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Anonymous Poster #1

Why Autotransformers Always Starts at Zero Position ?

01/02/2013 8:53 AM

Why autotransformers always starts at zero position ?

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#1

Re: Why autotransformers always starts at zero position ?

01/02/2013 9:05 AM
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#4
In reply to #1

Re: Why autotransformers always starts at zero position ?

01/02/2013 9:16 AM

Here I couldn't find the answer. I heard that if an autotransformer switches on at non zero position, it can get damaged. Can anyone explain the reason.

What is the advantage of starting from zero after switching on.

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#2

Re: Why autotransformers always starts at zero position ?

01/02/2013 9:06 AM

They have to start somewhere....

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#3

Re: Why autotransformers always starts at zero position ?

01/02/2013 9:11 AM

They can't count backwards!

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#5

Re: Why autotransformers always starts at zero position ?

01/02/2013 9:17 AM

I think you are talking about variac. And the reason is because it offers max possible range of output voltage without adding much on cost. But if one dont like it,He or She can put a small stopper on its knob, Thaugh will add little on cost.

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#6
In reply to #5

Re: Why autotransformers always starts at zero position ?

01/02/2013 9:31 AM

Yes, about variac. For example if we need to set at 200V, we have to make the knob at zero position before switching on. After getting the power switch on, we slowly increases the voltage into 200V.

In almost all tests I saw the switching on procedure like this.

My question is why we can't set the knob at 200V and then switch on the variac directly.

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#7
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Re: Why autotransformers always starts at zero position ?

01/02/2013 9:46 AM

By always starting at zero, and slowly adjusting to the correct setting, one cannot blow the living crap out of a device by feeding it more power than it can tolerate.

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#31
In reply to #7

Re: Why autotransformers always starts at zero position ?

01/06/2013 7:31 PM

I saw this on a big printer one time:

"Nicht sticken der fingern in das machinegewrken oder sparken und fumin commencen".

It applies in this case too, as you pointed out.

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#8
In reply to #6

Re: Why autotransformers always starts at zero position ?

01/02/2013 10:04 AM

As Lyn says. If you wind it up slowly and there's a fault, it'll usually start sparking/smoking/making loud humming noises or whatever before you've got to full power, so that you can turn it off before it explodes/bursts into flames etc.

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#9

Re: Why Autotransformers Always Starts at Zero Position ?

01/02/2013 10:06 AM

Does it go to eleven?

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#10
In reply to #9

Re: Why Autotransformers Always Starts at Zero Position ?

01/02/2013 10:12 AM

11???

How bout 13!

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#11
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Re: Why Autotransformers Always Starts at Zero Position ?

01/02/2013 11:10 AM
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#27
In reply to #11

Re: Why Autotransformers Always Starts at Zero Position ?

01/03/2013 7:02 PM

Is that one arc-minute and twenty five seconds or one foot and twenty-five inches?

Unfortunately, here at work, the net nannies have blocked all things YouTube. I'll have to wait until I get home to watch the link.

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#28
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Re: Why Autotransformers Always Starts at Zero Position ?

01/03/2013 7:25 PM

Just plain old minutes and seconds - as well as a kind of joke about another scene in Spinal Tap, in which a mistake is made (by one of the band) and a stage-set model of Stonehenge comes out at 1/12th scale.

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#29
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Re: Why Autotransformers Always Starts at Zero Position ?

01/03/2013 7:52 PM

Spinal Tap was a funny movie. I actually saw it in the movie theatre back in my undergrad days. And I never forgot when Graham Chapman came to the school for a lecture gig one evening. Not only a wry, funny kind of guy, he was quite intelligent and interesting. He fielded numerous questions from the audience, not surprising that a bunch of geeks in an engineering school knew just about every line in the Grail as well as most of the Circus skits.

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#14
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Re: Why Autotransformers Always Starts at Zero Position ?

01/02/2013 1:20 PM
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#15
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Re: Why Autotransformers Always Starts at Zero Position ?

01/02/2013 1:45 PM

Guy orders a pizza.

Kid behind counter says, "You want that cut into 6 or 8 slices"?

Guy says, "Make it 6, I can't eat 8 slices".

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#17
In reply to #15

Re: Why Autotransformers Always Starts at Zero Position ?

01/02/2013 5:19 PM

You laugh about the pizza example, but I lived it! My mother ALWAYS asked the waiter or counter person how many slices came in whatever size we were ordering. If they said a number that wasn't what she thought was right, I just learned to ask them to cut it differently. It saved time compared to attempting to convince her that it was just an arbitrary number without a unit definition.

Hmmm... I wonder if that had anything to do with me becoming an engineer?

