DC motor: the principle of operation. DC Motor: Device

The first of all the rotating electric machines invented in the 19th century is a DC motor. The principle of its operation is known from the middle of the last century, and until now, DC motors continue to faithfully serve a man, driving many useful machines and mechanisms.

First DFT

Since the 30s of the 19th century, in their development they have gone through several stages. The fact is that before the appearance at the end of the century before last of the machine alternators, the only source of electricity was a galvanic cell. Therefore, all the first electric motors could work only on direct current.

What was the first DC motor? The principle of operation and arrangement of engines built in the first half of the 19th century was as follows. The polarity inductor was a set of fixed permanent magnets or rod electromagnets that did not have a common closed magnetic circuit. An arboreal armature formed several separate rod electromagnets on a common axis, rotated by forces of repulsion and attraction to the poles of the inductor. Typical of their representatives were the engines of U. Ricci (1833) and B. Jacobi (1834), equipped with mechanical current switches in electromagnets with armature contacts in the armature winding circuit.

How the engine of Jacobi worked

What was the principle of this machine? The Jacobi direct current motor and its analogs had a pulsating electromagnetic moment. During the time of approaching the opposite poles of the armature and inductor under the action of the magnetic force of attraction, the instant of the engine quickly reached a maximum. Then, at the location of the armature poles opposite the inductor poles, the mechanical switch interrupted the current in the armature electromagnets. The moment fell to zero. Due to the inertia of the armature and the propelled mechanism, the armature poles came out from under the inductor's poles, at that moment a current of the opposite direction was fed from the commutator, their polarity also changed to the opposite direction, and the attraction force to the nearest pole of the inductor was replaced by a repulsive force. Thus, the Jacobi engine was rotated by successive thrusts.

An annular anchor appears

In the rod electromagnets of the Jacobi motor armature, the current was periodically turned off, the magnetic field created by them disappeared, and its energy was converted into thermal losses in the windings. Thus, the electromechanical transformation of the electric current source of the armature (galvanic cell) into a mechanical one occurred in it with interruptions. We needed an engine with a continuous closed winding, the current in which would flow continuously during the entire time of its operation.

And such fuhtufn was created in 1860 by A. Pacinotti. What was the difference between its DC motor and its predecessors? The principle of operation and the Pachinotti engine arrangement are as follows. As an anchor, he used a steel ring with spokes fixed to a vertical shaft. However, the anchor did not have clearly expressed poles. He became an implicit pole.

Between the spokes of the ring, the coils of the armature winding were wound, the ends of which were connected in series at the armature itself, and the taps connected to the collector plates located along the circumference at the bottom of the motor shaft, the number of which was equal to the number of coils, were made from the connection points of each two coils. The entire armature winding was closed to itself, and the successive points of the connection of its coils were connected to adjacent collector plates, along which a pair of current-carrying rollers slid.

The annular anchor was placed between the poles of the two stationary electromagnets of the stator inductor, so that the lines of force of the magnetic field of excitation created by them entered the outer cylindrical surface of the motor armature at the north pole of excitation, passed along the annular anchor without moving into its inner opening, South pole.

How Pachinotti engine worked

What was his principle of operation? The Pachinotti direct current engine worked exactly like the modern DPT.

In the magnetic field of the pole of the inductor with a given polarity, there was always a certain number of conductors of the armature winding with a current of constant direction, the direction of the armature current under different poles of the inductor being opposite. This was achieved by placing current-carrying rollers playing the role of brushes in the space between the poles of the inductor. Therefore, the instantaneous current of the armature flowed into the winding through the roller, the collector plate and the tap-off connected to it, which was also in the space between the poles, then flowed in opposite directions along the two half winding branches, and finally flowed through the branch line, collector plate and roller in another interpolar Gap. In this case, the armature coils themselves under the inductor's poles changed, but the current direction in them remained unchanged.

According to Ampere's law, for each conductor of the armature coil in the magnetic field of the inductor pole, a force was acting, the direction of which is determined by the well-known "left hand" rule. With respect to the axis of the motor, this force created a torque, and the sum of the moments from all such forces gives the total torque of the DCT, which, even with several plates of the collector, is almost constant.

DFT with a ring armature and a Grammar winding

As has often happened in the history of science and technology, the invention of A. Pacinotti has not found application. It was for 10 years forgotten, until in 1870 it was independently repeated by the Franco-German inventor Z. Gramm in a similar design of a direct current generator. In these machines, the axis of rotation was already horizontal, and carbon brushes were used sliding over the collector plates of an almost modern design. By the 70s of the 19th century the principle of reversibility of electric machines had become well known, and the Gram machine was used as a generator and a DC motor. The principle of its operation is already described above.

In spite of the fact that the invention of the annular anchor was an important step in the development of the DPT, its winding (called the Gram channel) had a significant drawback. In the magnetic field of the inductor poles, there were only those conductors (called active conductors) that lay beneath these poles on the outer cylindrical surface of the armature. It was to them that Ampere's magnetic forces were applied , creating a torque about the axis of the engine. The same inactive conductors that passed through the hole of the annular anchor did not participate in the creation of the moment. They just uselessly scattered electricity in the form of heat losses.

