Types of AC Motor Overview
Electricity and electrical devices are extremely important in the modern world. It is difficult to imagine life without them. The motor is one such electrical invention that enabled humanity to advance significantly. When it comes to motor usage, AC motors are much more common than DC motors. In daily life, there are numerous applications for various types of AC motors. Depending on their specifications, different types of AC motors are categorized.
An electrical device that converts electrical current into mechanical current is known as an electric motor. Electric motors play a huge role in daily life. Electric motors are extremely efficient machines that primarily depend on the motor's size and state of operation. Unlike other electrical devices, the maintenance of electric motors does not involve the use of fuel or engine oil. The two main categories of electric motors are AC motors and DC motors. Because AC motors are typically more expensive than DC motors, they are used less frequently in daily life. The main reasons why AC motors are used are their quiet operation and high degree of practicability.
What are AC motors?
An AC motor is one that transforms alternating current into mechanical energy. The AC motors are induction motors.
The term "AC motors" refers to an electric motor with a stator and a coil that is the "transformed motor of the current." To convert electric power into mechanical current, it is delivered with a transformed current (Wang, Reitz, & Yaz, 2018). Three-stage motors are typically used for the conversion of large amounts of power, though AC motors can be single-phase or three-phase. Small power conversions are done using single-phase AC motors.
Due to their efficiency, viability, and quiet operation, AC motors are shown to be a reliable source of power for a wide range of applications. A copper-wound stator that is magnetically driven and has a rotating field of magnets made over it makes up the basic structure of AC motors (Lee, 2021). The IE4 and IE3 specifications, as well as international motor efficiency standards are met by AC induction motors.
Also read more about Magnetic Effect of Electric Current and Working Principle of Transformer.
Types of AC Motors and their Working Principle
Following are the different types of AC motors-
1. Induction Motors
Induction motors, also referred to as asynchronous motors, are frequently used as AC electric motors. In this motor, electromagnetic induction from the rotating magnetic field of the stator winding is used to generate the electric current in the rotor necessary to produce torque. The induction motor's rotor is essentially a wound type or squirrel cage rotor.
Working Principle
Through the use of brushes, a DC motor delivers electricity to both the stator and the rotor. One is the only supply we give the stator in an induction motor. A magnetic flux is created in the stator as a result of the current flowing in the coil when we supply electricity to the stator winding.
Each coil in the rotor winding is short-circuited by design. The short-circuited coil in the rotor is cut by the magnetic flux from the stator. The current will begin to flow through the rotor coils as they are short-circuited. This is referred to as Faraday's law of electromagnetic induction.
Another magnetic flux is produced in the rotor when current passes through the rotor coil. Both the stator and the rotor currently have two fluxes. The stator flux will advance faster than the rotor flux. This produces the torque necessary for the rotor to rotate in the same direction as the rotating magnetic field.
2. Synchronous Motors
An AC motor in which, at steady state, the rotation of a shaft is in synchrony with the frequency of the applied current is referred to as a "synchronous motor." It functions as an AC motor, but in this case the shaft's total number of turns is equal to the integer multiple of the applied current's frequency.
The induction current is not necessary for the synchronous motor to function. These motors have multiphase AC electromagnets on the stator, which create a rotating magnetic field, unlike induction motors.
In this case, the rotor is a permanent magnet that rotates synchronously with the frequency of the current applied to it and is synchronized with the rotating magnetic field.
Working Principle
The interaction of the magnetic fields of the rotor and the stator is what makes synchronous motors work. The stator has 3 phase windings and is supplied with 3 phase power.
Thus, a three-phase rotating magnetic field is generated by the stator winding. DC power is used to power the rotor. The stator winding-generated rotating magnetic field is where the rotor enters and begins to rotate synchronously.
The frequency of the supply current now determines the motor's speed. The frequency of the applied current regulates the speed of the synchronous motor. Ns=60f/P=120f/p is the formula used to calculate a synchronous motor's speed, where,
- f = be the frequency of the AC current (Hz)
- p = be the total number of poles per phase
- P = be the total pair number of poles per phase
The motor desynchronizes if a load that is greater than the breakdown load is applied. The ability to choose the winding direction is a benefit of the three phase stator winding.
