Machine Parts

The process to determine the transformer constants referred to the low voltage side if only one set of high voltage measurement is taken , is to short circuit the low voltage side. Then the watt meter W, a voltage V and an ammeter A are connected in the high voltage side i.e. primary winding. A low voltage usually 5% to 10% of the rated supply voltage is applied to the primary and is continuously increase till full load current will flow both in primary and secondary winding.      As 5% of rated voltage is necessary to overcome the impedance which is the only limiting component   of current flow through each winding. The impedance is formed by the resistance and reactance of two windings.      Now the total impedance of the transformer is given by Z T =V 1 /I 1 . Where V1 is the applied voltage and I1 is the rated current. Since flux is directly proportional to voltage, the mutual flux in the transformer under the short circuit test is very small. So,the iron or c

Factors that control the speed of a DC motor : The speed of a motor is given by the equation S=(V-IaRa)/kɸ It is obvious that the speed can be controlled by varying ·          Flux/pole(ɸ) (Flux control) ·          Resistance Ra of armature circuit(Rheoststic control) ·          Applied voltage V(Voltage control) Advantage of field control method: The advantage of field control method is to increase the speed above the base speed. Disadvantage of field control method: The disadvantage of field control method is excessive sparking at the brushes caused by the distortion of the magnetic field resulting from the weakened shunt field.

The copper loss of a transformer can be determined by short circuit test. In this test A watt meter W, voltmeter V and   an ammeter A are connected in the high   voltage winding. The low voltage winding is solidly short circuited by a thick conductor as shown in Fig 1.   A low voltage usually 5 to 10% of normal primary voltage at correct frequency is applied to the primary and is cautiously increased till full load currents are flowing both in primary and secondary. Since, in this test, the applied voltage is a small percentage of the normal voltage, the mutual flux ? produced is also a small percentage of its normal value. Hence core losses are very small with the result that the watt meter reading represent the full load Cu loss or I2R loss for the whole transformer i.e. both primary Cu loss and secondary Cu loss.                                         And the full load core loss of   a transformer can be determined by open circuit test. In this test A wattmeter W

Justify the statement: Stability of operation is an important factor in the parallel operation of generators. Shunt generators share the load proportionately. Should anything to occur to upset this proportionate division of load, it would be desirable for the generators to restore automatically the division of load. Assuming that the speed governor of one of the motors driving the generators is such that it will not hold the speed constant and because of this condition the speed of generator 2 will increase. The increase in speed will produce a large r induced voltage in generator 2,which in turn will cause generator 2 to deliver more load. Of course ,if generator 2 delivers more of the load, generator 1 delivers less. This larger load upon generator 2 will cause more current to flow in the armature of generator 2,producing a larger I a R a drop, a larger drop due to armature reaction, and a larger reduction in armature current, all of which tends to oppose the increas

With two generators connected in parallel, adjustments can be made for one generator to deliver all or any part of the load. If the resistance of the field rheostat of a generator is lowered, the resulting increase in field current will cause the generated voltage to increase. This increase in generated voltage causes the load delivered by the generator to increase. Naturally if one generator takes more load, the other generator drops some load. Hence by adjusting the field rheostats of the generators, the load can be shifted from one generator to the other or divided in any proportion desired.