An autotransformer is not like the traditional voltage transformer. Normally a voltage transformer has two distinct windings. These windings are called the primary and the secondary.
But with an autotransformer things are a bit different because it only has one single winding that actually does the job of both the primary and the secondary.
Now this single winding is pretty clever because it has taps at various points along its length. This allows it to take a part of the primary voltage and send it over to what we call the secondary load.
It still uses the typical magnetic core that you would expect but since there is only one winding, both the primary and secondary circuits are all connected together.
Meaning in an autotransformer you have both the primary and secondary parts linked together not just electrically but also magnetically. One of the really great things about this design is that it can be made at a lower cost yet still providing the same VA rating that you would get from a traditional transformer.
But there is a downside to this setup, you do not get that isolation between the primary and secondary windings that you would normally find in a regular transformer that has two separate windings.
Let me explain how the winding works in an autotransformer. So the part of the winding that we call the primary is hooked up to the AC power source. Now because the secondary is actually part of this same primary winding, both parts are connected such that it allows them to work together.
Now you can also use an autotransformer to adjust the supply voltage either up or down just by switching around the tapped winding connections.
Imagine this, when you have the primary as the entire winding and it is connected to a power supply and then you connect the secondary circuit across just a portion of that winding, what happens is that the voltage at the secondary side gets “stepped-down.”
We can understand this through the following example:
Understanding Autotransformer Working Concept
When the primary current which is called IP, moves through the single winding in the direction of the arrow shown in the diagram, the secondary current known as IS, flows in the completely opposite direction.
Because of this, in the section of the winding that produces the secondary voltage labeled as VS, the current coming out of that winding ends up being the difference between IP and IS.
Now about the Autotransformer, it doesn’t have to stick to just one single tapping point. You can totally build it with multiple tapping points if needed.
This lets the Autotransformer provide various voltage levels along its winding or even boost the supply voltage compared to the original supply voltage marked as VP in the diagram.
This can be understood by seeing the following diagram.
Autotransformer with Multiple Tapping Points
The usual way to label the windings on an autotransformer is by using big capital letters. For instance you might see letters like A, B, Z, and so on to show the supply end.
Most of the time, the shared neutral connection gets marked with either N or sometimes a small n. When it comes to marking the secondary tapping points, numbers are added as suffixes to identify each one.
These numbers are used for all the tapping points along the primary winding of the autotransformer.
Typically the numbering starts at 1 and then goes up in order for each tapping point, just like in the following diagram.
How Tapping Terminals are Marked in Autotransformer
So an autotransformer is basically a device that helps us tweak line voltages. It can either change the voltage to a different level or keep it steady at the same level.
When we are making a small adjustment to the voltage whether we are increasing it a bit or decreasing it a bit, the transformer ratio is pretty small because the primary voltage (VP) and the secondary voltage (VS) are almost the same. This also means that the currents on both sides which we call IP and IS, are nearly equal too.
Now because of this close match in currents, the part of the winding that deals with the difference between these two currents does not need to be very big.
We can use a much smaller conductor for that part since the currents involved are quite low. This is a great way to save money compared to using a regular double wound transformer which would need bigger conductors.
On top of that when we look at things like regulation, leakage inductance, and even the physical size of an autotransformer, we find that for the same VA or KVA rating, an autotransformer is actually less bulky and has better performance than a double wound transformer.
The Cost Benefits of Autotransformers
Autotransformers are definitely a lot more affordable compared to the traditional double wound transformers that have the same VA rating. When you are trying to figure out whether to go with an autotransformer or stick with a double wound type it is pretty common to look at how much they cost in comparison.
To do this comparison we usually take a close look at how much copper we save in the winding process. If we define the ratio “n” as the ratio of the lower voltage to the higher voltage it turns out that we can calculate the savings in copper as n multiplied by 100 percent.
For instance if we were to look at the following two different autotransformers we could calculate how much copper we are saving based on this ratio.
Solving an Autotransformer Problem #1
Lets say we want to use an autotransformer to increase a voltage from 200 volts to 240 volts. The transformer we are working with has a total of 2200 turns on its primary winding. Our task is to calculate where the primary tapping point should be located as well as the primary and secondary currents when the output is rated at 10 KVA. Additionally we want to look into how much copper wire we can save in this process.
We will begin by Determining the Position of the Primary Tapping Point
The autotransformer steps up voltage from 200V to 240V. The position of the primary tapping point corresponds to the fraction of the total winding that represents the primary voltage.
Primary turns = (Primary voltage / Secondary voltage) × Total turns
By Substituting the values we get:
Primary turns = (200 / 240) × 22000 = 18333.33 turns (approximately 18333 turns).
Now let us Calculate Primary and Secondary Currents
Secondary Current (I2):
I2 = Output power / Secondary voltage
I2 = 10000 / 240 = 41.67 A
Primary Current (I1):
I1 = Output power / Primary voltage
I1 = 10000 / 200 = 50 A
Finally Let us Calculate Economy of Copper Saved
The economy of copper saved in an autotransformer depends on the ratio of the voltage difference to the secondary voltage.
