Talk:Autotransformer

Nature of an autotransformer

An autotransformer is an electrical transformer with only one winding sounds a bit confusing. One winding means one loop of wire in my ears.

I guess it is supposed to be:

An autotransformer is an electrical transformer with only one coil.

Am I right? --Abdull 12:21, 24 July 2005 (UTC)

Your interpretation is correct, but engineers don't use that term ("coil") in that way; instead, they use the word winding. "One loop of wire" is referred to as "one turn", so a winding may have one or more turns of wire.

Atlant 21:27, 24 July 2005 (UTC)

Diagram

This diagram appears to be of a variable autotransformer. Should it not be captioned as such?--Light current 20:06, 15 January 2006 (UTC)

Duagram shows only step down operation. Is it not true that auto-transformers can also step up? --Light current 02:32, 16 January 2006 (UTC)

Autotransformers are used for step up, step down or adjust either direction. The pictured variac is connected for 120V input and 0 to 280V output. --C J Cowie 14:05, 16 January 2006 (UTC)

It must be your Variac, otherwise, how could you tell (from that picture)? :-)
Atlant 14:18, 16 January 2006 (UTC)

I suppose you could connect it the other way round, but what I was implying is that auto transformers (variable ones anyway) have an over unity ratio as a max (say 120%) then you can tap all the way down from that thro unity ratio toward zero voltage.--Light current 17:02, 16 January 2006 (UTC)

I think that an over unity max ratio is a common configuration for variacs. Mine, the one pictured, has several fixed taps with a max ratio of 233%. It was originally a 3-gang unit that could be wye connected for 480V use. I "let the smoke out of it" at work and eventually took home a section that I restored to reasonably serviceable condition. --C J Cowie 17:17, 16 January 2006 (UTC)

Yes, lots of Variacs had a tap that allowed a small step-up function; this greatly facilitated their use in testing electronic equipment as you could then easily vary the input mains voltage (to some piece of equipment-under-test) over the entire specified range of operating voltages. I've never seen one that allowed the range that C J Cowie is speaking of, though I have no doubt that they existed.
Atlant 17:22, 16 January 2006 (UTC)

I can redraw it to show some form of overrange capability. BillC 20:22, 16 January 2006 (UTC)

Thats very kind of you to offer to do that BillC . BTW nice selection of variacs here [1]:-)--Light current 21:50, 16 January 2006 (UTC)

Well, I did it. I'm not altogether convinced that the new one is clearer than the old, but the SVG image is uploaded to the Commons under GFDL, so if anyone want to pick it up and edit themselves, they can do so. BillC 23:19, 20 January 2006 (UTC)

I think the new diagram looks great. Very clear and obvious. Thanks BillC!--Light current 23:36, 20 January 2006 (UTC)

Questions from an IP editor

what about efficency?
is it suitable for high power applications?
what disadvantages has it over normal transformers? — Preceding unsigned comment added by 84.222.74.193 (talk • contribs) 17:35, 28 May 2006 (UTC)

"what about efficency?" -- all else being equal an autotransformer should generally be more efficient than an equivilent regular transformer because only part of the power is actually transfered through the inductive action of the transformer. Actual efficiencies will depend on the specifics of a given autotransformer (just as they will for a regular transformer)

"is it suitable for high power applications?" -- as the article says yes, an autotransformer can be designed for any power level.

"what disadvantages has it over normal transformers?" -- lack of isolation and nasty failure modes mainly.

-- Plugwash (talk) 05:35, 16 November 2012 (UTC)

Proposed merge

I placed the merge tags on this article and Buck-boost transformer. A buck-boost transformer is a specific type of autotransformer. Although a buck-boost transformer may use a transformer with multiple windings, they are connected together electrically to form an autotransformer. I think the buck-boost transformer article could be covered with a section in this article. Please post comments below. Thanks. HeirloomGardener 21:53, 18 May 2007 (UTC)

Please see discussion at Talk:Buck-boost transformer. Decision was to not merge. HeirloomGardener 22:30, 20 May 2007 (UTC)

UK railroad usage

I read that the new UK 'High Speed 1' rail line from London to the Channel Tunnel uses electrification on the 2 x 25 kV autotransformer system, as opposed to the conventional 25 kV system. Could anyone kindly explain what that means, please? Halibut, October 30th 2007. — Preceding unsigned comment added by 193.129.220.13 (talk • contribs) 19:12, 30 October 2007 (UTC)

Equations

Surely there should be some transformer equations so that an autotransformer can be compared to a normal tranformer? Thanks PaulE — Preceding unsigned comment added by 165.228.240.251 (talk • contribs) 23:21, 1 May 2008 (UTC)