From the annular anchor to the drum

In 1873 the well-known German electrical engineer F. Gefner-Altenek managed to eliminate this drawback of the ring anchor. How did its DC motor function? The principle of operation, the device of its inductor-stator is the same as that of a motor with a ring winding. But the design of the anchor and its winding have changed.

Gefner-Altenek drew attention to the fact that the direction of the armature current flowing from the fixed brushes in the conductors of the Gram's winding under the neighboring excitation poles is always opposite, i.e. They can be included in the composition of coils located on the outer cylindrical surface of the coil with a width (pitch) equal to the pole division (part of the circumference of the armature per one excitation pole).

In this case, the hole in the annular anchor becomes unnecessary, and it becomes a solid cylinder (drum). Such a winding and the anchor itself were given the name of a drum. The copper consumption in it for an identical number of active conductors is much less than in the Gramm winding.

Anchor becomes dentate

In the machines of Gram and Gefner-Altenek, the surface of the anchor was smooth, and the conductors of its winding were located in the gap between it and the poles of the inductor. The distance between the concave cylindrical surface of the excitation pole and the convex surface of the anchor reached a few millimeters. Therefore, to create the desired magnitude of the magnetic field, it was required to use excitation coils with a large magnetomotive force (with a large number of turns). This significantly increased the dimensions and weight of the engines. In addition, on the smooth surface of the armature of its coil was difficult to attach. But how to be? After all, to act on a conductor with a current of Ampé force, it must be located in points of space with a large magnetic field (with a large magnetic induction).

It turned out that this is not necessary. The American inventor of the machine gun H. Maxim showed that if the drum anchor is toothed, and the coils of the drum winding formed between the teeth, the gap between it and the poles of excitation can be reduced to fractions of a millimeter. This made it possible to substantially reduce the size of the excitation coils, but the torque of the DFT did not decrease in the least.

How does such a DC motor function? The principle of operation is based on the fact that, with a dentate armature, the magnetic force is applied not to conductors in its grooves (there is practically no magnetic field in them), but to the prongs themselves. In this case, the presence of current in the conductor in the groove is crucial for the emergence of this force.

How to get rid of eddy currents

Another major improvement was made by the famous inventor T. Edison. What did he add to the DC motor? The principle of action remained unchanged, but the material from which his anchor was made changed. Instead of the former massive, it became laminated of thin electrically insulated steel sheets. This made it possible to reduce the magnitude of the eddy currents (Foucault currents) in the armature, which increased the efficiency of the engine.

Principle of operation of the DC motor

Briefly it can be formulated as follows: when connecting the winding of the anchor of an excited motor to a power source, a large current appears in it, called the starting current and exceeding its nominal value several times. Moreover, under the poles of excitation of the opposite polarity, the direction of the currents in the conductors of the armature winding is also the opposite, as shown in the figure below. According to the rule of the "left hand", Ampere forces are acting on these conductors, directed counter-clockwise and dragging the anchor. In this case, an electromotive force (counter-EMF) is directed in the conductors of the armature winding , directed opposite to the voltage of the power source. As the anchor disperses, the anti-EMF in its winding also grows. Accordingly, the armature current decreases from the starting current to the value corresponding to the operating point on the motor characteristic.

To increase the rotation speed of the armature, it is necessary either to increase the current in its winding, or to reduce the counter-emf in it. The latter can be achieved by decreasing the magnetic field of excitation by reducing the current in the excitation winding. This method of controlling the speed of DPT has become widespread.

The principle of the DC motor with independent excitation

With the connection of the excitation winding (OB) leads to a separate power supply (independent OB), powerful DFTs are usually performed in order to more conveniently regulate the excitation current (in order to change the rotation speed). According to their properties, DPT with an independent OB are almost identical to DPT with OB, which is connected in parallel to the armature winding.

Parallel excitation of DFT

The principle of operation of the direct current motor of parallel excitation is determined by its mechanical characteristic, i.e. The dependence of the speed of rotation on the load moment on its shaft. For such an engine, the change in speed during the transition from idle rotation to the nominal load moment is from 2 to 10%. Such mechanical characteristics are called rigid.

Thus, the principle of the direct current motor with parallel excitation determines its application in drives with a constant speed of rotation with a large range of load variation. However, it is also widely used in an adjustable variable speed drive. In order to regulate its speed, a change in both the armature current and the excitation current can be applied.

Consecutive excitation of DFT

The principle of operation of the DC motor of series excitation, as well as parallel, is determined by its mechanical characteristic, which in this case is soft, because The engine speed varies considerably with changes in load. Where is the most advantageous to use such a DC motor? The principle of operation of the traction engine, the speed of which must decrease when the composition of the lifts is overcome and return to the nominal when moving along the plain, completely corresponds to the characteristics of the DCT with the OB, in series connected to the armature winding. Therefore, a significant number of electric locomotives are equipped with such devices all over the world.

The principle of operation of the DC motor with series excitation is also realized by traction motors of pulsating current, which are, in fact, the same DPT with a sequential OB, but specially designed to work with a rectified electric current already on board with considerable pulsations.