A typical AC electric motor is an induction motor, usually referred to as an asynchronous motor. In an induction motor, the spinning magnetic field of the stator winding is used to electromagnetically induce the electric current needed to create torque in the rotor. An induction motor’s rotor can be either a wound type rotor or a squirrel cage rotor.
Because they run at a speed lower than their synchronous speed, induction motors are known as “asynchronous motors.” Therefore, the first thing to comprehend is what synchronous speed is.
Squirrel-cage induction motors with three phases are frequently employed as industrial drives because they are easy to start, dependable, and affordable. Smaller loads like home appliances like fans frequently employ single-phase induction motors. Induction motors are increasingly employed in variable-speed service using variable-frequency drives (VFD), despite being historically used in fixed-speed service. For current and future induction motors used in applications requiring variable-torque centrifugal fans, pumps, and compressor loads, VFDs provide particularly significant energy savings prospects. In applications requiring both fixed-speed and variable-frequency drives, squirrel-cage induction motors are utilised extensively.
So far, the most common form of motor utilised in residential, commercial, and industrial applications is an induction motor.
These three three-phase AC induction motors’ distinguishing characteristics are:
- simple, rudimentary design
- inexpensive and low-maintenance
- High dependability and expertise
- No need for a second starting motor, as they are not related.
A common type of AC electric motor is the induction motor. In an induction motor, the spinning magnetic field of the stator winding is used to electromagnetically induce the electric current needed to create torque in the rotor. An induction motor’s rotor can be either a wound type rotor or a squirrel cage rotor.
Induction motors are also known as asynchronous motors and are used in many applications. This is due to the fact that an induction motor’s speed is always lower than synchronous speed. Synchronous speed refers to the speed of the stator’s spinning magnetic field.
A motor that uses electromagnetic induction to operate, or one that causes the rotor to rotate, is known as an induction motor. Electric energy is transformed into mechanical energy in this manner.
It is divided into two parts:
- Stator: A fixed component
The stator is constructed from a variety of stampings that include spaces for three-phase windings. It has a certain number of poles for winding. The windings are separated by 120 degrees geometrically. Squirrel cage rotors and wound rotors are the two types of rotors used in induction motors. The machine doesn’t need any DC field current to operate. Rotor voltage is not physically connected by wires, but is instead induced in the rotor windings.
- Rotor: Moving component
The revolving component of the electromagnetic circuit is called the rotor. The squirrel cage rotor is the most prevalent form of rotor. The rotor is made up of a cylindrical laminated core with parallel slots for the conductors that are axially positioned. Each slot carries a copper, aluminum, or alloy bar.
The anchor form of the rotors utilized in very early electrical equipment serves as the inspiration for this term. Although the rotor performs this function in three-phase induction motors, the magnetic field would stimulate the anchor’s winding in electrical equipment.
The physical stator of an induction motor is identical to that of a synchronous machine, but the rotor development is different. An induction motor can be used as a generator or a motor. However, they are mostly employed as induction motors.
An electromagnetic field is produced when an electric current is transmitted, leading to flux in the stator and another flux in the rotor. The rotor rotates as a result of this.
Because they do not operate at synchronous speed, induction motors are often referred to as asynchronous motors.
Depending on whether an induction motor is single phase or three phase, several types of induction motors may be identified.
Single Phase Induction Motor
Single phase induction motors come in the following varieties:
- Split Phase Induction Motor
- Shaded Pole Induction Motor
- Capacitor Start Induction Motor
- Capacitor Start and Capacitor Run Induction Motor
Application of Single Phase Induction Motor
- Small fans
- High-speed vacuum cleaners
- Electric shavers
- Drilling machines
Why Can’t a 1-Phase Induction Motor Start By Itself?
A single phase supply causes a one-phase induction motor to produce a pulsing magnetic field rather than a rotating one. A conductor’s current supply creates a flux that may be divided into two halves, each of which rotates at a constant speed in the opposite direction.
As a result, the net flux, the current flowing in the induced conductors of the rotor, and the torque all become zero. A single-phase induction motor is not self-starting as a result.
This motor may be briefly changed into a 2-phase motor during beginning in order to solve this problem and make it self-starting.
Three Phase Induction Motor
Three-phase induction motors come in the following varieties:
- Squirrel Cage Induction Motor
- Slip Ring Induction Motor
Application of Three Phase Induction Motor
- Large capacity exhaust fans
- Driving lathe machines
- Crushers Oil
- extracting mills
- Textile and etc.
Why Does a Three-Phase Induction Motor Self Start?
Three single-phase lines with a 120° phase difference make up a 3-phase motor. Therefore, the rotating magnetic field also has a phase difference, and the rotor will revolve as a result.
For instance, if we think of phases a, b, and c as three, the rotor will move toward phase a once phase a becomes magnetic. The rotor will get magnetized when phase “b” becomes magnetized in the following second phase, and phase “c” will follow. The rotor will continue to revolve in this manner.
The Basic Operation Of An Induction Motor
Supply must be provided for both the rotor winding and the stator winding in a DC motor. However, only the stator winding of an induction motor is fed with an AC supply.
- Due to the AC supply, alternating flux is created around the stator winding. This fluctuating flux spins at a synchronous rate. The term “Rotating Magnetic Field” refers to the rotating flux (RMF).
- According to Faraday’s law of electromagnetic induction, the speed difference between the rotor conductors and stator RMF induces an emf in the rotor conductors. Short circuiting of the rotor conductors causes induced emf to induce rotor current. Because of this, these motors are referred to as induction motors.
- Induced current will now cause an alternating flux to surround the rotor. The stator flux is greater than this rotor flux. According to Lenz’s law, the direction of induced rotor current tends to be in the opposite direction of what caused it to be produced.
- The rotor will strive to catch up with the stator RMF because the relative velocity between the revolving stator flux and the rotor is what generates rotor current. To reduce the relative velocity, the rotor rotates in the same direction as the stator flux. The rotor, however, is never able to catch up to the synchronous speed. This is how an induction motor functions fundamentally, whether it is a single phase or three phase model.
Synchronous speed of induction Motor
Synchronous speed is the rate of rotation of a rotary machine’s magnetic field, and it is dependent on the machine’s frequency and number of poles. Always operating at a lower speed than its synchronous speed is the induction motor.
The rotor will revolve because of the flux created in the stator by the revolving magnetic field in the stator. The rotor will never attain its rotational magnetic field speed because of the delay between the flux current in the rotor and the flux current in the stator (i.e. the synchronous speed).
Synchronous speed is the term used to describe the rotational speed of a spinning magnetic field.
Synchronous speed Ns=120f/P
where f = the supply frequency
P is the Number of poles.