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#24
In reply to #17

Re: Why Autotransformers Always Starts at Zero Position ?

01/03/2013 11:00 AM

The pizza example... there is something to this.

A pizza joint I used to frequenty made pretty good pies, but it was not uncommon for them to mix up orders. I started asking for my order to be sliced into squares instead of the more common pie shape, thus confirming at a glance at the food pickup window whether or not my order had my ticket on it.

I discovered my family could eat 24 pieces of pizza, no trouble, especially if we all started at zero!

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#12

Re: Why Autotransformers Always Starts at Zero Position ?

01/02/2013 12:23 PM

Do they?

Check the auto transformers of ADB (Adrien de Backer)

They have 5 connections. 4 taps and a slider. If you connect the 2 middle taps to your your feed voltage, they do not start at zero.

These rotating autotransformers have been designed originally to control the output of theatre and movie- light projectors. The Zero position has the filaments pre- heated (or warm).

Most of the auto transformers of different makes I have seen come with this TAP configuration.

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#13

Re: Why Autotransformers Always Starts at Zero Position ?

01/02/2013 12:50 PM

Lots of good answers here. I believe the answers show significant wisdom and a healthy respect for using a safe approach.

But I can't help but wonder if the answer that is being sought is really due to the fact that an autotransformer is a basic concept that is repeated in many applications. For instance, an ignition coil. And, with respect to the the LaPlace response to an step function (or impulse if the switch bounces), such as throwing the switch ON at some dial setting other than zero, there could be quite a high voltage output.....at least for a few microseconds.

Perhaps this is the reason why some care needs to be exercised. Anyone agree?

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#16

Re: Why Autotransformers Always Starts at Zero Position ?

01/02/2013 2:06 PM

Most variable transformers (autotransformers) will switch on at any voltage.

It is usually advisable to switch on at a low voltage to prevent damage if there is a low resistance or short circuit on the output.

Note: There should be a fuse or circuit breaker in the autotransformer output or there may be damage to autotransformer or load if the output has too low a resistance or is shorted.

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#18

Re: Why Autotransformers Always Starts at Zero Position ?

01/02/2013 10:44 PM

As a basic rule, going from zero at switching-on you cannot go wrong. While having a ratio pre-dialed produces a load rush-in current that you have no control over. Rush-in and magnetization currents are important factors in the sizing.

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#19

Re: Why Auto-transformers Always Starts at Zero Position ?

01/02/2013 11:17 PM

They don't.

But bench top (VARIACS) have poor regulation, so the output voltage can change markedly with load. Starting low and winding up stops components being fried.

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#20
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Re: Why Auto-transformers Always Starts at Zero Position ?

01/02/2013 11:41 PM

Thanks. In a roundabout way the same thing.

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#21

Re: Why Autotransformers Always Starts at Zero Position ?

01/03/2013 4:56 AM

They do not always start at a zero position. Forinstance step down transformers in mains power supply 11KV down to 240V will be autotransformers by design and therefore do not start at zero.

To start a variac at zero is a good habit as the highly inductive load can lead to blowing the mains fuse due to the high rush-in current. And you have to walk to the buildings fuse box and reset the fuse.

But I have never experienced issues with the 1KVA variacs we had on our working desks for the development of switchmode power supplies. They were turned on and off at rated voltage and loads till up to 5 to 800W or with no load at all, but without any issues for years. And we seldom started from zero volts. Only sometimes from reduced output voltagse to test the start-up behavior of the circuit design. For high rating variacs the issue is might be different and it it will definitly depend of the design parameters of the variac (Iron core size, current saturation, etc.)

For sure there will be differences for variacs or autotransformers from real "low cost" sources (I do not refer to specific countries here!) and professional transformer makers. A 200grm. "Tourist accessory shop" grade 500W step up transformer 110 to 220V is for sure not a really reliable thing - put your fingers on it and the heat will tell.

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#26
In reply to #21

Re: Why Autotransformers Always Starts at Zero Position ?

01/03/2013 12:52 PM

step down transformers in mains power supply 11KV down to 240V will be autotransformers by design

Really? I suppose I haven't looked closely enough at that design. Does that mean that the wire used on the 240 VAC side is many times larger than the rest of the winding? If so, there must be a good reason for it that has escaped this discussion.

As it turns out, many of the old magnetic ballast were step-up-autotransformers as well. In that case, they depended on a high impedance (before the lamp started) so that when a sudden change in current occured, a high voltage spike would be produced on the output (on purpose) in order to strike the arc (ionize gas in the bulb). All of the wire used in these autotransformers was the same wire size like the other autotransformers I have seen. But once the lamp starts, it loads down the high open circuit voltage and the effective circuit changes as the lamp impedence changes from a few megaohms down to a few hundred ohms.

