Principle of operation of the electric motor. Principle of operation of the AC motor. Physics, Grade 9

Today it is impossible to imagine human civilization and a high-tech society without electricity. One of the main devices that provide the operation of electrical appliances is the engine. This machine has found the widest distribution: from industry (fans, crushers, compressors) to household use (washing machines, drills, etc.). But what is the principle of the electric motor?

Appointment

The principle of operation of the electric motor and its main goals are to transfer to the working bodies the mechanical energy necessary for the fulfillment of technological processes. The engine itself generates it at the expense of electricity consumed from the grid. In essence, the principle of the electric motor is to convert electrical energy into mechanical energy. The amount of mechanical energy produced by him for one unit of time is called power.

Types of engines

Depending on the characteristics of the supply network, you can distinguish two main types of the engine: on a constant and alternating current. The most common DC machines are motors with sequential, independent and mixed excitation. Examples of alternating current motors can be synchronous and asynchronous machines. Despite the apparent diversity, the device and principle of operation of an electric motor of any designation are based on the interaction of a conductor with current and a magnetic field or a permanent magnet (a ferromagnetic object) with a magnetic field.

A frame with a current - a prototype of the engine

The main point in such an issue, as the principle of the electric motor, can be called the appearance of torque. Consider this phenomenon can be an example of a frame with a current, which consists of two conductors and a magnet. To the conductors, the current is fed through contact rings, which are fixed on the axis of the rotating frame. In accordance with the famous rule of the left hand, forces will act on the frame, which will create a torque about the axis. It under the action of this total force will rotate in a counter-clockwise direction. It is known that this torque is directly proportional to the magnetic induction (B), the current strength (I), the frame area (S) and depends on the angle between the field lines and the axis of the latter. However, under the action of a moment changing in its direction, the frame will perform oscillatory movements. What to do to create a permanent direction? There are two options:

• Change the direction of the electric current in the frame and the position of the conductors relative to the poles of the magnet;
• Change the direction of the field itself, while the frame rotates in the same direction.

The first option is used for DC motors. And the second is the principle of the AC motor.

Changing the current direction relative to the magnet

In order to change the direction of motion of charged particles in the conductor of the current frame, it is necessary to have a device that specifies this direction, depending on the arrangement of the conductors. This design is realized due to the use of sliding contacts, which serve to lead to the current frame. When replacing one ring with two, when the frame turns half a turn, the direction of the current changes to the opposite direction, and the torque keeps it. It is important to take into account that one ring is assembled from two halves, which are isolated from each other.

The design of the DC machine

The above example is the operating principle of a direct current motor. The real machine, naturally, has a more complex construction, where dozens of frames forming the armature winding are used. The conductors of this winding are located in special grooves in a cylindrical ferromagnetic core. The ends of the windings are connected to the isolated rings that form the collector. The winding, the collector and the core are an anchor rotating in the bearings on the body of the engine itself. The magnetic excitation field is created by the poles of permanent magnets, which are located in the body. The winding is connected to the mains and it can be switched on independently of the armature circuit or in series. In the first case, the motor will have an independent excitation, in the second case it will be sequential. There is also a mixed-excitation design when two types of winding connections are used at once.

Synchronous machine

The principle of the synchronous electric motor is the need to create a rotating magnetic field. Then you need to place in this field streamlined wires unchanged in the direction of the current. The principle of operation of the synchronous electric motor, which has become very widespread in industry, is based on the above example with a current frame. The rotating field created by the magnet is formed by a winding system that is connected to the mains. Usually three-phase windings are used, however, the principle of operation of a single-phase AC motor will not differ from the three-phase, except that the number of phases themselves, which is unimportant when considering design features. The windings are placed in the stator grooves with a certain shift along the circumference. This is done to create a rotating magnetic field in the formed air gap.

Synchronism

A very important point is the synchronous operation of the electric motor of the above construction. When the magnetic field interacts with the current in the winding of the rotor, the process of rotation of the motor itself is formed, which will be synchronous with respect to the rotation of the magnetic field formed on the stator. Synchronism will be maintained until the maximum moment is reached, which is caused by the resistance. If the load increases, the machine may exit synchronism.

Asynchronous motor

The principle of the electric motor asynchronous is the presence of a rotating magnetic field and closed frames (contours) on the rotor - a rotating part. The magnetic field is formed in the same way as in a synchronous motor - with the help of stator windings located in the stator grooves, which are connected to the AC voltage network. The windings of the rotor consist of a dozen closed contour frames and usually have two types of execution: phase and short-circuited. The principle of operation of the AC motor in both versions is the same, only the design is changed. In the case of a short-circuited rotor (also known as a "squirrel cage"), the winding is poured with molten aluminum into the grooves. When the winding is manufactured, the phase ends of each phase are output to the outside by means of sliding contact rings, since this will allow the inclusion of additional resistors in the circuit, which are necessary for regulating the engine speed.

Traction machine

The principle of operation of the traction motor is similar to a DC motor. From the mains supply current is fed to the step-up transformer. Then three-phase alternating current is transmitted to special traction substations. There is a rectifier. It converts AC to DC. According to the scheme, it is carried by one of its polarity to the contact wires, the second - directly to the rails. It should be remembered that many traction mechanisms operate at a frequency different from the steady-state (50 Hz). Therefore, use a chastotnik for the electric motor, the principle of which is to convert the frequencies and control this characteristic.

According to the raised pantograph, the voltage is supplied to the chambers, where the starting rheostats and contactors are located. With the help of controllers rheostats are connected to traction motors, which are located on the axles of the trolleys. From them, current flows through the buses onto the rails, and then returns to the traction substation, thus closing the electrical circuit.