Audio Frequency Transformer

Audio Frequency Transformer

Small iron-core transformer used in the audio-frequency range of 20 to 20,000 Hz are called audio-frequency transtormers. The use of these transformers in electronic circults employed for communications, measurements and control is quite common for the purpose of coupling load to the voltage source. Primarily, the functions of audio-frequency transformers are,

(1) for stepping up the voltage in amplifiers to obtain the required voltage gain and

(2) for decreasing or increasing the load impedance as seen by the voltage source to achieve the impedance matching. They are also employed sometimes for providing a path for dc through primary while isolating it from the secondary.

Induction Regulators

Induction Regulator

An induction voltage regulator enables a smooth variation of the output voltage, whereas in a tap-changer transformer, the output voltage can be controlled only in discrete steps. In induction voltage regulator, the output voltage is controlled by varying the angle between the magnetic axes of primary and secondary windings. In tap-changer, the output voltage is regulated by altering the turns ratio.

An induction voltage regulator may be single-phase or three-phase and in both the types, the rotor moves only through a limited angle, usually 0° to 180° (equal to one pole pitch).

Single-phase Induction Regulator

Single-phase Induction Regulator It consists of a stator and a rotor and for convenience in operation, the rotor shaft is vertical. Since the rotor does not rotate continuously, induction voltage regulators in larger sizes, may be immersed in oil for cooling purposes. A handwheel, through rack and pinion arrangement, controls the rotor movement and, therefore, the output voltage. For automatic voltage regulation, the rotor movement is carried out by a small driving motor and a voltage relay sensitive to the output voltage variations. If the output voltage deviates from the desired value, the relay operates the driving motor in one direction or the other and stops it when the desired voltage is attained.

The primary winding is placed in the rotor slots, since it has to carry smaller current and has smaller conductor area. No slip rings are required to feed the primary winding, since the rotor movement is restricted from 0° to 180°. Flexible leads are suficient to feed the power to primary winding. The secondary winding, connected in series with the outgoing lines, is housed in the stator slots, due to larger conductor area. In addition to the primary winding,the rotor also carries a short circuited compensating winding whose magnetic axis is always 90°away from that of the primary winding.

Three-Phase induction Regulator

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A three-phase induction voltage regulator resembles closely a three-phase slip-ring induction motor. The rotor carries the primary winding and the stator has the secondary winding as in the case of single-phase type. Three-phase primary winding is connected in star whereas the 3-phase secondary is connected in series with the load as large sizes, the induction regulator is oil-immersed in a tank like an ordinary transformer Rotor movement is carried out in the same manner as in a single-phase induction regulator.

When the induction voltage regulator is connected to 3-phase supply, three-phase currents in the three phase primary winding produce a constant amplitude rotating magnetie field as in a 3-phase induction motor. This rotating magnetic field induces e.m.fs, in the secondary winding whose magnitudes depend only on the ratio of primary to secondary turns and are independent of the rotor position. When the rotor position is changed with respect to stator, the magnitude of secondary e.m.fs. remains constant but their phase is altered with respect to primary voltages.

On Load TapChanger

On-load Tap-changer

This tap-changer is used for daily or short period voltage alterations. The output voltage can be regulated with the changer, without any supply interruptions. During the operation of an on-load tap changer;

(1) the main cireuit should not be opened otherwise dangerous sparking will occur and

(2) no part of the tapped winding should get short-circuited.

One form of elementary on load tap-changer is illustrated in Fig. 1.56 (a). The centre tapped reactor C prevents the tapped winding from getting short-circuited. The transformer tappings are connected to the correspondingly marked segments 1 to 5. Two movable lingers A and B. connected to centre-tapped reactor uia switches x and y, make contact with any one o the segments under normal operation.

