Induction Motor: Working Principles, Basics & Types Explained

Induction motors are the workhorse of modern industry, playing a major role in one-third to one-half of the electrical apparatus used in industries nowadays, either as the prime mover or agent. In this blog, we go over the basics about what an induction motor is, how it works and types of induction motors to get an overall idea about these critical machines. Induction motors are the most common among all electric motors.

What is an Induction Motor? 

These motors rely on the principle of electromagnetic induction, which is used to induce an electric current in the rotor due to the magnetic field created by those windings in the stator. As we know, basic two main parts to the construction of an induction motor:

1. Stator: The fixed half of the machine, including the coils that generate a rotating magnetic field when an alternating current (AC) is applied.
2. Rotor: This is the rotating part housed inside the stator. The rotor may be squirrel cage type or wound type.

The stator magnetic field interacts with induced currents in the rotor to generate shaft torque via a corresponding rearward-rotating magnetic field in the rotor.

How Does Induction Motor Work? 

The operation of an induction motor is based on Faraday’s law of electromagnetic induction and Lenz's law. Here's a step-by-step breakdown of the working principle:

1. AC Supply to Stator: When an AC voltage is applied to the stator windings, a rotating magnetic field (RMF) around the stator is created. This field rotates at a synchronous speed (Ns), which is determined by the frequency of the AC supply and the number of poles in the motor.
2. Induction in the Rotor: The rotating magnetic field passes through the air gap and cuts the rotor conductors. According to Faraday’s law, this induces an electromotive force (EMF) in the rotor conductors. Since the rotor is a closed circuit, currents are induced in the rotor.
3. Production of Torque: The induced currents in the rotor produce their own magnetic field, which interacts with the stator's rotating magnetic field. According to Lenz's law, the rotor will start to rotate in the direction of the rotating magnetic field to reduce the relative motion. This interaction between the two fields generates a torque that causes the rotor to turn.
4. Asynchronous Operation: The rotor never reaches synchronous speed (the speed of the stator’s rotating magnetic field) because, at synchronous speed, there would be no relative motion between the rotor and the magnetic field, and thus no induced EMF or current in the rotor. The difference between synchronous speed and rotor speed is called slip.

Types of Induction Motor

Based on the rotor type and the number of phases, there are the following types of induction motor. 

Based on Rotor Type

• Squirrel Cage Induction Motor: This type of induction motor has a rotor made of conductive bars shorted at both ends by end rings, forming a cage-like structure. It is the most common type due to its simple and rugged construction, low cost, and reliable operation. However, it has limited starting torque and speed control capabilities.
• Wound Rotor Induction Motor: This type has a rotor winding similar to the stator winding and is connected to external resistors or controllers via slip rings. This configuration allows for better control of starting torque and speed, making it suitable for applications requiring variable speed and high starting torque. However, it is more expensive and requires more maintenance.

Based on the Number of Phases

• Single-Phase Induction Motor: These motors are used in residential and small commercial applications where three-phase power is not available. They are typically used in household appliances, fans, and small machinery. Single-phase motors require additional starting mechanisms, such as capacitors or auxiliary windings, because they do not produce a rotating magnetic field at startup.
• Three-Phase Induction Motor: These are the most commonly used motors in industrial and commercial applications due to their efficiency, reliability, and ability to produce a self-starting rotating magnetic field. Three-phase motors do not require any external starting mechanism and can handle higher power loads.

Advantages of Induction Motor

Some of the advantages of induction motors are listed below. 

• Simplicity and Ruggedness: With fewer moving parts, induction motors are less prone to mechanical failures and require minimal maintenance.
• Cost-Effective: They are generally cheaper to manufacture and maintain compared to other types of motors.
• High Efficiency: Induction motors are highly efficient, especially at full load, which translates to energy savings.
• Wide Range of Applications: They are versatile and can be used in a variety of environments, from industrial settings to household appliances.

Disadvantages of Induction Motor

While induction motors are widely used due to their numerous advantages, they also come with certain disadvantages. Here are five notable disadvantages of induction motors:

• Starting Torque Limitations: Induction motors, especially squirrel cage types, often have lower starting torque compared to other types of motors. This can make them unsuitable for applications requiring high starting torque, such as cranes or heavy machinery, without additional starting mechanisms like capacitors or special starters.
• Poor Speed Control: Speed control of induction motors is less straightforward compared to DC motors. While methods like Variable Frequency Drives (VFDs) exist, they add complexity and cost to the system. Simple speed control methods like voltage variation are inefficient and not very effective, especially for precision applications.
• High Inrush Current: Induction motors typically draw a high inrush current when starting, which can be several times their full-load current. This can cause voltage drops in the power supply system and necessitate the use of special starting methods (e.g., star-delta starters, soft starters) to mitigate the impact.
• Lower Power Factor: Induction motors often operate at a lower power factor, particularly at partial loads. A lower power factor results in higher apparent power consumption, which can increase the demand charges on an electrical system. Power factor correction devices may be required to compensate for this, adding to the overall system cost.
• Dependence on Slip: The speed of an induction motor depends on the slip, which is the difference between the synchronous speed of the magnetic field and the actual rotor speed. This dependence on slip means that the motor's speed cannot be precisely controlled and varies with the load. Under varying load conditions, maintaining a consistent speed can be challenging without advanced control systems.

Application of Induction Motor

Induction motors are employed in numerous applications due to their robustness and efficiency. Here are some common uses:

• Industrial Drives: Induction motors power conveyors, pumps, compressors, and other industrial machinery.
• HVAC Systems: They are used in heating, ventilation, and air conditioning systems to drive fans and blowers.
• Household Appliances: Washing machines, refrigerators, air conditioners, and other household appliances often use single-phase induction motors.
• Electric Vehicles: Induction motors are increasingly used in electric vehicles for their efficiency and reliability.

Examples of Induction Motor

• Conveyors and Material Handling Systems
• Pumps and Compressors
• Fans and Blowers
• Machine Tools
• Cranes and Hoists
• Household Appliances
• Kitchen Appliances
• Vacuum Cleaners
• Fans
• Elevators and Escalators
• Industrial HVAC Systems
• Electric Vehicles

Read Also:  FEATURES AND ADVANTAGES OF USING ELECTRONIC SOFT STARTERS FOR A MOTOR STARTING

Conclusion

Induction motors, with their simple construction and reliable operation, are indispensable in both industrial and domestic applications. Understanding how induction motors work, their types, and various aspects such as starting methods and speed control can help in selecting the right motor for specific needs and ensuring optimal performance. As technology advances, induction motors continue to evolve, becoming more efficient and versatile, solidifying their role as a cornerstone of modern electrical engineering. Browse Schneider eShop to find the best induction motors of various types.