The transformer equations are exactly the same as for any other transformer, the only significant difference is that the currents in the common part of the winding are in opposite phase so partially cancel (a 1:1 transformer would only have the magnetising current in the common winding) This means that an autotransformer with a small ratio can be far smaller and lighter than the equivialent double-wound transformer. Wide ration transformers have a much smaller advantage and are not commonly found. As a rough guide, a 2:1 autotransformer is about half the mass of an isolating transformer of similar construction quality.149.254.58.232 (talk) 17:50, 18 February 2009 (UTC)

Er, don't think so. It would certainly have half the mass of copper, but I doubt that the mass of iron would change very much. 109.145.21.107 (talk) 17:33, 21 February 2012 (UTC)

Depends. The mass of iron could decrease signifcantly as the aperture required to hold the two windings of a double wound transformer would now only have to hold one winding. Whether it would be halved is another matter. DieSwartzPunkt (talk) 16:14, 22 February 2012 (UTC)

Tapping location preference

in autotransformer tapping are on high tension or lower tension side?.and why ?.. —Preceding unsigned comment added by 124.30.148.114 (talk) 07:32, 24 July 2008 (UTC)

In very high (100s of kV) voltage transformers the tappings are in the earthy end of the common winding as this is far cheaper to construct and the alowable ratio change is small. Otherwise, it is optional but costs usually require that the taps are in the winding that sees voltage variations as a transformer core is most efficient when fully fluxed. For example, the common Variac has its constant voltage input connected to fixed taps and the variable output is from the slider. There is nothing to physically stop you putting the input to the variable side but you run the risk of not having enough turns in curcuit to avoid the core saturating, this is usually followed by smoke emission and failure.149.254.58.232 (talk) 17:50, 18 February 2009 (UTC)

Autotransformer vs. two inductors

What is the difference between the autotransformer and 2 inductors in series ( with no magnetic coupling ) in a step-down application? James. —Preceding unsigned comment added by 80.225.193.212 (talk) 23:23, 4 September 2009 (UTC)

Two inductors in series would essentially be a voltage divider which is very different from an autotransformer. With a voltage divider (whether resistive, inductive or capacitive) the current running down the divider (and thus "wasted") has to be much greater than the maximum current delivered to the load to maintain a stable output voltage.
With the autotransformer on the other hand when idle only a minimal ammount of current flows down through the transformer and when on load some of the current to the load is delivered directly while some is delivered through transformer action. Plugwash (talk) 16:31, 8 June 2011 (UTC)

In AC circuits, inductors don't necessarily "waste" power. You are going to see a non-unity power factor when driving a load connected in this way. This is indirectly wasteful. --Kvng (talk) 14:32, 10 June 2011 (UTC)

Right, the inductors don't directly waste "real power", only "reactive power" which doesn't really represent energy. Nevertheless increasing your current demand by a factor of 10 or so to produce a reasonablly stable output voltage is far from desirable even if that current doesn't represent real power.

Just wondering, on this subject, how is an autotransformer different from a normal transformer, but with the grounds of the primary and secondary connected together? Electrically they'd be the same, but not magnetically, so does that make a difference? 188.29.165.174 (talk) 14:16, 3 October 2013 (UTC)

Microphone transformers

I've often seen loudspeaker autotransfomers in 100V loudspeaker systems, but every microphone matching transformer I've seen had separate windings, i.e. they were not autotransformers at all. Unless someone has info that such things really do exist, I'll remove the microphone reference in a few days.--Harumphy (talk) 10:49, 24 August 2008 (UTC)

Microphone impedance matching transformers are occasionally found wound with two wires twisted together. After construction they are connected with the windings in series to give an accurate centre tap. Tend to be used to convert an unbalanced feed to a ballanced one to reduce noise pickup on long cables. The twisted winding improves the accuracy of the balance over the wide audio frequency range. The type using separate windings is often cheaper but tends to have a narrower bandwidth.
The same principle is used in radio work to match unbalanced coaxial cable to ballanced twin feeders.149.254.58.232 (talk) 18:00, 18 February 2009 (UTC)

Railway autotransformer

Can anyone explain this to me or find a reference?

In UK railway applications, it is common to power the trains at 25kV AC. To increase the distance between Electricity Supply Grid feeder points they can be arranged to supply a 25-0-25 kV supply with the third wire (opposite phase) out of reach of the train's overhead collector pantograph. The OV point of the supply is connected to the rail while one 25kV point is connected to the overhead contact wire. At frequent (about 5km) intervals an autotransformer links the contact wire to rail to third supply conductor. This system reduces induced interference into external equipment and reduces cost. A variant is occasionally seen where the supply conductor is at a different voltage to the contact wire with the autotransformer ratio modified to suit.

