Machine Parts

Torque versus armature   current characteristics of DC motor : The torque developed   by the motor would therefore be directly dependent upon the armature current as indicated by the torque equation ,T=K ΦIa. Increasing the armature current would increase the torque and vice versa.   Shunt motor : For the shunt motor shunt field is connected accross the line, the flux in the machine will remain essentially constant . So, for shunt motor, T=KΦfIa Compound   Motor: The current in the armature must also pass through the series field winding. The series winding may be connected comulatively were the flux due the series field will aid the shunt field flux. In cumulative compound motor the series field flux is additive with the field flux. But in the differentially compound motor the series field flux is substractive with main field flux. Total flux does not remain constant in compound motor as it does in the shunt motor. Cumulative compound motor T=k(Φf+Φs)Ia

When power is firs applied to a motor, the armature is not rotating. Under these conditions the counter Emf is zero and only factor limiting the current in the armature is the armature resistance. The armature resistance of a motor is very small. Therefore the current through the armature would be very small. Therefore the current through the armature would be very large if power were applied to the motor. This current might produce an excessive voltage drop, affecting other equipment. Additional resistance must be connected in series with the armature to limit the current .Until the motor rotation can build up the Counter Emf. The resistance is gradually cut out as the meter rotation builds up the Counter Emf. What will happen if a shunt motor is directly connected with load? The flux of the shunt motor remains constant. If a shunt motor is directly connected to the supply line, the current increases. The increases the Armature current decrease the counter emf . The