ACTION  COMMUTATOR: Why output of commutator / dc generator is dc?

commutator arrangement

If  somehow,connection of the coil side to the external load is reverse at the same instant the current in the coil sides reverses,as shown in below figure,the current through the load will be direct current.  This is what a commutator does. Above fig. shows a commutator having two segments C1 and C2.  It consists of a cylindrical metal ring cut into two halves or segments C1 and C2  respectively separated by a thin sheet of mica.

commutating action

The commutator is mounted on,  but insulated from,  the rotor shaft.  The ends of coil sides AB and CD are connected to the segments C1 and C2 respectively as shown in Figure.  Two stationary carbon brushes rest on the commutator and lead current to the external load.  With this arrangement,  the commutator at all times connects the coil side under S-pole to the +ve brush and that under N-pole to the -ve brush

the coil sides AB and CD are under N.pole and s pole respectively.  Note that segment Cl connects the coil side AB to point P of the load resistance R and the segment C2 connects the coil side CD to point Q  of the load.  Also note the direction of current through load.  It is from Q to P
After half a revolution of the loop(i e.180 rotation), the coil side AB is under S-pole and the coil side CD under N-pole as shown in Fig.The current in the coil sides now flow in the reverse direction but the segments C1 and C2,  have also moved through 180 i e.  segment C1 is now in contact with +ve brush and segment C2 in contact with -ve brush Note that commutator has reversed the coil connections to the load i.e coil side AB is now connected point Q and coil side CD is now connected point P of the load Also note the direction of current through the load It is again from Q to P
                                                                

DC voltage output across commutator of dc generator

                      

thus alternating voltage generated in the loop will apear as direct voltage across the brushes reader may note that e.m.f generated in the armature winding generator is alternating one.  

It is by the use of commutator that we convert the generated alternating e.m.f into direct voltage.  The purpose of brushes is simply to lead current rotating loop or winding to the external stationary load

The variation of voltage across the brushes with the angular displacement of the loop will be as shown in Figure.This is not as steady direct voltage but has a pulsating  character.
 
It is because the voltage appearing across the brushes varies from zero to maximum value and back to zero twice for each revolution of the loop.  A pulsating direct voltage such as is produced by a single loop is not suitable for many commercial uses.  What we require is the steady direct voltage.  This can be achieved by using a large number of coils connected in series.  The resulting arrangement is called armature winding

Transformers are constructed so that their characteristics match the application for which they are intended. The differences in construction may involve the size of the windings or the relationship between the primary and secondary windings. Transformer types are also designated by the function the transformer serves in a circuit, such as an isolation transformer.

Fig: distribution transformer photo credit bhalachandra deshmukh

Distribution Transformer:
Distribution transformers are generally used in electrical power distribution and transmission
systems. This class of transformer has the highest power, or volt-ampere ratings, and the highest continuous voltage rating. The power rating is normally determined by the type of cooling methods the transformer may use. Some commonly-used methods of cooling are by using oil or some other heat-conducting material.Ampere rating is increased in a distribution transformer by increasing the size of the primary and secondary windings;voltage ratings are increased by increasing the voltage rating of the insulation used in making the transformer.

Power Transformer:
Power transformers are used in electronic circuits and come in many different types and applications.Electronics or power transformers are sometimes considered to be those with ratings of 300 volt-amperes and below. These transformers normally provide power to the power supply of an electronic device, such as in power amplifiers in audio receivers.

Control Transformer:
Control transformers are generally used in electronic circuits that require constant voltage or constant current with a low power or volt-amp rating.Various filtering devices, such as capacitors, are used to minimize the variations in the output.This results in a more constant voltage or current.

Auto Transformer:
The auto transformer is generally used in low power applications where a variable voltage is required.The auto transformer is a special type of power transformer.It consists of only one winding.By tapping or connecting at certain points along the winding,different voltages can be obtained

Isolation Transformer:
Isolation transformers are normally low power transformers used to isolate noise from or to ground electronic circuits.Since a transformer cannot pass DC voltage from primary to secondary,any DC voltage (such as noise) cannot be passed,and the transformer acts to isolate this noise.

Potential Transformer:
The instrument potential transformer (PT) steps down voltage of a circuit to a low value that can be effectively and safely used for operation of instruments such as ammeters, voltmeters,watt meters,and relays used for various protective purposes.

Current Transformer:
The instrument current transformer (CT) steps down the current of a circuit to a lower value and is used in the same types of equipment as a potential transformer.This is done by constructing the secondary coil consisting of many turns of wire,around the primary coil, which contains only a few turns of wire. In this manner,measurements of high values of current can be obtained. 

Transformer Noise
The"hum"caused by energized power transformer, under no-load conditions,originates in the core
where the laminations tend to vibrate by magnetic forces. The noise is transmitted  through the oil to the tank side and thence to the surroundings.

1. Magnetostriction (occurrence of dimensional changes both parallel to, and perpendicular to the direction of magnetisation)
2. The mechanical vibrations caused by the laminations, depending upon the tightness
of clamping, size, gauge, associated structural parts, etc.
3. The mechanical vibration of tank walls.
4. The damping.

1. prevention of vibration of core-plate by the use of a lower flux density and giving
attention to constructional feature (such as clamping bolts, proportions and
dimensions of the 'steps' in plate width, tightness of clamping and uniformity of plates).
2. Using cushions, padding, or oil barriers to sound insulate the transformer from tank.
3. Designing suitably the tank and stiffeners to check tank wall vibration.
4. sound insulating the tank from the ground or surrounding air.
However, the noise problem cannot be solved completely.

The construction of this type of circuit breaker is similar to that of a large air circuit breaker used for low-voltage applications, except that they are all electrically operated.

Compressed-air circuit breakers, or air-blast circuit breakers, depend on a stream of compressed air directed toward the separable contacts of the breaker to interrupt the arc formed when the breaker is opened. 

Air-blast circuit breakers have recently been developed for use in extra high-voltage applications with standard ratings up to 765,000 volts.

Oil circuit breakers (OCBs) are circuit breakers that have their contacts immersed in oil. Current interruption takes place in oil which cools the arc developed and thereby quenches the arc. The poles of small oil circuit breakers can be placed in one oil tank; however, the large high-voltage circuit breakers have each pole in a separate oil tank. The oil tanks in oil circuit breakers are normally sealed. 

The electrical connections between the contacts and external circuits are made
through porcelain bushings.


Circuit Breaker Control
As we have discussed, circuit breakers may be remotely operated. In order to operate the breakers from a remote location, there must be an electrical control circuit incorporated. Figure shows a simple control circuit for a remotely-operated breaker.

REFERENCE:DOE FUNDAMENTALS HANDBOOK
ELECTRICAL SCIENCE