Current and auto transformers (working principle)

As we know transformers are device for converting one AC voltage to the another AC voltage with same frequency. One transformer is made of core, primary windings and secondary windings.

In general there are two main classes voltage and power transformers. Let Milan – one of the members of the Electrical Engineering Community Blog tell us about it.

Principle of one classic transformer

Picture 1. Principle of one classic transformer

Picture 1 shows one AC transformer. The main thing is ratio between primary and secondary windings. The ratio can by step-up or step-down depending if the secondary windings are greater in amount than primary.

Current transformers

These types are commonly used in AC regulation in aircraft electrical system and also for protection between generator and its connection to the electrical distribution. As we mentioned before, the key is ratio between primary and secondary windings.

Current transformer

Picture 2. Current transformer

Picture 2. shows one current transformer. On this illustration we can see secondary windings on a toroidal strip-wound core of silicon-iron. On the picture is also mentioned H1 and H2 which represents polarity.

So, how does it work? Key is that the primary winding is conductor of power system through the core. Principle of transforming current into another level is same as power or voltage regulation.

In aircraft electrical system we have incorporated few current transformers in one device. This type of more than one transformer is common in current protection of AC generator.

Current transformer in protection of AC generator

Picture 3. Current transformer in protection of AC generator

One example of that centralized transformers are given in picture 3 above. On the input we have connection via three phases. The transformer gives comparison of current on three phases by incorporating differential current transformers for all phases.

If there is current unbalance between phases, the transformer will disconnect the generator from distribution. This way of protection is essential because it protect generator, and check its own ability for normal working operation.


In aircraft electrical system there is need for step-up and step-down voltage transformation. This is allow with capability of auto-transformers. Picture 4 illustrates one typical circuit of auto-transformer.

Auto-transformer working principle

Picture 4. Auto-transformer working principle

The working principle goes like this, one winding forms primary and secondary parts. In this case the number of primary windings are less than secondary and we have stepped-up voltage. On the input, the current will flow through the portion of the winding connected by these terminals. Current flow make magnetic flux.

On the output windings we have voltage made by self induction. The sum of output voltage is calculated by number of windings and give us output voltage. In this case output voltage is greater than input.
So when the load is connected to the output, current will flow from secondary windings and will be opposition to the primary windings of input terminal. If we want to calculate, the mathematics will be:

Input: 120V * 15A = 1800W;
5 A is from B and C
Output is 90V * 20A = 1800W
If we use elementary electrical calculation (1st Ohm law) we see that form 5A*90V on the input of secondary windings we have 20A*90V on load. So from 450 W we produced 1800W by implementing auto-transformer. By auto-transformers we can adjust value of needed power on various level.

Auto-transformers on aircraft electrical system

Typical deployment is on producing 36VAC or 26VAC. Aircraft AC generators have enough power for producing 208/400Hz power, and the small part of that power goes to the 36/26VAC. That is made by implementing auto-transformer in the circuit of generator electrical distribution.

One of most common use of auto-transformers are in auto-pilot system. There are few auto-transformers, because we need different voltages. One of requirement for voltage values are given in the Table 1.

Voltage requirements for auto-pilot system

Table 1. Voltage requirements for auto-pilot system

So, how can we make this voltages? It is produced by number of auto-transformers. In the Picture 5 is given one schematic auto-pilot electrical circuit with auto-transformers T4, T5, T6.

Generating various AC voltages for auto-pilot system

Picture 5. Generating various AC voltages for auto-pilot system

At the end we can conclude

The deployment of transformers and auto-transformers are not so obviously at first look at aircraft electrical system. But, this devices are essential for many systems.

One of many uses is illustrated on auto-pilot system, but we also have need for different voltage levels in inertial navigation, radars, and other systems.

Beside this types, we have transformer rectified units for generating the DC for AC voltages, but principle goes on different way.

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