The transformers are classified according to the construction, cores, magnetic path and function etc.
According to the magnetic paths
These are as follows:
Core type: Having only one magnetic path.
Shell type: Having two magnetic paths.
Berry type: Having distributed magnetic paths.
According to the transformation ratio
These are classified as follows:
Step up: The transformers which changes the low voltage into high voltage are known as step up transformers.
Step down: The transformers which changes the high voltage into low voltage are known as step down transformers.
According to the number of phases
These can be classified according to the number of phases as under:
- Single phase transformer.
- Two phase transformer.
- Three phase transformer.
- Six phase transformer.
According to the principle
Two winding transformer: In this transformer the one winding which is known as primary is energised and voltage due to mutual induction induces in the secondary winding to which load is connected.
Single winding transformer: In this case the transformer has only one winding, a portion of which is common to primary and secondary both, and emf is induced due to self induction.
According to the function
Transmission transformer: These are used in the power generating stations to step up the voltage for transmission.
Distribution transformer: These are used in the substation for distributing the supply to the different lighting and power loads.
What is meant by the three phase transformer?
A three phase transformer is that which operates on three phase supply. The voltage may be 440 V, 6. kV, 11 kV, 66 kV, 132 kW, 200 kV, 400 kV and even more. Generally, the core type construction is used. The winding is done only on ;the internal three limbs. Each limb carries the primary and secondary both. So there are three sets of primary winding and three sets of secondary windings. The windings can have either cylindrical or sandwitch type of coils.
What is Autotransformer. Also state the advantages, disadvantages and uses of the auto transformer?
An autotransformer works on the principle of self induction i.e., whenever the current changes in a coil. the change of flux linking with the same coil induces a voltage in the same coil. This induced e.m.f. is used for the practical purpose. Such transformer is known as the auto transformer.
In the auto transformer the two windings primary and secondary are connected electrically as well as mechanically. Truly speaking a part of the single continuous winding is common for both the primary and secondary purposes. An autotransformer has one continuous winding with tappings for different voltage as shown in Fig. 18.22. The secondary voltage depends upon the number of turns i.e.
Fig. 18.22. auto transformer.
This transformer can be used for step up and step down transformer.
Advantages:
- Only one winding is used for primary and secondary.
- Less copper is required.
- Less cost being only one winding.
- less weight and small in size.
Disadvantages:
- Two windings are not electrically separate so danger of shock is apparent.
- Secondary may give shock even if on load and without load.
Uses.The transformers are very commonly and efficiently used where the transformation ratio is nearly equal to one. These are used:
- In starter for starting the three phase induction motors. When a low voltage is required at the time of starting and full voltage in running condition.
- As a booster in the feeders to raise the line voltage.
- As a stabilizer for getting constant voltage for the load like T. V. sets, refrigerators etc.
- As a continuously variable voltage device, it finds useful application in electrical testing laboratories.
- As a balancer to obtain a neutral in three wire A.C. distribution as in case of D.C. three wire distribution circuits.
What is three-phase autotransformer?
In case of three-phase autotransformer, the three sets Of windings are placed over each limb. The tappings are taken from the suitable points as shown in Fig. 18.23. The secondary phase voltage can be calculated as
Then the line voltage will be
The windings are connected with a common star and finally the voltage is obtained. These are generally used with the three phase induction motor for starting purpose. The tappings are so adjusted so that the suitable voltage for starting of the motor is obtained.
What is Scott connections of a Three-phase transformer?
In case of welding and furnaces etc. two phase supply is needed. so the three phase supply can be transferred into two phase by means of the Scott connection. as shown below in Fig. 18.21. In this case two single phase transformers having identical ratings and provided with different tappings are used. A middle tapping is taken from one transformer’s primary winding. It is known as the main transformer. The other single phase transformer has tapping at 86.6% and is known as the teaser. These are connected as in star making the connection as T. Three phase supply is given to the L1, L2 , and L3 terminals.
