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Electric motors. DC electric motors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Electric motors
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DC motor called an electric machine in which the excitation winding is located on the cores of the poles and is powered by direct current.
Design
The magnetic flux passes from the north pole N through the armature to the south pole S and from it through the yoke again to the north pole. The cores of the poles and the yoke are also made of ferromagnetic materials. The DC electric machine consists of two main parts:
- fixed part (inductor);
- rotating part (anchor with drum winding).
On fig. 16.1 shows a structural diagram of a DC machine.
Rice. 16.1. Structural scheme of direct current.
The inductor consists of a cylindrical frame 1 made of ferromagnetic material and poles with an excitation winding 2 fixed on the frame. The excitation winding creates the main magnetic flux.
The magnetic flux can be created by permanent magnets mounted on the frame. The armature consists of the following elements: core 3, winding 4, laid in the grooves of the core, collector 5.
To reduce the losses of the DC machine to the vortex points, the armature core is assembled from sheets of electrical steel isolated from each other.
Consider the operation of a DC machine on a model (Fig. 16.2), where 1 are the poles of the inductor, 2 are the armature, 3 are conductors, 4 are contact brushes.
Rice. 16.2. DC machine model
The conductors of the armature winding are located on the surface of the armature. Contact brushes are placed on the lines of the geometric neutral, drawn in the middle between the poles. The armature of the machine rotates in the direction indicated by the arrow. The direction of the EMF induced in the conductors of the armature winding is determined by the right hand rule.
On fig. 16.2, the cross indicates the EMF directed from us, the dots indicate the EMF directed towards us. The conductors are interconnected so that the EMF in them is added. For this, the end of the conductor located in the zone of one pole is connected in series with the end of the conductor located in the zone of the pole of opposite polarity.
Two conductors connected in series form one turn or one coil. The EMF of conductors located in the zone of one pole is different in magnitude. The greatest EMF is induced in the conductor located under the middle of the pole, the EMF equal to zero - in the conductor located on the geometric neutral line. If all the winding conductors are connected in series according to a certain rule, then the resulting EMF of the armature winding is zero, there is no current in the winding. Contact brushes divide the armature winding into two parallel branches. In the upper parallel branch, an EMF of one direction is induced, in the lower parallel branch, in the opposite direction. The EMF removed by the contact brushes is equal to the sum of the electromotive forces of the conductors located between the brushes. In parallel branches, the same emfs act, directed opposite to each other.
Operating principle
The EMF removed by the contact brushes is equal to the sum of the electromotive forces of the conductors located between the brushes. In parallel branches, the same emfs act, directed opposite to each other.
On fig. 16.3 shows the equivalent circuit of the armature winding.
Rice. 16.3. The equivalent circuit of the armature winding of a DC machine
When connected to the armature resistance winding, the same currents arise in parallel branches 
through resistance RH current flows IЯ. EMF of the armature winding is proportional to the frequency of rotation of the armature n2 and the magnetic flux of the inductor Ф:
(16.1)
where Ce - constant.
In real DC electrical machines, a special contact device is used - a collector. The collector is installed on the same shaft with the armature core and consists of separate copper plates isolated from each other and from the armature shaft. Each of the plates is connected to one or more conductors of the armature winding. Fixed contact brushes are superimposed on the collector. With the help of contact brushes, the rotating armature winding is connected to the DC network or to the load.
Under the action of a voltage applied to the armature of the motor, a current I will appear in the armature windingя. When the current interacts with the magnetic field of the inductor, an electromagnetic torque occurs
(16.2)
where Cм - coefficient depending on the design of the engine.
On fig. 16.4 schematically shows a DC motor, the armature winding conductor is highlighted.
Rice. 16.4. Diagram of a DC motor with a dedicated armature winding conductor
The current in the conductor is directed away from us. The direction of the electromagnetic torque is determined by the left hand rule. The anchor rotates counterclockwise. In the conductors of the armature winding, an EMF is induced, the direction of which is determined by the right hand rule. This EMF is directed against the armature current, it is called counter-EMF. In steady state, the electromagnetic torque MEm balanced by the opposing braking torque M2 rotating mechanism:
Rice. 16.5. Motor armature equivalent circuit
On fig. 16.5 shows the equivalent circuit of the motor armature winding.
Rice. 16.5. Motor armature equivalent circuit
EMF is directed opposite to the armature current. According to Kirchhoff's second law
whence
(16.3)
Equation (16.3) is called the basic equation of the engine. From equation (16.3) one can obtain the formulas:
(16.4)
(16.5)
The magnetic flux Ф depends on the excitation current Iвcreated in the excitation winding.
From formula (16.5) it can be seen that the speed of the DC motor n2 can be adjusted in the following ways:
- Method 1 - by changing the excitation current using a rheostat in the excitation winding circuit;
- Method 2 - by changing the voltage U at the terminals of the armature winding.
- Method 3 - by changing the magnetic flux of the machine.
To change the direction of rotation of the motor to the reverse (reverse the motor), it is necessary to change the direction of the current in the armature or inductor winding.
Author: Koryakin-Chernyak S.L.
See other articles Section Electric motors.
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