Synchronous and asynchronous electric motors

Synchronous and asynchronous electric motors form the basis for the classification of all motors. No matter how good were DC motors, but they have a very unreliable collector assembly with brushes: it sparks and often goes out of order. Therefore, scientists began to look for a method of "brushless" operation of electric motors. In this they were helped by the famous Faraday experience, which allowed to receive alternating current: when magnet began to move into the coil, a current appeared, and then, when the movement of the magnet ceased, the current ceased. If we move the magnet back and forth continuously or rotate it, we get the real alternating current, and without any collectors, directly from the coil. To do this, we took three coils with cores, placed them in a circle at an angle of 1200, and inside the circle began to rotate the magnet - permanent or electric. The magnet could be rotated very quickly, which made it possible to obtain sufficiently large currents. So the generator of a variable three-phase current was invented - each coil gave the phase. The current in these phases increased and fell alternately with a shift of 1200.

Synchronous and asynchronous electric motors form the basis for the classification of all electric motors. a - stator, b - rotor of induction motor

The electric motor, which can be powered by such a three-phase current, is fundamentally the same as the generator. The same coils, the same magnet - the rotor. If you connect the generator to the motor, then when the pole of the generator magnet passes by any coil, the largest current appears in it, which magnetizes the corresponding motor coil. It is to this coil that the same pole of the magnet of the electric motor tends, exactly repeating the rotation of the generator magnet. This is how the synchronous motor works, in which the rotor-magnet moves synchronously with the rotor-magnet of the generator. But often the rotation of the rotor-magnet of the synchronous motor meets great resistance, and it can stop, get off the rhythm.

To prevent this from happening, scientists invented an asynchronous, or as it is now called, asynchronous electric motor, where the rotor can lag behind the rotating magnetic field. Imagine that instead of a permanent magnet, the rotor consists of a coil, such as a DC motor, only with a short-circuited collector. Strictly speaking, the collector is simply not needed, and the turns of the coil can be made in the form of rods, connected by rings at the ends. The construction of such a rotor was the most widespread and was called short-circuited, since indeed each rod-turn is short-circuited.

The asynchronous motor operates as follows. A rotating magnetic field of stationary stator coils begins to induce electricity in the windings or rods of the fixed rotor, turning them into electromagnets, which are carried away by the magnetic field of the stator and begin to rotate.

It was then that it became clear what synchronous and asynchronous electric motors are. If in the first magnet-rotor exactly repeats the rotation of the magnetic field, then in the second such a repetition is impossible in principle. If the rotor with windings rotates at the same speed as the magnetic field, then there will come a time when the current will not be inducted in the windings, since there will be no relative movement of the magnetic field and windings. Completely demagnetized, the rotor begins to lag behind the rotating magnetic field. But with the lag, the relative rotation of the rotor and field begins again and again the rotor becomes a magnet and again begins to catch up with the magnetic field.

The rotor of an asynchronous electric motor always lags behind a rotating magnetic field. This backlog is small and for short-circuited electric motors does not exceed a few percent.