM
Joined: Sep 2005
Posts: 202
WFO Offline
"People say to me, "it's the amps that kill you, not the volts."

But you're not going to get the amps without the volts to "push" it.

If you've ever grabbed the spark plug of your lawnmower while it's running, you know that you have just taken several thousand volts and you're still kicking (and cursing). But the current is so small that no damage is done (unless you're 80 yrs. old with a pacemaker).

Now go put your thumb and little finger across the terminals of your car battery. You're well aware that there is the potential for hundreds of amps there, but not enough volts to push it through your epidermis.

How much current is available from a source depends on the source and its design. The condenser (capacitor) in your car can knock the snot out of you with no real damage. Get across a 100 Kvar capacitor on a distribution line and they'll be lighting cigarettes off your ass for days.

Story time.
My boss and I were going to have to cross an electric fence and were looking for the best place to get across. I wondered out loud if the fence was even hot. He said there was an easy way to test it. Get a long piece of grass, hold it at one end with the other end touching the wire. Slowly run it closer until you feel a tingle.
So Ol' WFO (also known as Mr. Gullible) started sliding the grass closer and closer to the wire. Just about when I thought I was close enough to have felt something, my boss reached over and touched my earlobe.

If you've never gotten several thousand volts through your earlobe, you just aren't living right. It's, well......breathtaking!

Moral of story. Electricity (and Bosses) are unforgiving and shouldn't be experimented with without a good solid basis in the fundamentals.

Re: Transformers: mind-boggling, mysterious beasts #60060 12/23/05 03:44 PM
Joined: Jul 2003
Posts: 141
Spark Master Flash Offline OP
Thanks, WFO...

I prefer to watch somebody pull the spark plug cap off a CR250 laid over on the throttle side WFO, just getting into the rev limiter.

As for your boss, if you weren't getting energized with the grass touching the wire, how did him touching your ear change anything? Was he energized, touching the wire? At first I thought he only startled you, then I started thinking you actually took the voltage. Pls. explain!

Re: Transformers: mind-boggling, mysterious beasts #60061 12/23/05 08:08 PM
Joined: Sep 2005
Posts: 202
WFO Offline
If you ride CR's, I guess you know where my name comes from.

I wish I could explain what happened. It DID knock the fool out of me and DIDN'T BOTHER HIM!!!

I have no explanation. I've even posted this story on forums before hoping someone else would come up with an answer. The ONLY thing he touched was me....nothing else.

I can't explain it. I don't understand it. But one thing it does say is that just about the time you think you've figured everything out about electricity, it jumps up and gets you.

So please be careful with your experiments. We can't afford to lose any dirt bike riders (even though you should be on a KX or YZ).

Re: Transformers: mind-boggling, mysterious beasts #60062 12/23/05 08:47 PM
Joined: Jul 2003
Posts: 141
Spark Master Flash Offline OP
How interesting!

I rode the predecessor to the KX250, the Kawasaki F-11M Prototype (same type Brad Lackey rode in FIM races for Kaw), and I've raced YZ125s and 250s, Hondas, even other brands you might not want to hear about.

Talk with you soon.

Re: Transformers: mind-boggling, mysterious beasts #60063 12/23/05 10:29 PM
Joined: Sep 2003
Posts: 649
winnie Offline

By 'the question you asked Larry', I presume you mean

Define "electrical isolation" if I can still get grounded and severely amp-loaded. If I'm getting electrocuted, how is electrical isolation helping me? How exactly is my entire electrocution experience getting all warm and fuzzy and comfortable with this 2nd tranny?

Electrical isolation in this case means that the secondary of the transformer is not 'galvanically' connected to the primary. In other words, there is no flow of electrical current between primary and secondary. Energy flows between the two, by the mutual magnetic coupling, but there is no electrically conductive path from primary to secondary.

This makes the secondary 'ungrounded'. If you were to ground either of the secondary terminals, there might be slight capacitive charging current, but not much current flow. It is only with _two_ ground faults that you would get significant current flow.

Remember that electricity does _not_ 'seek ground', and simply touching an energized conductor is not sufficient for a shock. Electricity 'seeks' to close the circuit, and find a path back to the source.

The power distribution systems that we generally use are _intentionally_ grounded and bonded, meaning that one of the transformer terminals is connected to ground. Because of this, any contact between one of the 'hot' terminals and ground, either directly, or through a load (such as a person) will result in current flow. But with an _ungrounded_ secondary, grounding a lead will not cause significant current flow, because there is no complete circuit back to the source.

Your variac is not isolated. This means that there is a shock risk between the output and _any_ grounded metal. But the step down transformer is isolated, meaning that the only shock risk would require that you contact both output terminals at the same time.

On top of this, the step down transformer means that the maximum output voltage will be perhaps 14V; 5V out of 14V is probably safer and more easily controlled than 5V out of 140V.


My guess is that your boss did feel the shock, but was probably ready for it. The current was quite low, intentionally limited by the design of the fence charger. You didn't feel the current passing through you, you felt the spark right at your earlobe.

Your boss could have also done something as easy as holding a bit of metal to your earlobe, say a key or a ring. If he held the metal tightly, he wouldn't feel the arc, but you sure would. I do this when I am getting out of my car on a cold, dry day; I use my key to discharge myself to the car frame.


Re: Transformers: mind-boggling, mysterious beasts #60064 12/23/05 10:40 PM
Joined: Oct 2000
Posts: 2,721
Scott35 Offline
Broom Pusher and

What's the difference in how badly I get shocked on the secondary vs. the primary, and why?

Long story short,

The Secondary side of the Isolation Transformer being used (between the VARIAC and the load item) is not Grounded - therefore it offers an isolation, so there is a reduced hazard for a shock to ground.

Getting caught between the two output leads of the Secondary still gives the "Barbequed Effect" to whom ever is unlucky enough to be the Conductor.

In reality, there will still be a potential to ground through the isolation transformer (with an ungrounded secondary side).
The potential ("Voltage") is a result of Capacitive Coupling, and will vary with distance.

Also, if the Primary Winding faults into the Secondary Winding - either through a loss of smoke (extended overload scenario), or flash-over scenario, the resultant connection becomes an Autotransformer - which both eliminates the Ground Isolation safety thing, and increases the Voltage on the Secondary side.

Do a search on this site for threads regarding operational characteristics of Transformers, Inductors and AC Generators.
We have had many such in-depth talks regarding the basics through the extremes of these beasts.

Do searches in the General area, the NEC area and the Technical Reference area.

One very important point of safety regarding Transformers:

Don't drop them on your foot! That really hurts! [Linked Image] [Linked Image]

Good luck! [Linked Image] [Linked Image]


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
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