Motors work by converting electrical power into mechanical energy. Motors are classified into two major types based on the current supply.
- AC motors
- DC motors
Three Phase Asynchronous Motors are also known as induction motors. They use AC to convert electrical energy into mechanical power. Unlike synchronous motors, the speed of the rotor and stator in an asynchronous motor is not synchronized. The speed of the rotor is always less than stator magnetic fields. Induction motors are the most common and widely used electric motors in the industries. Let us discuss basic information, construction, and applications of three phase asynchronous motors.
3 Phase Asynchronous Motor – Overview
A 3-phase asynchronous motor is also known as an induction motor. It is a machine that has three phase power supply and operates on three AC (alternating current) of the same frequency. They do not demand additional starting devices for operation. Therefore, 3 phase asynchronous motors are also known as self-starting motors. The 3 phase power supply generates electromagnetic induction in the stator of the motor. The rotor of three phase asynchronous motor having a magnetic field rotates through the torque produced by the stator winding.
Three Phase Asynchronous Motor
These days, asynchronous motors are commonly used in many industrial and commercials sectors because of their impressive features. 3 phase asynchronous motors are efficient, high speed, and long-lasting. The speed range of the three-phase induction motor varies from 1500 rpm to 3000 rpm, depending upon the number of poles. These motors are widely used in the industries because it provides 1.5x more power than mediocre single-phase motors.
What Is The Meaning Of Asynchronous Motor?
Asynchronous electric motors work on the principle of electromagnetic induction, where magnetic fields induce a current in the rotor. The rotor of the three phase asynchronous motor opposes the rotating magnetic fields of the stator. Thus, the rotor starts rotating in the same direction as the stator. However, the speed of the rotor is always less than the stator. The field difference between rotor and stator induces a current in the rotor.
Why Is It Called An Asynchronous Motor?
As the asynchronous motor operates on electromagnetic induction, there is always a difference between rotor flux and magnetic flux. The relative speed difference induces a current in the rotor conductor, which flows in the direction of the magnetic field. Due to the difference between the relative speed of the rotor and the stator, it is called an asynchronous motor. This means, that if the speed of the rotor surpasses stator magnetic fields, there will be no induction.
Construction Of Three Phases Asynchronous Motor:
The major parts of the three phase asynchronous motor are Stator and Rotor. These two parts work together to produce mechanical energy. The stator and rotor do not have any electrical connection between them. Here is the construction of a 3 phase asynchronous electric motor.
The non-moveable stationary part of the asynchronous motor is called Stator. The stator is a driving force that rotates the rotor in the direction of the magnetic field. The stator of three phase asynchronous motor consists of the following significant parts;
- Stator Frame
- Stator Core
- Stator Winding
As the name suggests, the frame is the outer part of the stator. Mostly the frame of the stator is made up of aluminum, 100% stainless steel, and fabricated steel. Manufacturers make the frame of large motors with metal fabrication. Casting is used for small motors. The primary function of the stator frame is to provide support to the other two parts of the stator (e.g., stator winding and stator core). It also ensures the stability of the inner parts of the stator. It has a fin-like arrangement for heat dissipation and ventilation of the motor.
There is a laminated structure known as the stator core inside the stator core of 3 phase asynchronous motor. The stator core of three phase asynchronous motor is made up of thin high-grade steel laminated stamping. The stamping is segmented collectively together to form a 0.4 to 0.5 mm thick ring. The lamination of the stator core is shielded from one another to disperse conduction. This phenomenon minimizes the eddy current loss and hysteresis. The stator core contains a different number of paired slots depending on the number of poles. For example, there are 2-pole and 3-slots motors, 4-pole and 3-slot motors, etc. The speed of the motor depends on the number of poles. The speed of the asynchronous motor will be less if the number of poles is greater, and it will be high if the numbers of poles are less. To determine the relationship between the number of poles and synchronous speed, we use the equation shown below,
Ns = 120f / P
Ns is synchronous speed
f is the Frequency
P is the number of poles
Inside the slots of the stator core, there is stator winding which provides a magnetic field in the stator. Each phase of the three phase asynchronous motor consists of two terminals. The stator contains six ends of the three-phase winding. These windings are connected to the terminal box. When the three-phase power supply is given to the motor, the stator windings get excited. These windings are connected in the star or delta connection in the terminal box. The stator windings are made up of 100% copper wire to make them highly resistant to adverse atmosphere factors such as grease, oil, water, heat, and moisture. Moreover, stator windings are insulated with varnish, which remains rigid and fixed in unfavorable temperature conditions.
