DC Machines   1- Introduction: A DC machine is a reverse machine, i.e. it is used as a generator or as a motor, so the installation of the generator or motor is the same (similar) in DC machines.   2- Installation of the DC Machine : Figure (1) shows a sector showing the installation of a DC machine consisting of: 1.                  External structure: It is made of cast steel, where it is preferred over cast iron because of its large coefficient of magnetic permeability, which makes the size of the structure small compared to whether it is made of solid iron, and it is distinguished from it by its mechanical properties, and the benefit of the structure is: (a) Carrying the magnetic poles that are fixed to it by means of anchor bolts or by means of a gonorrhea joint. (b) Complement the magnetic circuit of the poles

Figure (1)

 

Magnetic Poles:

     They are electromagnetic electrodes that are composed of the iron core that is made of the steel of generators and in the form of chips isolated from each other to reduce the cyclonic currents and the magnetic obstruction caused by the current of the product, then they are assembled and connected with each other, then they end with the electrode shoe to facilitate the passage and regularity of the magnetic field through the air gap, then they are wrapped on them by well-insulated insulated copper coils, and then these coils arrive successively with each other so that they give a successive magnetic field, i.e. north and then south ... Thus, Figure (1) shows the installation and how to wrap the pole pillows.

 

The rotator consists of:

1.                  The product (conclusion organ): It is made of chips from the steel of generators in the form of circuits isolated from each other (to reduce cyclonic currents and magnetic obstruction) and from the axis of rotation, and forms ducts in their outer surroundings to place copper conductors after isolating them, and the electrical current is generated by the conductors as a result of the movement of the product, as for the magnetic poles, and the resulting current is alternating. Figure (2) shows the shape of the sheets of the conclusion organ.

Figure (2)

(b) Uniform Member: It consists of a copper cylinder composed of copper segments isolated from each other and from the axis of rotation, and these segments are connected to the ends of the product coils. The benefit of the unifying member is to unify the inferred variable current in the product coils into a DC current in the generator while  reflecting  the direction of current in the product coils in the engine.

 

1-                              Carbon Brushes: Figure (3)

 It is made of pressed carbon or red brass and fixed to the unifying member by the mattress house installed on the brush holder in the face of the generator and so that it is always in contact with the unifying member, so it is necessary to press it by a yay and the surface of the brushes is well connected to the surface of the unifier member to avoid the spark that is produced as a result of not having a good contact with the unifying member, and the benefit of carbon brushes is to deliver the current produced by the product and the unifying member to the outer circuit (load circuit) and their number is even.

 

Figure (3)

 

3- DC generators

Generator: It is an electrical machine that converts mechanical energy into electrical energy.

Generator theory:  Based on Faraday's law of electric generator, which states that "if a conductor cuts through a magnetic field, it generates an electric motive force (E.M.F.), with  the conclusion that its value is as great as possible when the cutting angle is 90 electrical."

Figure ( 4) shows the structure and working theory of the DC generator. It consists of four main parts, which we have already talked about, which are:

1.                  Magnetic Poles (Alarm Member): Produces a magnetic field

2.                  Wire Rolls (Product): Generates Variable Electrical Current

3.                  Uniform Member: Unifies the electric current generated by the product

4.                  Brushes: Collecting the current from the unifying member and delivering it to the electrical circuit

 

 

Figure (4)

 

Generator working theory: Figure (4) shows a simple DC generator with two permanent magnetic poles between which a rectangular coil (product) moves between its ends, two ends of which are connected to the halves of a copper cylinder representing the unifying member and collecting the current from the unifying member by two brushes.

When the coil is rotated by an external motor, it cuts the lines of the magnetic field and derives an electric motive force (E.M.F.), which causes the passage of a current from the side under the south pole to half of the copper cylinder (the unitifying member) to a mattress, thus making this mattress positive and the other mattress negative.

With the rotation of the coil, this side is located under the north pole and with it half of the copper cylinder of the unification member, while the other side is located under the south pole where the current comes out of it to the positive mattress, and from this it is clear that the positive mattress is continuously positive and the negative is constantly negative, and thus we get a rectified DC current, i.e. uniform direction, but its value is not constant, and to obtain a uniform current of uniform value, the number of coils must be increased, as well as the number of sectors of the unification member.

The types of DC generators can be divided in terms of electrode feeding, which are:

1- Generator with permanent magnets (magneto)        

The poles of this generator are made of dry steel that retains its magnetism for a long time, and this type is used in telephones, cars, bicycles, and electrical measuring devices (majors).

2- Generator with electromagnetic poles: It is divided into two types.

(a) Independent feeding generator (external)

The electrodes of this generator are fed from an external electrical source from the generator as a battery or other DC generator (called a feeder) and this method is used in school laboratories and in feeding the feeder of variable current generators in power stations where the control of the product voltage is smooth.

(b) Psychologically nourished (self-nourished) birth:

The electrodes of this generator are fed by the same DC current coming out of the product, provided that there is residual magnetism in the electrodes. Nutrition is done in three ways:

1.           The electrode coils are connected in series with the product, and in this case it is called a sequence generator.

2.           The electrode coils are connected in parallel with the product, and in this case it is called a parallel generator.

3.           Part of the electrode coils are connected in line with the product and the other part is connected in parallel with it, and in this case it is called a composite generator.