A diagram would really help - what purpose does the autotransformer serve here? It might be a good example to keep in the article, if it made sense. --Wtshymanski (talk) 16:02, 13 January 2009 (UTC)

I believe it's relatively new in UK railway use (CTRL only); the design is based on that used in French LGV construction with the very prominent third supply wire hung along the outside of the pylons. —Sladen (talk) 09:10, 28 January 2009 (UTC)

Re the above, the purpose is to increase the possible distance between bulk power supply points (they often cost upwards of £10million in 2008 money) and the level of energy that can be availabe, epecially for heavy freight trains (I'm a Network Rail Engineer involved in system design) They are becoming common on the West Coast route and are an option for new electification. Cost, as usual, is the final determinant of the system to use. The transformer allows power distribution at 50kV yet use at 25kV, this is the reason for use as effective volts drop per conductor is halved. The original uses in Europe were in Germany/Switzerland/Austria in the 20s - long before the French used AC trains. The article has been checked by other engineers, it does make sense, the technology is complex.149.254.58.232 (talk) 17:32, 18 February 2009 (UTC)

Ah, so the autotransformers are not variable, but are simply Fix 2:1 step down tapped three points: -25kV, 0kV, +25kV with the upstream transmission system delivering between the -25kV and +25kV taps and the train using the current between the 0kV and +25kV taps? The 'auto' being (one) rather that (self-operated)? —Sladen (talk) 18:12, 23 May 2009 (UTC)

I've come to this article from a Railway Electrification point of view (having just corrected List of current systems for electric rail traction‎ as it looks like someone had confused this type of 25-0-25 kV supply for a 50 kV overhead line in India. I do understand regular transformers, and also from that how an Autotransformer works, and I remember reading the Split-phase electric power article several months ago.

Okay, I've read and re-read this several times, and reading Sladen's last comment above now makes just a little more sense. I would like to understand it properly, so how does this sound:

Fig. 1

1. Instead of a vanilla (two wire) single phase 25kV supply, the main transformer actually gives 50kV with a grounded centre tap, rather in the manner of a split phase supply (Fig. 1).
2. The grounded centre tap is connected to the thoroughly earthed running rails, so that both legs are now at 25kV and 180° out of phase to each other.
3. One leg is connected to the overhead line, whilst the other is carried somewhere above it, possibly on taller support posts in a similar way that the Pennsylvania Railroad used for it's 138kV 25Hz 25Hz Traction Power System transmission lines.
4. A few of score furlongs down the line, the first leg would have dropped to say 15kV due to trains on the line, but the spare leg should still be at or close to 25kV.
5. Now we get to the magic of the autotransformer, and this is where things get a bit fuzzy, but I think what should happen now is:
6. The voltage across the two legs is now 40kV, so those are connected across the single winding of the autotransformer.
7. The grounded rails are then connected to a tap, which is about 5/8 from the overhead line leg, and therefore 25kV away from it, with the spare leg now down to 15kV.
8. What has really happened is that the overhead line now really sees a 25kV supply at each section end, and the spare leg has dropped by 10kV over the section length, but since no train uses it directly, no-one cares.
9. This process could be used for several sections in each direction.

Where did that explanation go wrong, and would the taps be moved about depending on load? Tim PF (talk) 00:32, 18 January 2011 (UTC)

AIUI you don't want or need to use different taps on different transformers, that is just asking for circulating currents as the load changes and you don't want the voltage to drop too much because you are throwing power away.

"A few of score furlongs down the line, the first leg would have dropped to say 15kV" -- That seems like a lot of volt drop to me but I don't know what they consider tolerable in railway electrification. Still i'll use your numbers

In the absense of the autotransformer the first leg would have dropped to say "15kV" but the autotransformers won't let that happen. They force the voltage on the two legs to stay roughly the same. So rather than the first leg dropping to "15kV" and the second staying at "25kV" the current would be roughly equally shared between the two legs and both would drop to say "20kV".

So-far this doesn't seem much of an advantage, after all we could have just made one leg twice the size. However the big advantage comes when you consider what is going on in the conductor bonded to the rails. With a single ended system all the current has to flow all the way back to the injection point through this conductor. To keep the voltages on the rails within tolable limits on a single ended system this conductor will have to be big and/or very good grounding for the rails will be needed. Whereas on a system with autotransformers the grounded conductor only has to carry significant current from the train to the nearest autotransformers. Plugwash (talk) 05:58, 16 November 2012 (UTC)

Car ignition coils are autotransformers?

I have been trying to improve the articles related to car ignition after finding what I consider to be some real 'howlers' of mistakes ie. using voltage when they should say current.

• One I thought was wrong, but wasn't 100% sure of was references to Ignition coils being autotransformers. I believe they have seperate primary and secondary windings, BUT are electrically connected at one end. (thus usually having only 3 connection). Perhaps by a very broad interpretation they can be defined as a type of autotransformer?.

