Machine Parts

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.

In a series motor the flux depends upon the current in the series field, which is the same current that flows through the armature. At light loads the armature current is small; therefore the flux and torque are small. An increase in the load increases both the armature current and the flux. The basic speed equation                                    S =        Vt   - IaRa   - IaRa           KФ Since the current in the series field is the armature current, the equation may be rewritten substituting Ia for Ф.                                        S =    Vt   - IaRa   - IaRa           KIa With no load connection to the motor the value of I is very small. The numerator of the speed equation is large, while the denominator is very small, indicating that the speed of   the motor   will be extremely large. In fact, the speed of the motor may reach such a value as to cause the centrifugal force to destroy the motor. It is

The speed of a motor is given by the relation.                    Where, R a = armature circuit resistance. So, the factors that control the speed of a dc motor are (i) flux/pole, Φ (ii) Resistance R a of armature circuit (iii) Applied voltage V                 Ward-Leonard system             The Ward-Leonard system uses the armature control method of lowering the speed without the use of series resistors. A generator is used solely to supply the motor armature with the required voltage. The output of a separately excited generator is connected to the armature of the variable-speed motor. The motor is also separately excited. Varying the setting of the generator field rheostat changes the output voltage of the generator. The output of the generator, being connected to the armature of the motor, thereby changes the motor speed.   This method is used in mills where it is necessary to increase or decrease the speed of an entire production line.

When a transformer primary is energized from a voltage source and the secondary is opened circuited, a small current will flow in the primary. This current is usually less than 5 percent of the rated full load value of the transformer. Since no current flows in the secondary winding . The primary may be considered as a coil with a large reactance due to the iron core. Thus causes a small current . But when load is applied to the secondary terminals. In accordance with Lenz’s law, the current that flows through the secondary winding must act in such a direction as to oppose the flux set up by the primary current. When the flux is momentarily reduced, the induced emf in the primary winding is also reduced and therefore more current flows, in the primary winding. When more load current flows in the secondary the primary current will again increase.flux

The shunt generator is not satisfactory for supplying power to remotely located points because not only does the terminal voltage of the generator drop as load increases but there is also a volt drop in the line. The greater the load, the greater the volt drop in line. The combined effect of the reduction in generator voltage plus the line drop makes the shunt generator unsatisfactory for transmission of power over long distances. To overcome the reduction in generator voltage we can use cumulative compound generator. The cumulative compound generator is the same as a shunt generator with the addition of a series field winding which produces flux to aid the flux of the shunt field. We can use cumulative compound generator for the following characteristics. ·          The drop in the terminal voltage is small. ·          Maintaining the constant terminal voltage.           The terminal voltage rises as load as is applied to generator