It would seem to me that all autotransformers were subject to this ballast like responce to being switched on, creating a large voltage spike as current changed rapidly. Did I miss something? Is it really in-rush current or inductive spike due to the sudden change in current in a highly inductive device?

The big difference is that a fluorescent lamp changes its impedence very rapidly but a fixed impedence will be exposed to the large inductive spike voltage as the current increases to the level at which ohms law is satified. The resulting curent is much more than the catch-all term of "inrush current". Remember, inductors don't like sudden current changes and they respond by changing the voltage to try to maintain the status quo current flow. That works the same way whether it is being switched ON or OFF. I believe that the dynamics are far more complex than has been explained here. Fortunately, we don't need to do all of those calculations, every time, to appreciate some common sense advice like start with the output set to a very low setting.

Lastly, an inline reactor in series is a great way to reduce the inrush current. It is recommended for protecting Variable Frequency Devices because the VFD must charge up its internal DC voltage supply before it can be used. And those things have big thirsty capacitors that drink up a lot of current before the inrush can diminish.

By sizing the iron core, you can reduce the inrush current but that also limits the useful current capacity of the autotransformer, so it is best to use another device to do that. That is where the inline reactor saves the day.

I never realized how many times autotransformers were preferred to isolated windings on a common core. I always belived that the inability to regulate the output voltage was thing that would limit their use. But if you have several taps, that could fix that.......hmmm interesting.

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#22

Re: Why Autotransformers Always Starts at Zero Position ?

01/03/2013 7:35 AM

Everyone seems to be looking at the load, what about the poor variac. Its tapping mechanism is simply a carbon brush sliding across a bare copper winding surface. If you want to avoid pitting and burning and preserve the life of those components, you do so by limiting the load by starting at the minimum possible.

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#23

Re: Why Autotransformers Always Starts at Zero Position ?

01/03/2013 8:22 AM

Is this just a "why is it so" question, or do you really want to start up at a set higher voltage every time? If so, why do you have a variable transformer and not a standard transformer with a set turns ratio to give you your desired voltage?

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#25

Re: Why Autotransformers Always Starts at Zero Position ?

01/03/2013 11:29 AM

Auto transformers come in two types. In one type the out put voltage is continuously variable over the entire range right from 0 to full voltabe. This is called a variac.In the other type, the transformer has fixed voltage taps like 110v,220v,415v etc depending upon your specific requirements. Hence, it is obvious that an autotransformer need not always start from zero voltage.In fact starting from zero is not for the transformer itself but for some loads which necessitate gradual application of the voltage.

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#30

Re: Why Autotransformers Always Starts at Zero Position ?

01/05/2013 6:08 PM

It is a similar question to

"Why Electric-Iron (for clothing) is set to off position after use?"

This is tip for safety.

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#32

Re: Why Autotransformers Always Starts at Zero Position ?

01/12/2013 8:52 PM

I assume you are refering to a variable Autotransformer used for testing. As mentioned by other contributers it is set to zero for safety reasons.

An autotransformer (sometimes called autostep down transformer) is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting on itself and not to any kind of automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps where electrical connections are made. Autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but autotransformers have the disadvantage of not providing electrical isolation.

Autotransformers are often used to step up or step down voltages in the 110-117-120 volt range and voltages in the 220-230-240 volt range-for example. providing 110 or 120V (with taps) from 230V input, allowing equipment designed for 100 or 120 volts to be used with a 230 volt supply (as in using US electrical equipment in with higher European voltages).

By exposing part of the winding coils and making the secondary connection through a sliding brush, a continuously variable turns ratio can be obtained, allowing for very smooth control of voltage. Applicable only for relatively low voltage designs, this device is known as a variable AC transformer (often referred to by the trademark name Variac). The output voltage is not limited to the discrete voltages represented by actual number of turns. The voltage can be smoothly varied between turns as the brush has a relatively high resistance (compared with a metal contact) and the actual output voltage is a function of the relative area of brush in contact with adjacent windings. Typically the primary connection connects to only a part of the winding allowing the output voltage to be varied smoothly from zero to above the input voltage and thus allowing the device to be used for testing electrical equipment at the limits of its specified voltage range.

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#33
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Re: Why Autotransformers Always Starts at Zero Position ?

01/12/2013 9:08 PM

Quite good at copy'n'paste, then?

If you authored the wiki, then apologies. If not, then please give credit where credit is due.

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#34
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Re: Why Autotransformers Always Starts at Zero Position ?

01/13/2013 5:49 PM
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