Most important questions of DC generator


Most important questions of DC generator

1. What is the need of Multiplex winding?

This is one of the most important DC generator. With a simple wave winding, there are only two parallel path whereas for lap winding the number of parallel path is equal to number of poles. Thus in case of 10 pole machines using simple winding the designer is restricted to either two parallel circuits or 10 parallel circuits. However sometimes it is desirable to increase the number of parallel path enabling the armature to carry large current and at the same time reducing the conductor current for this purpose Multiplex winding are used. Thus with duplex wave winding the above 10 pole machine will have 4 parallel path. Likewise the duplex lap winding machine will have 20 parallel path.

2. What are the advantages of using a large number of poles in DC machine?

The advantages of using a large number of poles in DC machine
1. The number of the flux per pole is reduced, this reduced the thickness of armature core and has the weight of the machine.
2. The length of the magnetic circuit is reduced resulting in the reduction of field.
3. The length of armature and conductor is reduced.
4. The current per brush is reduced. The reduced the length of commutator.
5. Too small a number of poles means a largest number of armature ampere-turn pole and high value of armature reaction. which require a longer air gap.

3. What are the types of rotating electrical machines?

The rotating electrical machines are two types mainly, one is DC machines and other is AC machine. AC machines on the basis of principle of operations are of two types i.e. synchronous machine and induction machines. Ac machines, on the basis of number of phase employed are also of two types i.e. single phase and three phase machines.

1. DC machines: A dc machines has a coil winding for pole on the stationary part, which give a field stationary in space in the air gap due to dc supply in the field winding wound on the poles. The armature has commutator type winding with stationary brushes for corrections to external circuits.

2. Synchronous Machines: A synchronous machine has a coil winding on the rotor and three phase winding on the stator. The rotor winding is fed with DC supply. The coil winding on the rotor create a field and, with the rotation of rotor, this will move in space at the speed of rotation of rotor. This build induced EMF in the stator three-phase winding. The magnitudes of these induced EMF depends on the strength of these fields and the frequency crosspondance the rotor speed in case of generator. In case of motor the three phase supply is fed to stator winding.

3. Induction machines: Induction machine has poly-phase winding on both the stator and rotor. The three phase winding on stator, when connected to three phase supply, develops a rotating field which is cut by rotor closed circuits, thereby induced voltage in the rotor winding, due to transformer action. This causes flow of current in the rotor winding. Due to interaction of failed with stator field electromagnetic torque is developed and makes the machine run as a motor.

4. What are the method of improving commutation?

There are three methods of obtaining sparkless commutation
1. Resistance commutation
2. Voltage commutation
3. Compensating winding

Resistance commutation
This method of improving commutation consists of using carbon brushes. This makes the contact resistance between commutator segments and brushes high. This high contact resistance has tendency to force the current in short circuited coil to change according to the commutation requirement, namely, to reverse and then built up in the reversed directions.

Voltage commutation
In this method, arrangement is made to induce a voltage in the coil undergoing commutation which will neutralize the reactance voltage. This injected voltage in the opposition to the reactance voltage. If the value of injected voltage is made equal to the reactance voltage, quick reversal of current in the short circuited coil will take place and there will be spark less commutation.

Compensating Windings
compensating windings are most effective means for eliminating the problems of armature reaction and flash over by balancing the armature mmf. Compensating winding are placed in slots provided in pole faces parallel to the rotor conductor. These windings are connected in series with armature winding. The direction of current in crossponding compensating winding must be opposite to that of in the armature winding, just below the pole faces. Thus, it provides an mmf equal and opposite to the armature mmf. In effect the compensating winding neutralizes the armature flux produced by the armature conductor laying under the pole faces.

5. What is commutations?

The process of reversal of current in the armature coil by means of brushes and commutator bar is called commutation process.
A machine is said to have poor or bad commutation if there is sparking at brushes and commutator surface. The bad commutation may be caused by mechanical or electrical condition. The mechanical condition include uneven, commutation surface, non uniform brushes pressure, vibration of brushes in the brushes holder etc. The Electrical condition include an increasing in voltage between the commutator segments, an increase in current density at the trailing of the edge of the brush.
Linear computation is an ideal computation and causes change of current in a coil undergoing commutation from positive current negative current uniformly.

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