Fig. 18.21. Scott connection
As the primaries are connected in tee form so these are also known as Tee connections. The secondaries are as usual. The two phase supply can be obtained from these two windings. The less or more voltage can be obtained from these two windings by connecting them in parallel or in series.
What are the instrument transformers?
It is always against safety to connect instruments or control apparatus directly to high voltage line. The normal range of instrument may also not be sufficient to measure high values of voltage and currents. In case of d.c„ shunts and multipliers are used to increase the range of the instruments. In a.c., the instrument transformers are used to reduce high voltage and currents of safe and lower values.
The main function of instrument transformer are as follows:
- These act as ratio devices, so that with their uses the use of low range instrument is possible.
- These also act as the instrument devices to protect the instrument and apparatus from high voltage.
The transformer in conjunction with the measuring instrument for measurement purposes, is known as the instrument transformer. These
Current transformer: The transformer used for measuring heavy current is called current transformer or C.T.
Potential transformer: The transformer used for measuring high voltage is known as the potential transformer or PT.
Advantages of instrument transformer
There are the following advantages:
- The size of the instrument is reduced or say moderate because the secondary of C.T. is designed for 5 A and of P.T. for 110 V.
- The replacement Of damaged instrument is easy.
- Several instruments can be operated from a single instrument transformer.
- Low consumption of metering circuit.
- Accessibility on H.T. is easy.
What is Current transformer?
The current transformer is used for measuring the heavy currents. It is a step up transformer. The primary winding consists of thicker conductor having less number of turns. Sometimes only a straight conductor also serves the purpose of primary winding. The secondary winding is done with thinner conductor having more number of turns.
The primary winding is connected in series with the line and the measuring instrument is connected across the secondary of the current transformer or C.T. It should be clearly understood that the secondary winding of the instrument should never be opened. It should always be short circuited either by an ammeter or thick wire. If the secondary is open, there is no current in the secondary winding hence the m.m.f. of primary will not be opposed and the cotes will have high flux which will cause high e.m.f. induced on the primary and secondary windings. This e.m.f. is dangerous and may give sever shock.
The secondary of the C.T. should be earthed to avoid the danger of shock to the operator. The ratio of turns of both the windings is given. Sometimes the dial of the instrument is given with a multiplier to obtain a correct reading, but nowadays the dials of the instruments are calibrated and marked to have the direct reading.
Fig. 18.25. Current transformer.
Sometimes nearly a coil is placed around the current carrying conductor. In this case the conductor itself behaves like the primary and the coil as secondary. The ammeter is connected across the coil. The current flowing in the conductor sets up the magnetic flux which links the secondary and thus inducing voltage in the coil, this e.m.f. causes the current to flow through the meter to deflect. Thus the reading is obtained as in Tong Tester, or Clip-on-meter etc.
In case of three phase line the readings are taken separately and individually for all the three phases. The reading can be taken without disturbing the conductor by means Of a split core type current transformer.
In case of three phase line if all three phases are placed inside the split core, the reading will be zero; because in the vector sum of all the three line fluxes will be zero. Thus the induction will not be possible to induce the voltage and meter will not give any deflection.
What is Potential transformer?
The instrument transformer which is used to measure the high voltage, is known as potential transformer (PT.) as shown in Fig. 18.25.
The primary of the P.T. is having more number Of turns Of fine wire and secondary is having less number of turns. so the PT. in principle is a step down transformer. The primary winding is connected across the line and secondary across the meter to measure the line voltage. The primary winding when connected to line, carry some current, which produces the magnetic flux. The secondary winding is linked with flux causing the induction of some voltage, (generally 110 V in case of PT.). This voltage defect the voltmeter on the secondary of the P.T.
The scale is directly calibrated to obtain the actual voltage. The secondary of the P.T. is always connected to earth. If two or more instruments are to be connected then the voltmeters are connected in parallel across the secondary of the PT. and ammeter in series with the secondary of CT. as shown in Fig. 18.26.
Fig. 18.26. Potential transformer.