Copper saving = (1 – Voltage difference / Secondary voltage) × 100%
Voltage difference = Secondary voltage – Primary voltage = 240 – 200 = 40 V
Copper saving = (1 – 40 / 240) × 100%
Copper saving = (1 – 0.1667) × 100% = 83.33%.
The Required Final Results are as follows:
- Primary Tapping Point: 18333 turns
- Primary Current: 50 A
- Secondary Current: 41.67 A
- Economy of Copper Saved: 83.33%
Disadvantages of Autotransformer
The biggest drawback here is that, unlike a traditional double wound transformer an autotransformer does not provide isolation between the primary and secondary windings.
This lack of isolation means that you cannot safely use an autotransformer to step down higher voltages to much lower voltages that would be suitable for smaller loads.
Now here’s another important point if the winding on the secondary side happens to become open-circuited, this means that the load current will stop flowing through the primary winding.
When this happens the transformer action essentially comes to a halt and you end up with the full primary voltage being applied directly to the secondary terminals which can be quite dangerous.
Additionally if there is a short-circuit condition on the secondary circuit, this would lead to a primary current that is significantly larger than what you would see with a traditional double wound transformer.
This increase in current is caused by the greater flux linkage which can end up damaging the autotransformer itself.
Another thing to consider is that because the neutral connection is shared between both the primary and secondary windings, if you earth the secondary winding it automatically means that you are also earthing the primary winding.
This lack of isolation can be problematic especially since double wound transformers are often used specifically to isolate equipment from earth for safety reasons.
Uses and Applications of an Autotransformer
Autotransformers are greatly versatile and serve a variety of purposes. They are often used for starting induction motors which is a common application.
Also they play a significant role in regulating the voltage of transmission lines to help maintain a stable power supply. Another important use of autotransformers is to step up or step down voltages especially in situations where the voltage ratio between the primary and secondary windings is nearly equal to one.
Interestingly we can build an an autotransformer by modifying a regular two-winding transformer. We can do this by connecting the primary winding and the secondary winding in series with each other.
How the connection is made determines the effect on the voltage. The secondary voltage can either increase or decrease the primary voltage depending on the arrangement of the windings.
What is a Variac
So we know that an autotransformer has a fixed or tapped secondary that gives us a voltage output at a certain level.
But we can also use these devices to create a variable AC voltage from a steady AC supply.
We often see this type of variable autotransformer in labs and science classrooms and we usually call it a Variac.
The way a variable autotransformer or Variac is built is pretty similar to a fixed autotransformer. It has one primary winding wrapped around a laminated magnetic core just like the fixed version.
The big difference is that instead of being stuck at one tapping point, we get the secondary voltage through a movable carbon brush.
This carbon brush can move around either by rotating or sliding along part of the primary winding that is exposed. As it moves, it makes contact and gives us the voltage level we want.
Basically a variable autotransformer has this movable tap in the form of a carbon brush that travels along the primary winding.
This movement alters how long the secondary winding is, letting us adjust the output voltage from the primary supply all the way down to zero volts.
We usually find that a variable autotransformer has a lot of primary windings. This lets us adjust the secondary voltage from several volts all the way down to tiny fractions of a volt for each turn.
This flexibility comes from the carbon brush or slider that stays in constant contact with one or more turns of the primary winding.
Since the turns of the primary coil are spread out evenly along its length, the output voltage changes directly based on how much we rotate it.
Understanding Variable Autotransformers
So now we know how variac works? It is really great because it can smoothly change the voltage that goes to whatever load you are using.
It can adjust this voltage from zero all the way up to the rated supply voltage.
Now here is something interesting, if you tap into the supply voltage at a certain point along the primary winding, it is actually possible for the output secondary voltage to be higher than the actual supply voltage itself.
People also use variable autotransformers for dimming lights and in those cases they often call them “dimmerstats.”
When you are in electrical or electronics workshops or laboratories variacs become super useful because they give you a variable AC supply that you can adjust as needed.
But here is an important thing to keep in mind. It is really necessary to have proper fuse protection in place.
This is to make sure that there is not a high supply voltage showing up at the secondary terminals when something goes wrong or during fault conditions.
The Benefits of Autotransformers
Autotransformers have a bunch of advantages when you compare them to the traditional double-wound transformers.
They usually show higher efficiency for the same VA rating which is pretty impressive.
They also take up less physical space which is a big plus if you are working with limited room.
Plus they use less copper in their construction which helps keep costs down when you look at double-wound transformers that have the same VA ratings.
On top of that their core and copper losses which are represented as I²R are kept to a minimum because of lower resistance and leakage reactance.
This means they can do a better job at voltage regulation compared to the standard two-winding transformers.
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