The direction of rotation cannot be determined in the case of single-phase winding, and the motor can start in either direction. To control the direction of rotation, these synchronous motors require initial configuration.
Read more about the Resistance Formula and Refractive Index Formula.
3. Single-Phase Motors
An AC motor called a single-phase induction motor transforms electrical energy into mechanical energy to carry out some physical work. Only one power phase is necessary for the single-phase induction motor to operate. These ac motor types are frequently employed in low power applications for both domestic and commercial uses.
Working Principle
A single-phase AC supply is given to the stator winding in these kinds of ac motors. This results in a sinusoidally pulsing magnetic field.
After a while, the field polarity changes, and the alternating current might not be able to supply the motor with the necessary winding. However, if the motor is powered externally, it will only rotate at a certain speed.
4. Three-Phase Motors
A three-phase motor is a type of electric motor that uses electromagnetic interactions to transform electrical energy into mechanical energy. The most popular type of motor for three-phase AC (alternating current) operation is an induction motor because they don't need any additional starting equipment. Self-starting induction motors are the name given to these kinds of motors. A three-phase induction motor consists of a stator and rotor.
Working Principle
The stator winding creates a constant magnetic field with 120 displacements in a constant magnitude that revolves at synchronous speed when these kinds of ac motors are powered by a three-phase supply.
The rotor conductors are cut by the variable magnetic field, which produces a current in them, in accordance with Faraday's theory of electromagnetic field laws. Current starts to flow through these rotor conductors as they are compressed.
Rotor conductors are positioned in the magnetic field of the stator. Thus, a mechanical force is acting on the rotor conductor in accordance with Lorenz force theory. All the rotor conductors work together to exert force on the rotor, which causes it to rotate in the same direction as the rotating magnetic field.
Lenz's law, which states that currents induced in rotors oppose the cause of their production and that in this case this resistance is rotating the magnetic field, can also be used to explain why this rotor conductor rotates. As a result, the rotor starts to spin in the same direction as the rotating stator's magnetic field. Because the rotor rotates as a result of the relative motion of the rotor and the stator magnetic field, if the rotor speed is greater than the stator speed, no current will be generated in the rotor.
Slip is the distinction between the stator and rotor fields. Due to this relative speed difference between the stator and rotor, the 3-phase motor is known as an asynchronous machine.
Also read about- Father of Physics and EMF Equation of DC Generator.
5. Squirrel Cage Induction Motor
A three-phase induction motor based on the electromagnetism law is called a squirrel cage induction motor. This is known as the "squirrel cage motor" because of the rotor inside, which is called that because it resembles a squirrel cage.
This rotor is a steel cylinder with a highly conductive metal on the surface (typically copper or aluminum). A rotating magnetic field is created when an alternating current passes through the windings of the stator.
As a result, the rotor winding generates a current that creates its own magnetic field. The squirrel cage rotor is subjected to a torque due to the interaction of magnetic fields produced by the stator and rotor winding.
The efficiency with which a squirrel cage motor's speed-torque characteristics can be altered is a significant benefit. The size of the bars in the rotor can be changed to achieve this. Due to their dependability, self-starting ability, and adaptability, these types of motors are utilized in a variety of industries.
Working Principle
When the stator winding receives a three-phase power source, a rotating magnetic field is created in space. This rotating magnetic field has a speed known as "synchronous speed."
Short-circuit currents start to flow in the rotor bars as a result of the stator's rotating magnetic field inducing voltages in the rotor bars. These rotor currents generate their own magnetic fields, which combine with the stator's magnetic field. The rotor now begins to follow the rotating magnetic field as the rotor field tries to resist its cause.
Because there is no longer any relative motion between the rotating magnetic field and the rotor when the rotor catches the rotating magnetic field, the rotor current falls to zero. As a result, the rotor experiences zero tangential force at that instant, briefly disintegrating.
The rotor current is re-induced as a result of the rotor's disintegration because the relative motion between the rotor and rotating magnetic field re-establishes.
As a result, the tangential force is once again applied to the rotor's rotation, which causes it to once again begin rotating the magnetic field. As a result, the rotor maintains a constant speed that is lower than the synchronous speed of the rotating magnetic field.
