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

If the counter emf   becomes equal to the applied voltage, then current through the armature conductor will be zero and for this armature of motor will not rotate. Thus again the counter emf of the motor will be zero. So the counter emf can never be equal or greater than terminal voltage of a motor. Explain the universality of the statement that motor action is always accompanied by generator action or vice versa:  When armature of motor rotates in magnetic field, then the conductors cut flux and for this reason a voltage is induced across the armature. This is the motor action accompanied by the generator action. In the case of a generator, current flows through the conductors of the armature. When a current flowing conductor is placed in a magnetic field, it rotates. Thus it is the generator action accompanied by motor action. What are the detrimental effects of armature reaction on the performance of a dc machine?    Mechanism of Solar Colle