Machine Parts

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 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