Single-layer Winding It is that winding in which one conductor or one coil side is placed in each armature slot as shown in Fig. 26.27. Such a winding is not much used. Two-layer Winding In this type of winding, there are two conductors or coil sides per slot arranged in two layers. Usually, one side of every coil lies in the upper half of one slot and other side lies in the lower half of some other slot at a distance of approximately one pitch away (Fig. 26.28). The transfer of the coil from one slot to another is usually made in a radial plane by means of a peculiar bend or twist at the back end as shown in Fig. 26.29. Such windings in which two coil sides occupy each slot are most commonly used for all medium-sized machines. Sometimes 4 or 6 or 8 coil sides are used in each slot in read more

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Armature Windings The mean­ing of the following terms used in connection with armature winding should be clearly kept in mind. Pole-pitch It may be variously defined as : i) The periphery of the armature divided by the number of poles of the generator i.e. the distance between two adjacent poles. ii) It is equal to the number of armature conductors (or armature slots) per pole. If there are 48 conductors and 4 poles, the pole pitch is 48/4 = 12. Conductor The length of a wire lying in the magnetic field and in which an e.m.f. is induced, is called a conductor (or inductor) as. for example, length AB or CO in Fig. 26.21 Coil and Winding Element With reference lo Fig. 26.21. the two conductors AB and CD along with their end connections constitute one coil of the armature winding. The read more

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Gate circuit should also be protected against overvoltages and over currents. Overvoltages across the gate circuit can cause false triggering of the SCR. Overcurrent may raise junction temperature beyond specified limit leading to its damage. Protection against over-voltages is achieved by connecting a zener diode ZD across the gate circuit. A resistor R2 connected in series with the gate circuit provides protection against overcurrents. A common problem in thyristor circuits is that they suffer from spurious, or noise, firing. Turning-on or turning-off of an SCR may induce trigger pulses in a nearby SCR. Sometimes transients in a power circuit may also cause unwanted signal to appear across the gate of a neighbouring SCR. These undesirable trigger pulses may turn on the SCR leading to read more

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As thyristor possesses high surge current capability, it can be used in an electronic crowbar circuit for overcurrent protection of power converters using SCRs. An electronic crowbar protection provides rapid isolation of the power converter before any damage occurs Fig. 4.28 illustrates the basic principle of electronic crowbar protection. A crowbar thyristor is connected across the input dc terminals. A current sensing resistor detects the value of converter current. If it exceeds preset value, gate circuit provides the signal to crowbar SCR and turns it on in a few microseconds. The input terminals are then short-circuited by crowbar SCR and it shunts away the converter overcurrent. The crowbar thyristor current depends upon the source voltage and its impedance. After some time, main read more

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Thyristors have small thermal time constants. Therefore, if a thyristor is subjected to overcurrent due to faults, short circuits or surge currents ; its junction temperature may exceed the rated value and the device may be damaged. There is thus a need for the overcurrent protection of SCRs. As in other electrical systems, overcurrent protection in thyristor circuits is achieved through the use of circuit breakers and fast-acting fuses as shown in Fig. 4.29. The type of protection used against overcurrent depends upon whether the supply system is weak or stiff. In a weak supply network, fault current is limited by the source impedance below the multi-cycle surge current rating of the thyristor. In machine tool and excavator drives, if the motor stalls due to overloads, the current is read more

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