Three Phase Induction Motor(What You Should Know)

Three three-phase induction motor is the most popular of all motor type because, it has a high power factor, which means high efficiency, and they are self-started no capacitor is needed.

A 3-phase induction motor is an electric motor that is designed to work on a 3-hase power supply and needs no starting capacitor like a 1-phase induction motor.

Induction Motor Parts

It’s important to understand the construction of a 3-phase induction motor to get a good understanding of its principle of working

This motor consists of two main parts

  • Motor stator
  • Motor rotor

3-Phase Induction Motor Stator

The stator is made of slots to construct a winding circuit which is connected to a phase electricity source.

The three-phase winding is arranged to produce a rotating magnetic field when connected to a power source.

Stator of 3 phase induction motor
The stator of the 3-phase induction motor

The rotor of the induction motor

Rotor is the rotating part of the motor, that is connected to the mechanical load.

It’s a cylindrical laminated core with parallel slots that can carry conductors.

The conductors are short-circuited by end rings and are made of copper or aluminum fitted in the slots.

Induction Motor Working Principle

A three-phase induction motor operates on the principle of electromagnetic induction, which was discovered by Michael Faraday. The working principle of a three-phase induction motor can be explained as follows:

  1. Stator: The motor has a stationary part called the stator. In the stator, there are typically three sets of windings, each physically separated by 120 degrees, and connected to a three-phase AC power supply.

  2. Rotor: Inside the motor, there is a rotating part called the rotor. The rotor can be of two main types: a squirrel-cage rotor or a wound rotor.

    • Squirrel-Cage Rotor: This type of rotor consists of laminated iron cores with conductive bars (usually made of aluminum or copper) embedded in them. These conductive bars are short-circuited at each end by end rings, forming a closed loop resembling a squirrel cage.

    • Wound Rotor: In a wound rotor, the rotor windings are made of wire coils. These windings can have variable resistance, allowing for external control of the rotor’s characteristics.

  3. Operation:

    a. Start: When three-phase AC power is applied to the stator windings, a rotating magnetic field is generated. The rotating magnetic field is created by the phase difference between the currents in the three stator windings. This magnetic field rotates at a synchronous speed determined by the supply frequency and the number of pole pairs in the motor.

    b. Induction: The rotating magnetic field induces voltage in the rotor windings (either the squirrel-cage or wound rotor). According to Faraday’s law of electromagnetic induction, a changing magnetic field induces a current in a nearby conductor. As the rotor conductors cut through the changing magnetic field, they have voltage induced in them.

    c. Rotor Motion: The induced voltage in the rotor windings causes a current to flow. This current, in turn, generates its own magnetic field, which interacts with the rotating magnetic field from the stator. The interaction of these magnetic fields creates a torque on the rotor, causing it to start rotating.

    d. Synchronization: The rotor rotates at a speed slightly less than the synchronous speed of the rotating magnetic field. This speed difference is called slip. The rotor must operate at a slip to generate torque and perform mechanical work. The higher the load on the motor, the greater the slip and the greater the torque produced.

  4. Operation at Rated Load: When the motor is operating at its rated load, it continues to run with the rotor slipping slightly behind the rotating magnetic field. This slip ensures that the rotor experiences a sufficient relative speed difference to generate the torque needed for the motor to perform mechanical work efficiently.

In summary, a three-phase induction motor operates by creating a rotating magnetic field in the stator, which induces current and generates a magnetic field in the rotor.

The interaction of these magnetic fields produces a torque that drives the rotor, allowing the motor to perform mechanical work.

The amount of slip between the rotor speed and the synchronous speed determines the motor’s performance and efficiency under different load conditions.

3 Phase Induction Motor Advantages

  • One of the important advantages of this motor is that it is self-starting. i.e. needs no starting capacitor like single phase induction motor.
  • Good speed regulation.
  • High full load efficiency (up to 97%).
  • Unlike the single-phase motor, a 3 phase motor power factor can reach up to 0.9
  • Easy rotation direction reversing, unlike 1 phase motor, 3 phase motor rotation direction can be controlled just by reversing two phases of the power source.


Although this motor type is the most common and widely used in almost all industries and applications, it has the following disadvantages:

  • High starting current, causes voltage drop and affects other loads on the same power supply, so this motor requires a starting method to overcome this problem.