According to the Transformer types article an Ignition Coil is a Resonant transfomer. So, any opinions?

• From Ignition system "This capacitor and the coil’s primary windings form an oscillating LC circuit." (referring to the capacitor across the points)"..extends the high voltage pulse at the output of the secondary windings."

This may well be so, as anytime you have an inductor and capacitor in parallel you have a 'tank circuit'. But is it significant, in this applicaton. The main point of the cap. is to reduce arcing and point burning, isn't it? Is the cct. actually tuned in any way to get a longer spark. ? See full quote below

"At the same time, current exits the coil's primary winding and begins to charge up the capacitor ("condenser") that lies across the now-open breaker points. This capacitor and the coil’s primary windings form an oscillating LC circuit. This LC circuit produces a damped, oscillating current which bounces energy between the capacitor’s electric field and the ignition coil’s magnetic field. The oscillating current in the coil’s primary, which produces an oscillating magnetic field in the coil, extends the high voltage pulse at the output of the secondary windings. This high voltage thus continues beyond the time of the initial field collapse pulse. The oscillation continues until the circuit’s energy is consumed."

My feeling is that this is geting too technical and makes the article less easy to understand.

The articles on the various seperate parts of the overall ignition system are also somewhat inconsistent, repetitive and perhaps again overly detailed.

Sorry for getting 'off-topic' (autotransformers) towards the end here, but the good people commenting seem to know their stuff. If I should address this elsewhere please let me know, i'm fairly new to Wikipedia editing.(first edit 11 September 2009)

Have edited Ignition system, Ignition coil, Contact breaker, Distributor, Spark plug recently so please take a look if you're interested.

--220.101.28.25 (talk) 15:58, 30 October 2009 (UTC)

Yes, automotive ignition coils are autotransformers. — Quicksilver^{T @} 06:10, 28 July 2011 (UTC)

Circuit diagram for my car clearly shows a double wound coil with a common connection. Thus not all ignition coils are autotransformers, but I accept that they may exist. 109.145.21.107 (talk) 17:40, 21 February 2012 (UTC)

I rather think that the turns ratio of an ignition coil is too high for an autotransformer to provide any advantage. Also, since the primary circuit is resonant (with the condenser across the contact breaker), a common secondary would interfere with that resonance. DieSwartzPunkt (talk) 16:17, 22 February 2012 (UTC)

UK electric grid

RE: The links between the UK 400 kV and 275 kV 'Super Grid' networks are normally three phase autotransformers with taps at the common neutral end.
According to the national grids 7 year statement the transformers they normally use between 400kv and 275kv do not have taps. The autotransformers that run at 400kv/132kv and 275kv/132kv do have tap changers however there is no mention of there location in the winding. CAJ (talk) 21:54, 18 January 2011 (UTC)

Wiping contact autotransformer

The article shows a photograph of an adjustable autotransformer with a wiping contact, often used as a piece of test equipemnt. This allows it to be adjusted on the fly. How does the wiping contact behave when it moves between adjacent taps? Does it a) break the connect to the first tap then make the connect to the adjacent tap or b) make the connection to the ajacent tap then break from the first tap ?

If case a) then there would be rotor positions where you have now output. From personal experience that doesn't seem to be the case.

If case b) then you have effectively created a shorted single turn ! That's generally considered a bad thing. Why isn't the autotransformer reduced to a smoking heap? :) You must have have something similar happening with the Commutator_(electric) of a DC motor. So this must not be a problem but I don't understand why. — Preceding unsigned comment added by 31.51.250.102 (talk) 16:04, 16 November 2012 (UTC)

According to the article the wiping contact is relatively high resistance. This means that (under light load at least) there will be come current flowing through the brush between contract points but presumablly the components can be sized so that this current is manageable. Plugwash (talk) 02:14, 14 December 2012 (UTC)

Autotransformer starter ... X-ray detectors.

The Autotransformer starter section mentions X-ray detectors ... I just read the referenced page and still can't see any connection.

I *think* the reference to X-ray detectors should just be removed, but this is far enough that I would prefer a second pair of eyes first -- if you agree that the reference is irrelevant, please just delete "and X-ray detectors"

That's been there and wrong since January 2019. And I've visited the page several times since and missed it every time. It's gone now. The perp has been blocked, it seems, as well. --Wtshymanski (talk) 17:06, 5 July 2021 (UTC)

Question

Just plain Bill [[2]] What is wrong with the links that I put?.Mohmad Abdul sahib 15:13, 24 September 2021 (UTC)

They are redundant, repeating a link in the text of the article. Right next to the image it says "making the secondary connection through a sliding brush..."

See MOS:LINKONCE. Since the image is adjacent to the text containing the existing link, there is no need for two more links to the same article in the caption. Just plain Bill (talk) 15:26, 24 September 2021 (UTC)