Machine with D.C. current

A machine with D.C. current is a electric Machine: electromechanical converter allowing the bidirectional conversion of energy enters an electrical installation traversed by a D.C. current and a mechanical device.

Under driving operation electrical energy is transformed into mechanical energy.
Under generating operation the mechanical energy is transformed into electrical energy. The machine behaves like a brake. The generator with D.C. current is also called Dynamo

Official inventor: Zénobe Gram. It was at the beginning a simple generator of D.C. current (for galvanoplastic applications, for example, accumulating being expensive).

Basic machine or machine with separate excitation

Summary description

An electric machine with D.C. current is made up:

Of a stator which is at the origin of the circulation of a longitudinal magnetic flux fixes created either by stator windings or by permanent magnets. This stator is also called inductive in reference to operation out of generator of this machine.
Of a rotor wound connected to a rotary collecting reversing the Polarity in each rotor rolling up at least once by turn in order to make circulate a transverse magnetic flux in squaring with stator flow. Rotor rollings up are also called windings armature, or commonly induced in reference to operation out of generator of this machine.

Physical constitution and principles

the diagram of this type of machine is thus the following:

the current I , injected via the brushes with the collector, crosses a rotor driver (=une rotor whorl) and changes direction (=commutation) with the right of the brushes. This makes it possible to maintain the magnetizing of the rotor perpendicular to that of the stator.

Provision of brushes on “line neutral” (=zone where the density of Flux is null), makes it possible to obtain the counter electromotive Force maximum. This line can nevertheless move by the magnetic reaction of armature (influence of stator flow on the induction field) according to whether the machine works with strong or with weak load. A Overpressure, due partly to the bad distribution of the tension between commutator segments and partly to the fast inversion of the current in the sections of driver at the time of the passage of these blades under the brushes, is then likely to appear at the boundaries of the whorl which commutates and to cause the progressive destruction of the collector. For stage that, i.e. to compensate for the reaction of armature, and also to improve commutation one uses auxiliary poles of compensation/commutation.

the existence of the couple is explained by the magnetic interaction between stator and rotor:

the stator field ( B_s on the diagram) is practically null on the drivers placed in notches and thus does not act on them. The origin of the couple remains the transverse magnetizing of the rotor, unchanged during its rotation (role of the collector). A stator pole acts on a rotor pole and the engine turns.
a traditional but simplistic manner to calculate the couple is to be based on the existence of a Force of Laplace (fictitious) created by the stator field ( B_s on the diagram) and acting on the rotor drivers crossed by the intensity I . This force ( F_L on the diagram) which results from this interaction is identical in module for two rotor drivers diametrically opposite but as these currents are in opposite direction thanks to the system brush-collector, the forces are also of opposite directions.

the force thus created is proportional to I and B_s . The engine torque T is thus him also proportional to these two sizes.
the conducting stem crossed to the rotor by the current I moves subjected to the stator field B_s . It is thus the seat of a counter electromotive Force (FCEM) induced (law of Faraday-Lenz) proportional to B_s and its rate of travel thus at the rotational frequency. The whole of these counter electromotive forces with for consequence appearance of a total counter electromotive force E at the boundaries of the rotor rolling up which is proportional to B_s and at the number of revolutions of the engine.
to make it possible the current I to continue to circulate, it will be necessary that the power supply of the engine delivers a tension higher than the counter electromotive force E induced with the rotor.

Idealized electric diagram

This diagram rudimentary is not valid in transitory mode.

R_i and R_e is respectively resistances of the rotor and the stator

This diagram corresponds to the following electric equations:

- with the stator: U_e = R_e . I_e (law of ohm) and the fields stator is worth B_s = k_e . I_e ( the least exact of the formulas of this paragraph because one moreover does not take account of non-linearities which are important and, one supposes that the machine comprises compensating windings/commutation which make this field independent of the rotor currents. In fact, one makes pass in these commutation/compensating windings a current such as it creates a field cancelling the field induced on the level of the brushes. This power is the current one being on in the rolling up of reinforcement because the field of commutation must vary same manner as the induced field. )
- with the rotor: U_i = E + R_i . I_i

In addition there are two electromechanical equations:

- the force against electromotive: E = Cte. B_s . Ω ( Ω = rotational frequency in rad/s).
- the electromechanical couple (driving or resistant): T = Cte. B_s . I_i

One can show that the constants are the same ones for the two lines, which implies:

E . I_i = T . Ω or “useful Electric output” = “mechanical Power”.

