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# Autotransformer Ratio

07/09/2010 12:57 AM

Auto transformers are economical only when transformation ratio is 1: 3 or near to it.

can anybody explain below question please

When we use auto transformer instead of power transformer. In above, sentence what is 1:3 ratio

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#1

### Re: autotransformer

07/09/2010 3:36 AM

Well, one example of a 1:3 ratio for an autotransformer would be 240VAC:80VAC.

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#2

### Re: Autotransformer Ratio

07/09/2010 1:50 PM

Autotransformer usage

1- In laboratory, it is commonly used to provide variable voltages over broad ranges.
2- It is used to boost the supply voltage by small amount to a distribution cable to correct for the voltage drop
3- It is used to interconnect power system of different voltages
4- With a number of tapping, it is used to start the induction motor.

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#3

### Re: Autotransformer Ratio

07/10/2010 2:10 AM

Your first statement is false! Autotransformers work perfectly well over a wide ratio of voltages. A 'Variac' is an autotransformer with a sliding contact; they commonly work with ratios less than 100:1 to 1:1.05. In my experience, the 'transformation ratio' is simply the voltage ratio, which is also the turns ratio. In the above, I'm using the convention that the first number is proportional to the input voltage, and the second number is proportional to the output voltage. This is opposite to the way PWSlack used it. To me, a 1:3 ratio means that a 240V input would produce a 720V output.

To be clear, I consider 'autotransformer' to mean any transformer where there is a single winding, and different voltages are obtained by connecting to different taps on that winding. Normally one end of the transformer will be common to both input and output. In the 'Variac' I referred to above, the common side of the input is connected to one end and the hot side of the input is connected about 95% of the way to the other end. The common side of the output is connected to the same end as the common of the input, and the hot side of the output is a slider that can connect to any one of the windings, including common. Thus the output voltage can vary in tiny steps from zero to several percent above the input voltage.

I have used one like this to vary the speed of a small bandsaw for quite a few years.

When used this way, the autotransformer IS a power transformer. It does NOT provide electrical isolation from input to output, as a two-winding transformer can.

Now when you say economical, an electronic device such as a dimmer may be considerably less expensive, but I don't think that is what you were talking about.

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#7

### Re: Autotransformer Ratio

07/10/2010 2:35 PM

GA

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#4

### Re: Autotransformer Ratio

07/10/2010 4:01 AM

Yes, The Difference between Auto transformer and Power Transformer is what explained earlier.

Turns ratio is the voltage ratio of the transformer, 1:3 in a Power transformer means the voltage ratio that is 3 times the input thats all.

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#5

### Re: Autotransformer Ratio

07/10/2010 9:30 AM

I learned sometime in 1960s that it was due to USA aid to UK in WWII & the solution was to obtain the autotransformer 2:1 ratio was wound.

And most of the autotransformers now are for use of 110V equipment on 220V supply.

A tapped- transformer normally used to roughly control power can not be included in this category; like speed control, Charge control etc.

Variacs can also be called autotransformers, though it is normally used in labs for testing & not for continuous operation.

Variacs used in power control are normally working for half of the input voltage, ie 110V on 220V stabilizers. A fixed 110V winding is connected in series with a 110V variac, so total travell of slider is for 110V.

I have also seen a variac in a regulated power supply [by HP] as a pre-regulator where it is used for a very limited range of voltage to keep voltage drop across series-pass transistor to within less than 5V.

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#6

### Re: Autotransformer Ratio

07/10/2010 12:30 PM

I believe that the ratio that you are stating is incomplete. The complete ratio is 1:1.3 (1 to 1.3) and the reason for the 1.3 is to allow for transformer losses. No transformer is 100% efficient. There are losses such as coupling coefficient losses ect. and a rule of thumb that I have seen before is that transformer efficiency is 70 to 80%. In a critical voltage environment you would regulate the output but in many applications the addition of .3 to the secondary ratio is quite sufficient.

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#8

### Re: Autotransformer Ratio

07/16/2010 5:26 PM

Auto-transformers are voltage converters that only has one continuous winding, where the input (primary) and the output (secondary) windings are electrically connected. The secondary side may also be multi-tapped depending on the usage. It can either be a step-up or step-down type voltage depending on its design and application.. Where the primary (input) winding shares a common wire with the secondary (output) of the transformer. The ratio of 1:3 or any proportion thereof reflects the number of turns between the primary and secondary, that is also directly proportional to the expected operating voltages. The size or gauge of the magnet wire used in the windings will determine the power capacity in VA or KVA of the transformer. This particular transformer is the most economical type and physically less bulkier capacity-wise.

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