Induction motor fed by ac current, typically three-phase. Three-phase system of windings creates a rotating magnetic field with an angular velocity of rotation: (1) where f – frequency of the mains, pn – number of pole pairs. Rotating magnetic field crosses the winding turns of the rotor. If the rate of rotation of the rotor is different from the speed of the field, the rotor winding begins to induce a current which interacts with the stator field, creates a moment. As a result, the engine has a mechanical characteristic, shown in Figure 1 Fig. 1. Mechanical characteristics of the induction motor can be seen from the characteristic speed in the work area slightly different from the idle speed, and it becomes clear that to effectively regulate the motor speed can be by changing the frequency of rotation of the stator or the frequency of the mains. The question remains open: how change the supply voltage? Typically, the engine runs on the knee of the magnetization curve, as shown in Figure 2.
Then it becomes clear that the increase in flow will lead to a sharp increase in current, while reducing to a decrease in moment. Fig. 2. Magnetization curve of the engine electrical balance equation for one phase can be written as follows: (2) If we neglect the voltage drop across the active resistance of stator phase, then the equation simplified: (3) Phase voltage varies as: (4) If we now equate the right sides of the above written equations, we get the expression: (5) After integrating this expression becomes: (6) or (6) From the expression that the amplitude-phase flow is directly proportional to voltage and inversely proportional to frequency, so to maintain a constant flow of needed change in voltage proportional to the frequency change.