In Fig. 1.56 (a), both the fingers are in contact with segment 1 and full winding is in circuit. Switches x, y are closed. One half of the total current flows through x,lower half of the reactor and then to the external circuit. The other half of the total current flows through y,

upper half of the reactor and then to the external circuit. It is seen that currents in the upper and lower halves of the reactor flow in opposite directions. Since the whole reactor is wound in the same direction, the m.m.f. produced by one-half is opposite to the m.m.f. produced by the second half. These m.m.fs. are equal and the net m.m.f. is practically zero ; therefore, the reactor is almost non-inductive and the impedance offered by it is very small. Consequently the voltage drop in the centre-tapped reactor is negligible.
When a change in voltage is required, the fingers A and B can be brought to segment 2 by adopting the following sequence of operations:
(1) Open switch y, Fig. 1.56 (b-i). The entire current must now flow through the lower half of the reactor. It, therefore, becomes highly inductive and there is a large voltage drop. It
should be noted that the reactor must be designed to handle full load current, momentarily.
(2) The finger B carries no current and can, therefore, be moved to segment 2, without any sparking [Fig. 1.56 (b-ii)]
(3) Close switch y, Fig. 1.56 (b-iii). The Transformer winding between taps 1 and 2 gets connected across the reactor. Since the impedance offered by the reactor is high for a
current flowing in only one direction, the local circulating current flowing through the reactor and tapped winding is quite small. In this manner, the reactor prevents the tapped winding from getting short-circuited. The terminal voltage will be mid-way between the potentials of
tappings 1 and 2.
(4)Open switch x. The entire current starts flowing through the upper half of the reactor, manifested by a large voltage drop, Fig. 1.56 (b-iv).
(5) Move the finger A from segment 1 to segment 2 and then close switch x. The winding between taps 1 and 2 is, therefore, completely out of circuit, Fig. 1.56 (b-v). If further change in voltage is required, the above sequence of operations is repeated.
For large power transformers, the switches x and y may be circuit-breakers.

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Another form of on-load tap-changer, also provided with a centre-tapped reactor, is illustrated in Fig.1.57. The function of the reactor is again to prevent the short-circuit of the tapped winding. The switches 1,2……5 are connected to the correspondingly marked taps. The switch S in Fig. 1.57 is closed during normal operation. With switches 2, 3, 4, 5 opened and switch 1 closed, the entire winding is in circuit. Here again the two halves of the reactor, carry half of the total current in opposite directions. In changing from tap 1 to tap 2, the following sequence of operations is carried out:

(1) Open switch S. Now total current flows through the upper half of the reactor and there is more voltage drop.

(2) Close switch 2. Winding between taps 1 and 2 is connected across the reactor.

(3) Open switch 1. The entire current now flows through the lower half of the reactor and there is more voltage drop.

(4) Close switch S. The total current now flows equally between the upper and lower halves of the reactor.

For changing from tap 2 to 3,the above sequence of operations is repeated.

No-Load(or Off-Load)Tap Changer

No-Load (or Off-load) Tap Changer This tap changer is used for seasonal voltage variations. An elementary form of no-load tap changer is illustrated in Fig. 1.55. It has six studs marked from one to six. The winding is tapped at six points, equal to the number of studs. The tapping leads are connected to six marked stationary correspondingly arranged in circle. The face plate carrying the six studs, can be mounted anywhere on the transformer, say on the yoke or on any other convenient place. The rotatable arm R can be rotated by means of handwheel, from outside the tank.

If the winding is tapped at 2.5% intervals,then with the rotatable arm R;

(a) at studs 1, 2; full winding is in circuit;

(b) at studs 2, 3: 97.5% of the winding is in circuit;

(c) at studs 3, 4; 95% of the winding is in circuit;

(d) at studs 4, 5; 92.5% of the winding is in circuit; and

(e) at studs 5, 6; 90% of the winding is in circuit.

Stop S fixes the final position and prevents the arm R from being rotated clockwise. In the absence of stop S, the arm R may come in contact with studs 1 and 6. In such a case, only the lower part of the winding is cut out of circuit and this is undesirable from mechanical-stress considerations.

The tap-changing must be carried out only after the transformer is disconnected from the supply. Suppose arm R is at studs 1 and 2. For bringing arm R at studs 2 and 3, the transformer is first de-energised and then the arm R is rotated to bridge studs 2 and 3. After this, transtormer is switched on to the supply and now 97.5% of the winding remains in circuit.

