Machine Parts

The advantage of electronic speed control l for dc motor:    We can control the speed of a dc motor by two method: ·          Field control ·          Armature control 1) Field control : To increase the speed above the base speed it is only necessary to increase the resistance of the rheostat in the shunt field circuit. Increasing the resistance decreases the field current which in turns decreases the field flux. And from the basic equation we know that If we decrease the field flux then the speed is increased. 2) Armature control : To operate the speeds lower than the base speed it is necessary to reduce the voltage applied to the armature .The voltage applied is reduced by adding a resistor in series with the armature. From the basic equation we know that the speed is directly proportional to the voltage impressed across the armature. since voltage impressed across armature circuit also determines the magnitude of counter emf the motor speed therefore

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.