I highly recommend Reading My Articles, Star Delta Starter of Motor.

  • Difficulty speed control of the induction motor is one of the disadvantages because it is a constant speed motor.
  • Poor starting torque is another disadvantage. This makes it not suitable for applications that require high starting torque.


Three-phase induction motors are widely used in various industrial and commercial applications due to their reliability, efficiency, and robustness. Here are some common applications of three-phase induction motors:

  1. Industrial Machinery: Three-phase induction motors are used in a wide range of industrial machinery, including conveyor belts, compressors, pumps, fans, agitators, and mixers. They provide the necessary power and torque to drive these machines efficiently.

  2. Electric Generators: Three-phase induction motors can be used as generators in applications where mechanical energy needs to be converted into electrical energy. This is common in wind turbines and some hydroelectric power plants.

  3. Air Conditioning and HVAC Systems: Many commercial and residential air conditioning and heating systems use three-phase induction motors to drive the fans and compressors. They offer efficient and reliable operation, especially in larger systems.

  4. Oil and Gas Industry: Three-phase induction motors are used in the oil and gas industry to power various equipment such as pumps, compressors, and drilling rigs. They are well-suited for harsh environments.

  5. Machine Tools: In manufacturing and machining processes, induction motors power machine tools like lathes, milling machines, and grinders. They provide precise control over the rotational speed and torque.

  6. Transportation: Electric trains, trams, and electric buses often use three-phase induction motors for propulsion. These motors offer high torque at low speeds, making them suitable for these applications.

  7. Mining Industry: Three-phase induction motors are used in mining equipment such as crushers, conveyors, and hoists. They can withstand the rugged and demanding conditions often found in mining operations.

  8. Water and Wastewater Treatment: Pumps and blowers used in water treatment and wastewater treatment plants are often driven by three-phase induction motors. They can handle varying load conditions efficiently.

  9. Paper and Textile Industry: Three-phase induction motors are used in machines for paper and textile manufacturing, including printers, winding machines, and looms.

  10. Agriculture: In agricultural applications, these motors are used in irrigation pumps, grain conveyors, and other machinery used in farming operations.

  11. Food Processing: Food processing equipment, such as mixers, grinders, and conveyors, often rely on three-phase induction motors for their continuous and reliable operation.

  12. Renewable Energy: In renewable energy systems like wind turbines, three-phase induction generators are used to convert mechanical energy from the wind into electrical energy.

  13. Elevators: Many elevators and escalators use three-phase induction motors for smooth and reliable vertical transportation.

  14. Automotive Industry: Some manufacturing processes in the automotive industry use three-phase induction motors for tasks like welding and assembly.

  15. Chemical and Petrochemical Industry: Equipment in the chemical and petrochemical industry, such as agitators, pumps, and compressors, often rely on three-phase induction motors.

In all of these applications, three-phase induction motors are favored for their simplicity, durability, and ability to handle a wide range of load conditions.

They play a crucial role in various industries, contributing to the efficient and reliable operation of machinery and equipment.

Motor Speed Control and Formula

The speed formula of this motor is N = Ns(1-S),

while Ns = (120 f)/P

Where :

  • Ns is the synchronous speed.
  • f is the frequency.
  • P is the number of poles of the motor.

controlling the motor speed can be done by one of the following methods

  • Frequency control or (V / f) control.
  • Number of poles Changing
  • Voltage Controlling.

Read My detailed article: Electric Motor Speed Formula, Calculator and Control

Motor Current Calculator

The current formula for a 3-phase induction motor is, I = P/(1.73*V*pf)


I: Motor current

P: Motor power in Watt

V: Motor operating voltage

pf: Motor power factor

You can find the motor’s data, voltage, power, and pf, on the motor nameplate. I have written a detailed article about Motor Name Plate, You Can Find It here, for more information.

If we don’t know the power factor of the motor, we assume it’s 0.85

I designed my Android app to be easy and help you so, no matter what motor power you have KW or HP, the app converts it to watt internally.

What Causes An Electric Motor to Start Slow?

There are different factors that affect the speed of the motor and cause it to slow down.  It is very important to know about these factors. As it will badly affect the performance of the motor. So we are discussing each factor in detail.