Parallel Operation of Single- Phase of Transformer

When electric power is supplied to a locality, city or an area, a single transformer, capable of handing the required power demand, is installed. In some cases, it may be preferable to install two or more transformers in parallel, instead of one large unit. Though two or more transformers may be expensive than one large unit, yet this scheme possesses certain advantages described below:

(1) With two or more transformers, the power system becomes more reliable. For instance if one transformer develops fault, it can be removed and the other transformers can maintain the flow of power, though at a reduced level.

(2) Transformers can be switched off or on, depending upon the power demand. In this manner, the transformer losses decrease and the system becomes more economical and efficient in operation.

(3) The cost of a standby (or spare) unit is much less when two or more transformers are installed.

In any case, with the passage of time, electric power demand may become more than the rated kVA capacity of the already existing transformer or transformers. Under such circumstances, the need for extra transformer arises. Since the supply voltage has to remain constant, the extra unit must be connected in parallel.

Note that the parallel operation of transformers requires that their primary windings as well as their secondary windings, are connected in parallel. In this article, only the parallel operation of single-phase transformers is considered.

The various conditions which must be fulilled for the satisfactory parallel operation of two or more single-phase transformers, are as follows :

(A) The transformers must have the same voltage ratios, i.e. with the primaries connected to the same voltage source, the secondary voltages of all transformers should equal in magnitude.

(B) The equivalent leakage impedances in ohms should be inversely proportional to their respective kVA ratings. In other words, the per unit leakage impedances of the transformers based on their own kVA ratings must be equal.

(C) The ratio of equivalent leakage reactance to equivalent resistance, i.e. x/r should be same for all the transformers.

(D) The transformers must be connected properly, so far as their polarities are concerned. Out of the conditions listed above, condition

(D) must be strictly fulfilled. If the secondary terminals are connected with wrong polarities, large circulating currents will flow and the transformers may get damaged. Condition (A) should be satisfied as accurately as possible; since different secondary voltages would give rise toi undesired circulating currents. For conditions (B) and (C), some deviation is permissible. Thus the fulfilment of condition (D) is essential, whereas the fulfilment of other conditions is desirable.

Fig. 1.47 shows two single-phase transformers in parallel, connected to the same voltage source on the primary side. Terminals with proper polarity markings have been connected both on the H.V. and L.V. sides. A further check on the polarities can be applied by connecting a voltmeter V in series with the two secondaries. Zero voltmeter reading indicates proper polarities. If voltmeter reads the sum of the two secondary voltages, the polarities are improper and can be corrected by reversing the secondary terminals of any one transformer.

Features of Rotating Electrical machines

Features of Electrical rotating machines

All the rotating electrical machines, used for generation purposes, electric drives or for control systems, have many common essential features from the construction point of view. For example, every rotating electrical machine must possess,

(1) stator (stationary member)

(2) rotor (rotating member)

(3) air-gap separating the stator and rotor and

(4) shaft, bearing, foundation etc. In addition to it, every electrical machine usually has

(a) exciting or field winding, which produces the working flux and

(b) armature woinding in which the working e.m.f. is induced by the working flux.

The current in a winding that varies as the machine is loaded is called load current. The current that produces only a working magnetic flux and does not vary with the load on the machine is called magnetizing current, exciting current or field current. The winding on the machine that carries only load current is called armature winding. The winding that handles only exciting current is called field winding. Current in the field winding is always dc. A winding which handles both the exciting current and load current is called the primary winding of that device. The primary winding is usually the power-input winding. The power-output winding for such machines is called the secondary winding.

The armature winding handles all the power that is being converted or transformed. The rating of armature winding is equal to the power rating of the machine. The field winding power rating is about to 1/2 to 2% of the rated power of the machine. The power input to dc field winding is dissipated as I²R loss in the field winding (once the required field current is established).

The armature windings of both the D.C. and A.C. machines have to deal with alternating current only-this is the reason why the armature structures of all rotating machines are laminated in order to reduce the eddy current losses. Further, almost all the rotating maichines have even number of alternate N and S poles (called hetropolar structure). If power is fed to or taken from the rotor it is obvious that sliding contacts are essential. All types of large rotating machines are provided with radial and axial ventilating ducts for cooling purposes.

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