Skip to content

Three-phase Induction Motor

July 12, 2020
Three-phase Induction Motor

On this topic, I will explain in detail the three-phase induction motor’s construction, before we talked about parts of the electric motor generally. However, on this topic, we will go deeper only on the three-phase induction motor’s construction.

Nikola Tesla invented the three-phase induction motor in 1886.

They are considered to be the most widespread in the industry today. In America, more than 50 million of these species are collected by 150 million horsepower. It is estimated to increase by one million motors every year.

Three-phase Induction Motor

The widespread of the induction motor is due to its advantages such as durability, the simplicity of installation, low price compared to other electric motors, and lack of maintenance.

Induction motor also has capabilities of horsepower ranging from a part to more than 10,000.

It does not need any means of excitation in addition to its high efficiency.

The advantages of operating it are excellent and satisfactory for a business that does not require a change in speed.

The disadvantages of three-phase induction motors are that; it is not easy to control its speed. Also, the starting current of this engine is six to eight times full load as well as the low power factor at light loads.

But we find that the advantages of this motor outweigh its defects in most industrial applications. Additionally, electronic controls and drives eassily overcome these defects.

Three-phase Induction Motor Construction

The three-phase induction motor has two main components, such as any other motor: The stator and the Rotor.

The Stator

The stator is composed of magnetic strips of magnetic iron. They range from 0.3 mm to 0.6 mm in size according to the size of the induction motor. An electric insulator isolates them from each other. Together they form a cylindrical object with a number of sewers inside in order to install the winding.

The aim of manufacturing the stator with this method is to reduce the iron heat generated by the eddy current. This is a result of the iron exposed to the variable magnetic field within the motor.

Large motors usually have ventilation holes between the slides in the stator.

After completing the stator’s manufactured in this way, we divide it into the required number of electrodes. Furthermore, we divide the sewage in all poles into three phases. Then we install the windings per each phase in the sewer under each pole. This is to separate each phase and the last 120 degrees electric.

At the end of the winding process, we’ll have three phases installed in the fixed member of each winding. The stator will feed these terminals after connecting them to either as a star or delta. This is the delta connection for three-phase induction motors and star connection for three-phase induction motors.

The Rotor

There are two different types of structure and similar in electrical properties.

We usually call the three-phase induction motor according to the type of its rotor to distinguish between the two types:

  • Wound rotor type
  • Squirrel cage rotor type
Wound Rotor Type

The rotor composes of concentric strips of magnetic iron isolated from each other, mounted on the motor’s shaft with many sewers to install on the winding.

The rotor divides into a number of poles. These are equal to the poles of the stator.

The sewer groups into three sections. Each section fits with the winding on one of the three sides between each phase and the last 120 degrees of electricity.

Usually, these three windings come in the shape of a star with the rotor shortening together the three limbs. The other three that connect to three sliding rings mounted on the same shaft through the sliding rings and there we use carbon brushes for sliding rings during rotation.

We connect the rotor coil to the outside of the motor to control the start of the motor’s rotation. Or to regulate its speed by connecting external resistors to these three sides.

Therefore, this type of induction motor has the potential to change the characteristics of its operation on a large scale by connecting the windings rotor through an external circuit.

Squirrel Cage Rotor Type

The squirrel cage rotor is very similar to the rotor in the wound rotor type in terms of mechanical structures. We use copper or aluminum rods, instead of placing windings in the sewer. There are two rings that shorten the ends of these bars.

This type doesn’t divide into a number of poles. However, it can automatically adapt to the number of poles and phases of the stator in which it will be installed.

Since it does not have slip rings, an external circuit cannot be connected. The speed can’t change or control its properties.