The rotating part within the stator is called the rotor. The rotor rotates and induces a current in the direction of magnetic fields. The rotor of a three phase asynchronous motor carries the current and moves the shaft of the motor.
Depending upon the difference in the construction of the rotor, the 3 phase asynchronous motors are classified into two categories.
- Squirrel Cage Induction Motor
- Slip Ring Induction Motor
Squirrel Cage Induction Motor
The rotor of three phase asynchronous motor looks like the cage of a squirrel. Thus, this motor is called a squirrel cage induction motor. The squirrel cage rotor of induction motor is a cylinder of steel laminations. These laminations contain premium quality conducting metal (aluminum or copper) installed on the exterior covering. A squirrel cage rotor has angular slots on the outer edge of the core. The slot minimizes magnetic locking between the rotor and stator. Moreover, the slots reduce high noise bearing and ensure smooth operation.
Circuit Diagram of Squirrel Cage Motor
The squirrel cage induction motor does not contain winding; instead, this motor operates on rotor bars. When the AC current drive through the stator, it generates rotating magnetic fields. These magnetic fields induce a current in the rotor bars. Therefore, short-circuit current flows in the rotor bars. About 80% of three phase asynchronous motors used in the industries are squirrel cages because the construction of these motors is robust and simple.
Slip Ring Induction Motor
Slip ring induction motors are also known as wounded type motors. The rotor of the slip ring induction motor is different from the three-phase squirrel cage motor. It has wound rotor and windings placed inside the slots. The number of poles of wound rotor winding is coincident with the number of stator windings. The three-phase winding in slip-ring type is covered from one end, while the other ends are connected to the slip rings on the shaft. These slip rings are connected to the external resistor to generate high starting torque. These motors have slip-rings and carbon brushes to connect external resistance or rheostat.
Circuit Diagram of Slip-ring Motor
The primary function of external resistance is to control the speed and starting torque rate of a three phase asynchronous motor. The external resistance is connected with the circuit of the rotor only during the initial stage because it provides a high starting torque. The external resistance increases friction and copper loss if it remains connected while the motor is running. They have complex motor construction; therefore, slip ring induction motors are applicable where high torque and adjustable speed are required.
The fan of the three phase asynchronous motors regulates the temperate of the internal parts of motors. The fan dissipates the extra heat and maintains the temperature gradient of the 3 phase asynchronous motor.
Working Principle Of Three Phase Asynchronous Motor:
Fundamentally, the 3 phase asynchronous motors work on the principle of electromagnetic induction. The stator windings and the rotor windings are the major parts of an induction motor. Let’s discuss the working principle of the 3 phase induction motor step by step:
- The stator winding produces magnetic fields inside the stator when the motor is connected to the three phase power supply. The rotating magnetic fields rotate at synchronous speed producing 120 displacements known as(NS). According to Faraday’s principle of electromagnetic induction, the rotating magnetic field (RMF) cuts the rotor windings. Thus, electromotive force (EMF) is induced in the rotor bar or winding. As the rotor conductor is short-circuited, the current starts flowing in the rotor.
- The rotor conductors are placed in the presence of the magnetic field of the stator. Thus, in consonance with the Lorenz principle of force, a mechanical force acts on the rotor winding. The mechanical force on the rotor conductors tends to generate torque in the rotor. Hence, the rotor starts moving in the direction of the producing magnetic field.
- The direction of rotation can also be elaborated with the help of Lenz’s law. According to this law, the induced current in the rotor opposes the magnetic field of the stator. The relative motion between the stator and rotor flux causes the rotor to move in the direction of the stator’s magnetic fields. In other words, the rotor moves in the direction of the stator to capture the stator flux. The speed of the rotor is always less than the speed of the stator. There will be no current induction if the rotor catches the synchronous speed. Due to the difference between the rotor and synchronous speed, it is called three phase asynchronous motor. The relative speed difference between rotor and stator is known as slip.
