The three-phase induction motor has a major advantage over other types of induction motor is a self-starting property which requires no need of starting device and provides high power factor, good speed regulation, and robust construction.
Working Principle of Three Phase Induction Motor:
The working principle of a three-phase induction motor is based on the production of a rotating magnetic field. The rotating magnetic field can be defined as the filed or flux having constant amplitude but whose axis is continuously in a plane with a certain speed. So when the arrangement is made to rotate a permanent magnet, then the resulting field is a rotating magnetic field but this requires the magnetic rotation to be physical rotation which is for single-phase induction motors.
In the three-phase induction motor, the rotating magnetic field is produced by supplying currents to a set of stationary windings, with the help of a three-phase AC power supply. The current-carrying windings produce the magnetic field or flux due to which the interaction of three magnetic fields or fluxes produced, resultant magnetic field or flux has a constant magnitude and its axis is rotating in space without the physical rotation of the three-phase windings. This type of magnetic field is a rotating magnetic field.
Rotating Magnetic Field Production of Three Phase Induction Motor:
A three-phase induction motor consists of three-phase windings as it remains stationary due to which it is called the stator. The three-phase winding is usually connected in star or delta fashion. The three-phase windings are displaced from each other by 1200. The windings are supplied by a balanced three-phase AC power supply.
The three-phase currents flow simultaneously through the windings are displaced from each other by 1200 electrically. Each alternating current produces its own flux which has sinusoidal type nature. Due to this, all three fluxes produced are sinusoidal in nature and are separated from each other by 1200. If the phase sequence of the windings is R-Y-B, then the mathematical equations for the instantaneous values of the three fluxes ϕR, ϕY and ϕB can be written as
ΦR = ϕmsin(wt) = ϕmsin
ΦY = ϕmsin(wt – 1200) = ϕmsin0 )
ΦB = ϕmsin(wt – 2400) = ϕmsin0 )
As the windings are identical and input AC power supply is balanced, the magnitude of each flux is ϕm. Due to phase sequence R-Y-B, flux ϕY lags ϕR by 1200 and flux ϕB lags ϕY by 1200. So ultimately, flux ϕB lags ϕR by 2400. The flux ϕR is always taken as a reference while writing the equations. Let ϕR, ϕY, and ϕB be the instantaneous values of the three fluxes which will produce the resultant flux ϕT which the phasor addition of these fluxes. And it can be written as
ΦT = ΦR + ΦY + ΦB
ΦT = 1.5ϕm
The resultant of these three alternating fluxes which are separated from each other by 1200, has a constant amplitude of 1.5ϕm where ϕm is the maximum amplitude of an individual flux due to any phase. And this resultant flux will always keep rotating in space at a certain speed. This shows that when the three-phase stationary windings are excited by a balanced three-phase AC power supply then the resulting field produced is rotating magnetic field. Though nothing is physically rotating, the field produced is rotating in space having constant amplitude.
The direction of Rotating Magnetic Field:
The direction of the rotating magnetic field is always from the axis of the leading phase of the three-phase winding towards the lagging phase of the winding. In a phase sequence of R-Y-B, phase R leads phase Y by 1200 and phase Y leads phase B by 1200. So, the rotating magnetic field rotates from the axis of R to the axis of Y and then the axis of B and so on. Hence, this direction of the rotating magnetic field is clockwise in nature. We can make this direction of rotating the magnetic field in anti-clockwise nature by reversing any two winding connections with each other while connecting to a three-phase AC power supply. Let’s say we reverse the connection of Y and B winding with the input power supply. Due to which the phase sequence will become R-B-Y and the rotation of the flux will become anti-clockwise. Thus, the rotating magnetic field axis will follow the direction from the R to B to the Y-axis which is of anti-clockwise nature. Thus by interchanging any two terminals of the three-phase winding when connecting it to a three-phase AC power supply, the direction of rotation of the rotating magnetic field gets reversed.
Slip Rings and Brush Assembly Concept of Three Phase Induction Motor:
When we need to connect the rotor of the motor to load or external circuit, brush assembly and slip rings are used. Consider a three-phase rotating star-connected winding in the motor and the need for the connection of three stationary star connected resistances to these windings is required. The windings must keep on rotating and external resistance must remain stationary and still the connection between must remain intact which is possible by slip rings and brushes.
The three rings made up of conducting material called slip rings are mounted on the same shaft with which winding is rotating. Each terminal of the winding is connected to an individual slip ring permanently. Thus, three ends of R-Y-B of winding are available at the three rotating slip rings. Then three brushes are used in which each brush is resting on the corresponding slip ring making contact with the slip ring but the brushes are stationary. Now, the stationary external circuits can be connected to the brushes which are nothing but the three ends of the windings.
Thus, the external stationary circuit can be connected to the rotating internal part of the machine with the help of the slip rings and brush assembly. We cannot only connect the external stationary circuit but the voltage injection can also be done by injecting the voltage to the rotating winding by connecting stationery supply to the brushes externally. Such slip rings and brush assembly plays an important role in the working of slip ring induction motors.