  • Electrical Load: Load is one of the main factors that slow down the motor. If an electrical load attached to the motor is greater than the rated load of the motor. It will slow down the motor. Therefore, it is important to keep in mind, to always attach a load to a motor that is less than the rated load of the motor. The phenomenon behind the electrical overload is that it will require more current to flow across the windings of the motor. Which will exceed the safety limit of an electric motor.
  • Low supply voltage: A supply voltage is a source for operating an electric motor. If the supply voltage is less than the rated input voltage, it will not produce the amount of enough current to produce the required torque in the rotor. While the motor will try to maintain its rated speed.  As a result, the motor will start to slow.
  • Low resistance: Low resistance causes degradation of insulation flux. As a result, it will cause overheating and flux leakage and the motor will start slow. It is worth mentioning to discuss overheating. Overheating is the main cause of motor failure and slowdown. Keep in mind that with every 10οC temperature increase, the insulation life decreases by 50%.
  • Motor installation: The speed of the motor also depends upon the installation of the motor. If a motor is installed on an uneven surface, it will result in vibrations. Similarly, loose bearing or any other issue can cause vibrations. As a result, the motor slows down.

What Happens If An Electric Motor Loses Power Under Load?

Three Phase Loss:

Electric motors are designed to rotate freely with the help of bearings. This makes it safe to rotate in case of power loss (i.e. 3 phase power loss).

The motor keeps rotating with the load connected. Its rotating speed decreases gradually to zero. Power is a combination of electric current and voltage. In case, power is cut off or an electric motor loses power under load.

It means that the amount of induced EMF or flux will reduce in the insulation winding. As a result, the motor will stop.

Single Phase Loss:

In case of phase loss (i.e. one phase loss) while the motor is under load, The motor will continue running at a lower speed and the current in the remaining phases will increase. If the motor has no phase loss protection, the increased current will burn the winding.

Do Electric Motors Slow Down With Age?

Yes, electric motors can slow down and experience a decrease in performance as they age. Several factors contribute to the aging and degradation of electric motors:

  1. Wear and Tear: Over time, the moving parts of an electric motor, such as bearings and brushes (in brushed motors), can wear out. This wear and tear can lead to increased friction and reduced efficiency, causing the motor to slow down.
  2. Insulation Degradation: The insulation materials used in the motor windings can degrade over time due to factors like heat, moisture, and electrical stress. This can lead to short circuits or reduced insulation resistance, affecting motor performance.
  3. Dirt and Contaminants: Dust, dirt, and contaminants can accumulate inside the motor, particularly in industrial environments. These foreign particles can interfere with the motor’s operation, leading to increased resistance and a slowdown in performance.
  4. Overheating: Continuous operation at elevated temperatures can cause thermal stress on the motor’s components, including the winding insulation and the bearings. Overheating can accelerate wear and reduce motor efficiency, potentially causing it to slow down.
  5. Electromagnetic Aging: Electrical stress, such as voltage spikes or unbalanced voltages, can lead to electromagnetic aging in the motor. This can affect the magnetic properties of the motor, leading to reduced performance.
  6. Lubrication Issues: In motors with bearings, inadequate or degraded lubrication can result in increased friction and wear on the bearings. This can cause the motor to slow down or exhibit increased noise during operation.
  7. Corrosion: In corrosive environments, the motor’s components can corrode over time. Corrosion can affect the mechanical and electrical integrity of the motor, leading to performance degradation.
  8. Load Variations: Motors subjected to frequent load variations or operating conditions beyond their design specifications may experience accelerated wear and reduced performance.

To mitigate these issues and prolong the life of electric motors, regular maintenance and proper operating conditions are essential.

Maintenance activities may include cleaning, lubrication, inspection of bearings and windings, and monitoring operating temperatures and vibrations.

Additionally, motors should be operated within their specified voltage, current, and temperature ranges to prevent premature aging and performance degradation.

What is a squirrel cage induction motor?

Squirrel cage induction motor is that AC induction motor type whose function is based on the electromagnetism principle. This motor can be a single or three-phase motor.

Why is it called a squirrel cage? This name of the squirrel cage motor comes from the fact that the rotor of the motor looks like the cage of a squirrel (squirrel-cage rotor).