- The slip of the AC asynchronous motor can be determined using the following formula;
Formula: s = (Ns – Nr) / Ns
Percentage slip = (Ns – Nr) / Ns x 100
- The slip is zero only when the speed of the rotor is equal to the stator. The value of slip in the three phase asynchronous motor is never zero because this condition never occurs in the case of induction motors. If the speed of the rotor and stator becomes equal, there will be zero relative motion. It results in no EMF and current in the rotor. Hence, the induction motor cannot run. For producing induced current, the Nr (Rotor speed) should always be less than NS (stator magnetic field speed). The 3 phase asynchronous motors are constant speed motors because the value of slip from no load to maximum load is 0.1 to 3 percent.
What Is The Difference Between Synchronous And Asynchronous Motors?
Synchronous motors are the type of electric motors in which the speed of the rotor is synchronized with the speed of stator magnetic fields. These motors do not have a slip; the value of the slip is always zero. It requires an external power supply to start the initial rotation. The rotor of synchronous motors demands current supply. The price of synchronous motors is always higher compared to asynchronous motors.
On contrary, the speed of the rotor is always less than stator magnetic fields. Three phase asynchronous motors have slip; so, the value of slip is never zero. Asynchronous motors are self-starting; therefore, there is no need for additional starting sources. Asynchronous motors do not require any current to rotate. These motors are cost-effective.
Some Significant Advantages Of Three Phase Asynchronous Motor:
3 phase Asynchronous motors are being widely used for various domestic and commercial purposes due to their phenomenal features. Let’s discuss some prominent advantages of a three-phase induction motor:
High Speed And Less Vibration:
The 3 phase asynchronous motors provide constant torque and speed range because there is no external resistance connected. These motors are highly efficient motors with an efficiency rate of 90% to 95%. Since the rotor of three phase asynchronous motors does not demand a current supply, the loss of energy is quite low. Moreover, the frictionless connection between the stator and rotor reduces loud vibrations.
The 3 phase asynchronous motors are independent and work on their own. An asynchronous motor does not require an external power supply for starting rotations. They produce their own starting torque through electromagnetic induction. Therefore, these motors are also called self-starting motors.
The three-phase induction motors are designed for the harsh climate and working conditions. These motors are long-lasting and have an extended life span. The AC induction motor does not have brushes; the maintenance rate is very low. The amateur reaction in asynchronous motors is less, which ensures long life. The absence of brushes reduces the chance of explosions and sparks at the work sites.
Robust Motor Construction:
The construction is the most prominent advantage of the AC induction motors. The motor construction of three phase asynchronous motors is robust. It does not have any complex slip ring and external power supply like other synchronous motors. Only wound-type asynchronous motors have slip rings to provide resistance to the rotor winding.
Unlike all synchronous and industrial motors, the 3 phase asynchronous motors are very affordable. The simple construction without slip rings and additional resistance is the reason behind the low price of three phase asynchronous motors.
Common Types Of Three Phase Asynchronous Motors:
There are several types of 3 phase asynchronous motors available in the marketplace. Some most common types of 3-phase asynchronous motor include;
- Y2, Y3, YE3 3 phase asynchronous motors
- MS series (B3, B5, B14, B35) aluminum shell three-phase motors
- YEJ series, YD/YDT series, and YVF2/YVP series
Industrial Applications Of Three Phase Asynchronous Motors:
The 3 phase asynchronous motors have a wide range of applications because these motors are highly efficient and easy to operate.
- Mostly three phase asynchronous motors are used in the industries where high loads and speed control are required. For example, elevators, compressors, and cranes.
- The three-phase slip ring motors are used in several industries such as steel mills, lifting industries, and heavy hardware/mechanic workshops.
- The squirrel cage induction motors have normal starting torque; therefore, these motors are used in pumps, fans, and blowers. It provides a normal range of starting rotation which gradually increases with a full load.
- The three phase asynchronous motors are also used in crushes, machining tools, and conveyors.
- The squirrel cage motors are applicable in grinding machines, pressing, and lathe machines.
The basic information, construction, and working principle of 3 phase Asynchronous motors conclude that these motors are the perfect fit for high load and constant speed. The simple construction and self-starting feature make these motors a better choice for industrial purposes. Xinnuo motor provides supreme quality and highly efficient 3 phase asynchronous motors. These motors are the perfect choice if you are looking for a low-cost, easy-to-maintain, and high-performance motor.