The word "engine" always gives rise to a representation of something moving, and even more often - rotating. Of course, that's why the engine is. The essence of any engine is to convert some form of energy into motion. Do you know which engine was the very first? Believe it was a sail. It will not be a mistake to say that the most common engine is an electric one. Among the variety of electric machines of this class, the most widely used is an induction motor, the principle of which is based on the use of two fundamental manifestations of electric energy - electromagnetism and electric induction.
To understand the principle of operation of an asynchronous electric motor, we recall the simplest experiments from the school physics course:
- if a current is passed through a conductor, a magnetic field proportional to the current appears around it;
- if you take two conductors with current, then a force field arises between them - they repel or attract, just like any pieces of magnets do;
- if one conductor with current moves in relation to another, which is not connected to the source, but its circuit is closed, then current arises in it.
The physical, or, more precisely, speculative, model of the process includes the concept of magnetic lines of force, by which we denote the presence of magnetic properties. So they stretch their “tentacles” in all directions, and if ferromagnetic material gets in the way , i.e. one in which the magnetic lines are concentrated, then the driving force acts on it. And if this material itself is a magnet, then the force interaction of magnetic fields is even stronger. It is possible to increase the magnetic field of the conductor by winding a large number of turns on a rod made of a ferromagnet - in the simplest case, this is an ordinary metal cylinder made of ferrous metal. It is not difficult to make the rod horseshoe-shaped, and put a winding on it. The ends of the “horseshoe” became the poles of a magnet, which, when the current through the coil is turned on, will create a powerful, maximum magnetic field at the center of the poles. Usually we visualize it in line drawings.
If we put a metal rod in the gap between the poles, and then start to rotate our magnetic “horseshoe”, we will notice that the rod will also rotate. Why is our device called an induction motor? The principle of operation of this electric machine, as was said, is based on the phenomenon of electromagnetic induction, i.e. the intersection of the magnetic lines of the inductor of the body of our rod. And “crossing” is possible only when the speed of rotation of the magnetic field of the inductor - “horseshoe” - is greater than the speed of rotation of the rod. This parameter of the motor - the asynchronism of rotation speeds - is called "slip" and can reach up to 7% of the nominal speed of the stator magnetic field.
The maximum of the magnetic field located in the gap of the “horseshoe” changes its spatial position when it is rotated. In accordance with the law of electromagnetic induction, an electric current appears in the rod together with its “life partner” - a magnetic field. Magnetic fields interlock, interact, and ... rotate the rotor. A real induction motor repeats the principle of operation exactly as described here. That's it, the circle is closed - we have two objects without a mechanical connection, but the rotation of one of them, which receives electrical energy, leads to the rotation of the other. The role of the invisible “rope” is performed by the magnetic fields of a horseshoe-shaped magnet and a rod.
To create the engine, there was almost nothing left - you need not rotate the “horseshoe”, but make the magnetic field rotate independently. In other words, the induction motor demonstrates the principle of operation by the fact that in it the maximum point of the magnetic field moves in the stator in a circle, as if it were our rotating electromagnet. The only thing left is to “spin” the magnetic field.
The problem of a rotating magnetic field is solved as follows:
- in a circle there are three pairs of poles - coils - at an angle of 120 degrees;
- a sinusoidal voltage is supplied to each coil from a separate source. It is believed that the full time cycle of a sinusoid (period) is 360 degrees. A positive maximum voltage occurs 90 degrees after a zero value - all as taught in school;
- the highlight is that the voltage maximums occur with a temporary shift by a third of the period (electricians say “three phases with a shift of 120 degrees”) - this is the principle of operation of an induction motor. The maximum magnetic field strength moves from the first pair of poles to the second, and then to the third. Then the process is repeated, which is equivalent to the movement, or rather, the "rotation" of the magnetic field.
So, simply and without noise, the electric energy of the stator turns into mechanical movement of the rotor. To create this wonderful engine, mankind took several decades, and it took us 15 minutes to confidently say: asynchronous motor? - nothing is easier.