Description of operation

Let us imagine an electric machine supplied with a source of tension U constant. When the engine turns in neutral (it does not make effort) it does not need there to provide of couple, I_i is very weak and U ≈ E . The number of revolutions is proportional to U.

operation out of engine

When one wants to make it work, by applying a resistive torque to its axis, that thus slows down it E decreases.
As U remains constant, the product R_i . I_i thus increases I_i increases, therefore the couple T also increases him and fights against the reduction speed: it is a engine torque .
The more it is slowed down, the more the current increases to fight against the reduction speed. This is why the engines with D.C. current can “roast” when the rotor is blocked, if the current of the source is not limited to a correct value.

operation out of generator

If a mechanical energy source tries to increase the speed of machine, (the load is involving: elevator for example), Ω thus increases E increases.
As U remains constant, the product R_i . I_i becomes negative and increases in absolute value, therefore I_i increases, therefore the couple T also increases him and fights against the increase speed: it is a braking moment .
The sign of the current having changed, the sign of the consumption changes him too. The machine consumes a negative power , therefore it provides power to the circuit. It became generating.

These two operating processes exist of course for the two directions of rotation of the machine. This one being able to pass without discontinuity of a direction of rotation or couple to the other. It is said whereas it functions in the four Quadrant S of the couple-speed plan.

Machine with constant excitation

It is the most frequent case: B_s is constant because it is created by permanent magnets or even because I_e is constant.

If one poses: Cte. B_s = K , the equations of the preceding paragraph becomes:

U = E + R_i . I_i
E = K . Ω
T = K . I_i

Engine series

The excitation series being reserved today for engines, it is not usual to use the term of machine to excitation series .

This type of engine is characterized by the fact that the stator is connected in series with the rotor.

Donc the same current crosses the rotor and the stator: I_i = I_e = I
and the voltage supply U = U_i + U_e
B_s = k_e . I

the equations of the machine become:

U = E + R_i . I + R_e . I = E + ( R_i + R_e ). I
E = K . k_e . I . Ω = K . I . Ω
T = K . I . k_e . I = K . I ²

the equations above make it possible to show that the engines with excitation series can develop a very strong couple in particular at low speed, this one being proportional to the square of the current. This is why they were used to produce engines of traction of engines until in the years 1975.
This type of machine presents however, because of its characteristics, a risk of overspeed and racing with vacuum.
Today, the principal applications are:

starters of cars.
universal driving (drilling machines, hand tooling, etc): the couple T = K . I ² remains of the same direction whatever the sign of I . One of the practical conditions so that an engine series is a universal engine is that its stator is laminated, because in this case inductive flow can be alternate. ( Note:: a drilling machine planned for connection on the alternative network 230V also functions in D.C. current: try to connect it on your battery of car, it is only 12V and it turns …)

Excitation Shunt

In the engine shunt the stator is assembled in parallel with the rotor. There is no more much of application to this assembly.

Donc the terminal voltage of the rotor is the same one as that at the boundaries of the stator: U_i = U_e = U

B_s = k_e . I_e K . U

the equations of the machine become:

U = E + R_i . I_i
E = K . U . Ω
T = K . U . I_i

Made up excitation or Compound

In the engine compound part of the stator is connected in series with the rotor and another is of parallel type or shunt.

This engine joins together the advantages of the two types of engine: the strong couple at low speed of the engine series and the absence of racing (overspeed) of the engine shunt.

Advantages and disadvantages

The principal advantage of the machines with D.C. current lies in their simple adaptation to the means making it possible to regulate or vary their speed, their couple and their direction of rotation: the variable speed transmissions. Even their direct connection with the source of energy: accumulator batteries, piles, etc

The main issue of these machines comes from the connection between the brushes, or “coals” and the rotary collector. As well as the collector even as indicated to him higher and the complexity of its realization. Moreover it should be noted that:

Plus the number of revolutions is high, plus the pressure of the brushes must increase to thus remain in contact with the collector more friction is important.
At the high speeds the brushes must thus be replaced very regularly.
the imposing collector of the ruptures of contact causes arc S, which use the switch quickly and generate parasite S in the feeding circuit, like by electromagnetic radiation.

Another problem limits high speeds of use of these engines when the rotor is wound, it is the phenomenon of “hoop removing”, the centrifugal force ending up breaking the bonds ensuring the behavior of the whole of whorls (hooping).

A certain number of these disadvantages were partially solved by achievements of engines without iron to the rotor, like the engines “discs” or the engines “bells”, which nevertheless always have brushes.

The disadvantages above were radically eliminated thanks to technology from the driving '' brushless '', also called “engine with D.C. current without brushes”, or engine without brushes.

Appendices

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