Explain the following terms as applied to induction motors: - Squirrel cage rotor - Wound rotor - Slip - Rotor frequency

Squirrel cage rotor: A squirrel cage rotor is a type of rotor commonly used in induction motors. It consists of a cylindrical core made of steel laminations with evenly spaced conductive bars or "squirrel cage" conductors placed in slots on the surface of the core. These conductors are typically made of aluminum or copper and are short-circuited at both ends. When the rotor is subjected to a rotating magnetic field produced by the stator, currents are induced in the conductors, creating a magnetic field that interacts with the stator field to produce torque and drive the motor.

Wound rotor: A wound rotor, also known as a slip ring rotor, is another type of rotor used in induction motors. It consists of a laminated core with a three-phase winding connected to slip rings on the shaft. The slip rings allow external resistors to be connected to the rotor winding, enabling control of the motor's speed and torque characteristics. Wound rotors are often used in applications where variable speed control is required.

Slip: Slip is a term used to describe the difference in speed between the rotating magnetic field produced by the stator and the rotor in an induction motor. The rotor speed is always slightly slower than the synchronous speed of the stator field, resulting in slip. Slip is necessary for the induction motor to generate torque and operate efficiently. The amount of slip is typically expressed as a percentage of the synchronous speed.

Rotor frequency: Rotor frequency refers to the frequency of the currents induced in the rotor of an induction motor. The rotor frequency is directly related to the slip of the motor, as it is the difference between the supply frequency and the rotor frequency that creates the torque necessary for the motor to operate. The rotor frequency is typically lower than the supply frequency due to the slip in the motor.

Squirrel Cage Rotor:

A squirrel cage rotor is a type of induction motor rotor that consists of a cylindrical iron core with evenly spaced copper bars embedded and connected at both ends by short-circuiting rings. These bars form a cage-like structure, hence the name "squirrel cage." Squirrel cage rotors are the most common type used in induction motors due to their low cost, simple construction, and durability.

Wound Rotor:

A wound rotor is another type of induction motor rotor. Unlike squirrel cage rotors, wound rotors have a set of coils wound on a laminated iron core. The coils are connected to slip rings mounted on the motor shaft. Brushes are used to make electrical contact with the slip rings, allowing external resistors to be connected to the rotor circuit. This allows for speed control and torque variation.

Slip:

Slip is a measure of the difference between the synchronous speed (Ns) of the rotating magnetic field in the motor and the actual speed (Nr) of the rotor. Slip is expressed as a percentage:

```
Slip = ((Ns - Nr) / Ns) - 100
```

Slip is essential for induction motors to function. It generates the torque that drives the rotor.

Rotor Frequency:

Rotor frequency (fr) is the frequency of the induced current in the rotor conductors. It is related to the slip and the stator frequency (fs) as follows:

```
fr = s - fs
```

Rotor frequency is important for determining the torque and power capabilities of the induction motor.