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

Function of 4-Point starter :    4-Point starter is mainly used to prevent the “unscheduled stoppage”. Here the field circuit and the electromagnet are not in series. So, if any reason there is less current flow through the field coil, it will prevent the unscheduled stoppage. The four point starter provides the motor with no voltage protection.If the power fail, the motor must be disconnected from the line. If not, full line voltage will be applied to the armature without the benefit of starting resistor. When power is restored. The   holding magnet, being connected across the line, release the arm when the voltage drops below a specific value, thus protecting the motor when power is restored.