How Does a Three-Phase Squirrel Cage Induction Motor Work?
A 3-phase squirrel cage induction motor is composed of four parts: a stator core, a rotor shaft, and end shields that house the bearings. The stator core is pressed into the frame while the end shields are bolted to the motor’s cast steel frame. The rotor shaft is supported by sleeve or ball bearings. This cutaway view of the motor illustrates the assembly process.
Transient start-up of a squirrel cage induction motor
A squirrel cage induction motor operates at a base speed and torque, but must attain a steady state through transient start-up. A torque-speed curve visualizes these conditions and identifies the motor operating conditions. Figure 1 depicts the various regions of the torque-speed curve. It also identifies the key conditions for the motor’s transient start-up. The torque-speed curve is not the same for every squirrel cage induction motor.
Unlike induction motors driven by a single-phase AC supply, squirrel cage motors operate on three-phase AC supplies. While single-phase motors require a “shove” to start up, three-phase motors can simulate the rotating field. Because of this difference, it is important to understand how squirrel cage motors transiently start up and maintain torque during this critical time.
Mutual induction principle
The mutual induction principle of squirrel cage induction motors maximizes electromagnetic induction. The rotor bars interact with the EMF produced by the stator, which usually consists of windings of wire carrying AC current. The direction of the AC current is changed in sync with a sinusoidal curve, and the resulting EMF follows this same oscillation. The opposing voltage generated by this rotating EMF pushes the rotor around, generating rotation.
The geometry of a squirrel cage induction motor is periodic and regular, resulting in a mutual induction matrix. This symmetry was exploited in the mesh generation strategy by subdividing each region into two. Figure 4.2.1 shows how the regions of the motor are defined. The mutual inductance matrix is then used to calculate the induction properties of each phase. Once the rotor and stator windings are identified, the mutual induction matrix is determined.
Skewing of laminations
The skewed laminations on squirrel cage induction motors have many advantages. They minimize noise and torque fluctuations, and reduce electromagnetic coupling between the rotor and stator. The maximum degree of skewedness is dependent on the design of the motor and the laminations. Skewed laminations also increase the length of copper bars, which increases the resistance. This reduces the starting current and torque.
Moreover, skewing the laminations in an induction motor can increase the speed and torque of the machine. The skewed rotors reduce tooth harmonics, which introduce oscillations in torque. In addition, skewed rotors reduce tooth harmonics, which cause crawling. This technique also decreases magnetic hum and makes the motor run more quietly.
These motors are available in several forms. The three-phase squirrel cage induction motor is one of the most common types. The name of the induction motor comes from the fact that it induces a current inside its rotor cage while operating. In contrast, other motors supply the rotor with current from an external source. They have two parts, a stator and a rotor, and are connected in series with a capacitor arrangement.
Synchronous speed of the field generated by the primary currents
The synchronous speed of the field generated by the primaries in a squirrel cage induction motor is determined by the rotational frequency of the rotor and stator. When the rotor is rotating at 50 Hz, the synchronous speed of the rotor is 2.5 Hz slower. If the synchronous speed is equal to the actual rotor speed, the motor is said to slip.
Induction motors use a three-phase AC power supply to drive their rotors. Single-phase motors require a starting voltage and initial “shove” to get them started, while squirrel cage motors require a three-phase AC supply. While these two types of motors are similar, the first one requires a “shove” to initiate rotation.
The squirrel-cage induction motor’s rotor is made up of steel laminations with copper and aluminum conductors embedded in the surface. The stator is a non-rotating component connected to an alternating current power source. The AC current in the stator changes direction in sync with a sinusoidal curve. As a result, the generated EMF follows the oscillations of the current. The opposing voltage and the rotating EMF interact to produce a torque on the rotor.
Jessica Watson is a PHD holder from the University of Washington. She studied behavior and interaction between squirrels and has presented her research in several wildlife conferences including TWS Annual Conference in Winnipeg.