In the previous post I have explained the operating principal of Induction motor which was essential for the start up, Now we are going to check its basic construction.
I am posting the basic construction because without the basic knowledge, its very difficult to move into pro, so give it your full attention and understand whats actually goingo n within Stator and Rotor of induction motor.
I have divided this tutorial in nine parts and in the whole tutorials, we will cover all about Induction motor, how it works and its controlling factors etc and in the coming tutorials I will explain its controlling using microcontroller. These are the few parts in which I have divided this tutorial :
- Basic Concepts of Induction Motor (Part 3)
- Types of Induction Motor (Part 4)
Like most motors, an induction-motor has a rigid outer portion (stator) and a rotating inner portion (rotor) that rotate inside with a cautiously engineered air gap between the two. Nearly all electrical motors make use of magnetic field rotation to spin their rotors.
A three phase induction-motor is the only type where the rotating magnetic field is formed naturally in the stator because of the nature of the supply. DC motors depend either on mechanical or electronic commutation to form rotating magnetic fields. A single-phase induction-motor depends on extra electrical components to produce this rotating magnetic field.
In any motor two pairs of electromagnets are formed. In an induction-motor, one pair is created in the stator due to AC-supply (connected to the stator windings). An EMF is induced in the rotor (just like the voltage is induced in the transformer secondary) due to alternating character of the supply voltage induces as per Lenz’s law, hence generating another set of electromagnets; for this reason the name induction-motor. Interaction between the magnetic fields of these electromagnets generates twisting force (or torque). So the motor rotates in the direction of the resultant torque.
Construction of Stator in Induction Motor
- Stator consists of numerous thin lamination (of aluminum or cast iron) that are punched & clamped together as shown in the below figure.
- It is a vacant cylinder with slots & coils (of insulated wires) are inserted in these slots. All combinations of coils collectively with the core they surround form a pair of poles when ac-supply is applied.
- Number of poles of an induction-motor depends on the internal connection of the stator windings. The stator-windings are connected directly to the power source. Internally they are connected in such a manner, that on applying AC supply, a rotating magnetic field is created.
Construction of Rotor in Induction Motor
- Rotor is made up of numerous thin steel- lamination with evenly spaced bars (made up of aluminum or copper). In squirrel cage rotor, these bars are joined at ends (mechanically and electrically) through rings.
- Almost ninety percent of induction motors have squirrel-cage rotors. Ŕotor has a simple, rugged structure and consists of a cylindrical core with (axially placed) parallel slots. These slots are used for carrying the conductors.Every slot has a copper, aluminum, or alloy bar.
- Using end rings both ends of these bars are permanently short-circuited, as shown in Figure 2.
- This total assembly resembles the look of a squirrel cage, which gives the rotor its name.
- Rotor slots are not exactly parallel to the shaft. They are given a skew for two main reasons:
- The first reason is to make the motor run quietly by reducing magnetic hum and to decrease slot harmonics.
- The second reason is to help reduce the locking tendency of the rotor. The rotor teeth tend to remain locked under the stator teeth due to direct magnetic attraction between the two. This happens when the number of stator teeth is equal to the number of rotor teeth.
- Using bearings on each end the rotor is mounted on the shaft; For driving the load usually one end of the shaft is kept longer than the other.
- Some motors may have an accessory shaft on the non-driving end for mounting speed or position sensing devices.
- Between the stator and the rotor, there exists an air gap, through which due to induction, the energy is transferred from the stator to the rotor.
- The generated torque forces the rotor and then the load to rotate. Regardless of the type of rotor used, the principle employed for rotation remains the same.