The 3-phase induction motor is quite popular, as it is commonly used for many industrial applications. The primary reason for their prevalence is reliability, simplicity, and long service life.
This motor is an example of an induction motor, also known as an asynchronous motor. This type of induction motor uses the principles of electromagnetic induction. Here is what a three-phase motor is and how it operates.
What is the meaning of 3-phase power?
Before learning about 3-phase AC induction motors, you must understand the 3-phase power. There are two types of power supply: the single power supply and the three-phase. In a single phase electric motor, one wire is used, and in 3-phase, three wires are used in a system to give sinusoidal voltage. However, each phase is transferred by 120 degrees, and the sum is always constant when you put together all the phases’ voltage.
The single-phase electric motor is appropriate for low power or residential applications, while the three-phase is for industrial use. Its prevalent use in industries is because it can dispense three times power while using 1.5 times as much wire. It makes the three-phase power efficient and an economic power supply.
What is a 3-phase motor?
These refer to a type of AC motor, a particular example of a polyphase motor. The motors can be either asynchronous (Induction motor) or synchronous motor. This type of motor comprises three major components: the stator, rotor, and enclosure.
- The stator comprises a chain of alloy steel laminations and has a wire cover to make induction coils. There is a single coil for every phase of the electrical power source. These coils are stimulated from the three-phase powerhouse.
- The rotor contains induction coils and metal bars to make a circuit. The rotor encircles the motor shaft and is the element that spins to produce the mechanical output of the motor.
- The enclosure clutches the rotor with its motor shaft on a set of bearings to lessen the friction of the rotating shaft. The enclosure comprises end caps to hold the bearing and houses a fan attached to the motor shaft that revolves when the motor shaft turns.
The rotating fan draws air from the outside of the enclosure and pushes air through the stator and rotor, cooling the motor elements. It also disperses heat generated in the coils due to coil resistance. In addition, it also has raised mechanical fins on the outside that conduct heat further to the external air.
The enclosure end cap also offers a spot to house the electrical connections for the 3-phase motor to the motor.
The Faraday Law
3 phase motors employ the principle of electromagnetic induction. English physicist Michael Faraday discovered this principle in 1830.
Faraday noted that when you place a conductor like a coil or loop of wire in a shifting magnetic field, an EMF (Induced electromotive force) is produced in the conductor. Also, he noticed that the current flowing in a conductor, such as a wire, would create a magnetic field. There will be a variation in the magnetic field as the current in the wire changes in magnitude or direction.
Its representation is in a mathematical formula that links the electric field curl to the rate of the shift in time of the magnetic flux.
How it works
A three-phase electrical energy source powers the stator. Every coil creates a magnetic field, and their poles (north or south) switch positions as the AC oscillates through an entire cycle.
The three phases of the AC are phase-shifted by 120 degrees. As a result, the magnetic polarity of the three coils is not all uniform at the exact moment. It results in the stator generating a Rotating Magnetic Field (RMF).
The rotor is in the middle of the stator coils, where the shifting magnetic field induces a current in the rotor coils. It then turns an opposing magnetic field that the rotor generates. The rotor field sees a way to align its polarity against the stator field. It results in a net torque applied to a motor shaft and commences spinning, aiming to get its field to alignment.
It’s vital to note that in the 3-phase induction motor, there is no direct bond to the rotor. The magnetic induction causes the motor rotation. The rotor aims to preserve alignment with the RMF of the stator using three-phase induction motors but never attains it.
The occurrence of making the rotor lag the speed of the RMF is known as slip, and the expression is:
Slip = (Ns – Nr) / Ns
Nr is the rotor speed, and Ns is the synchronous speed of the stator’s RMF (rotating field).
As mentioned earlier, the three-phase AC induction motor comprises the stator and rotor. A current is put through the stator during operation to induce a magnetic field that causes rotor rotation. The motors are mostly used in the industrial sector as they can produce more power compared to single-phase motors.