Working principle of AC motors
During the positive half cycle of an electromagnet, the rotor freely alternates between the two stator poles, which have a small air gap between them. The electromagnets attach directly to the AC supply source. The production of magnetic poles and the supply of AC give the electromagnets a boost.
- Case 1: Taking pole A into consideration, pole B aids in achieving the polarity of the south pole. According to the magnetism law, the opposite poles attract one another. The south pole of the rotor is drawn to pole A.
- Case 2: During the negative half of the AC supply's cycle, the polarity is switched. So, Pole A receives the south polarity, while Pole B receives the north polarity. In this case, like poles are facing each other. The magnet's repellent force tries to steer the room away from that location. As the rotor is diverted, the magnet's attractive force becomes visible.
Read more about the Refractive Index Formula.
Construction of AC Motors
The three-phase induction motor is made up primarily of two components:
Stator
To reduce eddy current and hysteresis loss, it is constructed with a steel frame that encloses a hollow, cylindrical cone made of thin silicon steel laminations. Numerous identical slots are carved on the inner periphery of the core.
These slots are used to house the stator conductors, which are isolated from one another and from the slots. A balanced star or delta winding is used to link the conductors. The number of poles for which the windings are wound depends on the speed requirement. When a high speed is required, fewer poles are wound, and vice versa.
Rotor
The rotor, which is located on the motor's shaft, can be connected to any mechanical load. A rotor can be of two different types:
- Squirrel cage rotor: The core of a squirrel-cage rotor is a laminated cylindrical core. For the conductors of the rotor, it also has parallel slots. The conductors of the rotor are thick bars made of copper or aluminum. One bar is given to each slot.
- Phase wound rotor: Laminated materials are used to create the rotor. The three-phase star-connected winding is installed in uniform slots that are present throughout the cylindrical core. Three insulated slip rings that are mounted on the motor shaft and connected to carbon brushes are where the open ends of the star winding are connected.
Also read more about the Electric Power Formula.
Advantages of AC Motors
The following are the benefits of AC motors:
- The cost of purchase may be low.
- The speed can change.
- low cost of maintenance.
- Power factor of an AC motor is high.
- It runs consistently.
- Simple in structure.
Disadvantages of AC Motors
The following are the drawbacks of AC motors:
- Because a back EMF is produced, it will produce eddy currents.
- Going at a slow speed impairment.
- Its positioning control is subpar.
Applications of AC Motors
The applications for AC motors are as follows-
1. Squirrel Cage Motors
- These are employed in the drives of industrial water pumps, tube wells, drills, lathes, etc.
- For the drives of cranes, punch presses, lathes, drills, etc., double cage motors are employed.
2. Phase wound motors
- When a high starting torque is needed, use this.
- Used to power mills, lifts, pumps, line shafts, and other devices.
- In the steel rolling industry, paper mills, and compressors, synchronous motors are used.
- Domestic electrical appliances such as washers, dryers, refrigerators, grinders, blowers, etc. use single-phase motors.
Also read about- SI Unit of Electric Flux.
How are AC motors different from DC motors?
- Nature of input current: The primary input power for AC motors is alternating current, which is its nature. But for DC motors, direct current provides the input power.
- Commutator: Commutators are only found in DC motors; they are absent from AC motors.
- Supply Phase: AC motors can run on single- or three-phase power, but DC motors can only be powered by a single phase.
- Input Terminals: Unlike DC motors, which only have two input terminals (+, -), AC motors have three input terminals (RYB).
- Brushes: DC motors have carbon brushes for spark-free commutation; AC motors do not.
- Armature Position: In AC motors, the armature is stationary while the magnetic field is rotating. In DC motors, the armature rotates, and the magnetic field is stationary.
- Starting: DC motors do not require external assistance during the start-up process, whereas AC motors do.
Conclusion
The rotor, which rotates, and the stator, which is stationary, are two crucial components of an AC motor. The main purpose of the AC motor's stator, which is inactive, is to transfer the magnet's rotary field so that it can connect to the rotor. In order to allow the interaction with external broadcasting of the system of rotating power, steel shafts that extend outside of the motor casing are pressed on to create the rotor assembly in AC motors. Controlling the speed of an AC motor involves changing the slip of the motor and the frequency of the AC signal. A good power to weight ratio, low cost, and minimal maintenance are all characteristics of AC motors.