Machine Parts

Phasing of a transformer :  Two dimensional vector is known as phasor.As the vector diagram of a transformer is drawn in phasor form and this is called the phasing of transformer. When load is applied to the secondary terminal the current that flows through the secondary terminal opposite to the primary current from lenz’s law.So flux is reduced,causes the induced emf in the primary winding is also reduced and therefore more current flows in the primary winding.This increased primary current will cause the flux to increase its original value.When more load current flow through secondary,the process is repeated. Phasor diagram with load : When load is connected the load current I 2 is   lagging E 2 .I 1 ̕   is the current that flows in the primary winding to balance the demagnetizing effect of I 2 .Since the flux remains constant I o   must be the same current that energizes the transformer at no load I 1 the actual current that   flows in the p

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 ordinary or commercial efficiency of a transformer is given by the ratio –(output in watts)/(Input in watts). But there are certain type of transformer whose performance can not judged by this efficiency. Distribution transformers have their primaries energized all the 24 hours,although their secondaries supply little or no load most of the time during the day except the   housing lighting period.It means that whereas core loss occurs only when the transformers are loaded.This is why the transformer’s all day efficiency is always less than the commercial efficiency.

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