What is a 3-Phase Induction Motor

In this article, we will explore a 3-phase induction motor (3-Phase IM), a type of asynchronous electric motor which converts three-phase input electrical energy into rotational mechanical energy at the output.

Definition of 3-Phase Induction Motor

A 3-phase induction motor is a type of electrical machine which converts three-phase electricity into mechanical energy by utilizing the principle of electromechanical energy conversion.

The 3-phase induction motors are the most common type of AC electric motors used in various industrial applications to drive mechanical loads.

A 3-phase induction motor is also known as an asynchronous electric motor.

Construction of 3-Phase Induction Motor

A three-phase induction motor is a type of rotating electrical machine, having two main parts namely, the stator and rotor.

The stator of a 3-phase induction motor is a stationary part of the machine. In a 3-phase induction motor, the stator is generally the outer part of the motor. The rotor is the internal part of the motor and it is free to rotate on its axis.

Let us now discuss the stator and rotor construction of a three-phase induction motor in detail.

(1). Stator of 3-Phase Induction Motor:

The stator of a 3-phase induction motor is a stationary part of the machine made up of cast iron. It has three major parts namely, the yoke, stator core, and stator winding.

The yoke is the outer cover of the stator and it provides protection against external threats to the internal parts of the machine. The stator core is a hollow cylinder made up of high-grade silicon steel. The stator core has slots cut on its inner periphery. In these stator slots, a three-phase winding made up of copper or aluminium conductors is placed.

(2). Rotor of 3-Phase Induction Motor:

In a three-phase induction motor, the rotor is an internal rotating part of the machine. There are two types of rotors used in a three-phase induction motor namely, squirrel-cage rotor and slip-ring rotor.

Let us discuss each of these types of 3-induction motor rotors individually.

(a). Squirrel-Cage Rotor:

The construction of a squirrel-cage rotor is depicted in the following figure.

This type of induction motor rotor has main parts namely, rotor shaft, conductor bars, and metal end rings.

In this rotor construction, the conductor bars made up of copper or aluminium are short-circuited by using two metal end rings. This whole assembly is mounted on a rotor shaft. It can be seen that the conductor bars are skewed to prevent magnetic cogging.

(b). Slip-Ring Rotor:

The construction of a slip-ring rotor or wound rotor is shown in the following figure.

The slip-ring rotor, also called the wound rotor, has a cylindrical core made up of silicon steel laminations. A number of slots are cut on the outer periphery of the rotor core to accommodate rotor windings.

In this type of rotor, three-phase windings are wound on the rotor core. An assembly of slip-rings and carbon brushes is used to excite the rotor winding, as shown in the figure above.

Working Principle of 3-Phase Induction Motor

A three-phase induction motor works on the principle of electromagnetic induction. According to this principle, when a 3-phase electric supply is given to the induction motor, it produces a rotating magnetic field in the machine. Due to the relative motion between the rotating magnetic field and the stationary rotor, an emf is inducted in the rotor winding, which is given by,

`\e=N (dϕ)/dt`

This induced emf forces an electric current to flow in the rotor conductors, causing a magnetic field to be produced.

Due to the interaction between two magnetic fields, stator magnetic field and rotor magnetic, a torque is developed that rotates the rotor.

This way a 3-phase induction motor works.

Advantages of 3-Phase Induction Motors

A three-phase induction motor has the following major advantages over other types of electric motors:

• Three-phase induction motors are self-starting.
• The 3-phase induction motors have robust construction and higher mechanical strength.
• 3-phase induction motors are relatively less complex to design and construct.
• 3-phase induction motors require less maintenance and hence less maintenance cost and downtime.
• 3-phase induction motors are economical choices for industries.

Disadvantages of 3-Phase Induction Motors

The major disadvantages of 3-phase induction motors are listed below:

• Three-phase induction motors draw a very high initial current (inrush current), typically around 4 to 8 times the normal operating current.
• 3-phase induction motors always have a lagging power factor.
• Under light load conditions, three-phase induction motors operate at a very poor power factor, typically between 0.3 lag and 0.5 lag.
• Three-phase induction motors are less flexible in terms of speed control and require specialized speed control mechanisms.

Applications of 3-Phase Induction Motors

The three-phase induction motors are the most commonly used type of electric motors in industries to drive different kinds of mechanical loads. Some common applications of three-phase induction motors are listed below:

• To drive pumps and submersibles.
• To operate the pressing machine, lathe machine, grinding machine, etc.
• To drive conveyors, hoists and lifts.
• To drive compressors.
• To drive flour mills.

This is all about three-phase induction motors, their construction, and their applications. In conclusion, a three-phase induction motor is the most widely used type of electric motor to drive various types of mechanical loads.