Machine Parts

Critical resistance :   The resistance of field-circuit for which around value build up process is possible is known as critical resistance .(After the value build up process is possible)    The significance of critical resistance in a self excited shunt generator:-   Due to residual magnetism in poles, some emf and hence current would be generated .When this current flow through the field coils will strengthen the magnetism of the poles.This will increases the pole flux which will further increase the generated emf that means further increase current which further increase the flux and so on. This phenomena will be happening till the generate voltage equal to terminal voltage. This phenomena also depends on the resistance of the field circuit. If field resistance is increased, then slope of the resistance line will   be increased, and hence the maximum voltage in which the generator will build up at a given speed decreases. If the field circuit resistance too much tha

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.

How does a four point starter overcome the objections of a three point starter? The speed of the motor is controlled by means of the field rheostat. To increase the speed of the motor it is necessary to set the field rheostat to a higher resistance value. The current through the shunt field is reduced and is the current through the Coil of the holding electromagnet. The reduced current through the coil reduces the strength of the magnet and makes it susceptible to line voltage variations. In the weakened condition a slight reduction in line voltage would further weaken the holding reduction in line voltage would further weaken the holding magnet, releasing the arm of the starter and thus disconnecting the mother from the line. Unscheduled stoppages of the motor make the three point starter quite unpopular. The four-point starter provides the motor with no-voltage protection. Should the power fail, the motor must be disconnected from the line. If not full line v

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