Three-Phase Induction Motors: Types, Construction, Working & Applications
What is a 3 Phase Induction Motor?
A three-phase induction motor (3-phase IM) is a type of AC electric motor that operates on a three-phase alternating current supply. These motors are the workhorses of the industry, widely used in various applications due to their robust construction and efficiency. They are classified into two main types based on their rotor design: the squirrel cage induction motor and the phase-wound (or slip-ring) induction motor.
Construction of a 3-Phase Induction Motor
Just like any other motor, a 3 phase induction motor also consists of a stator and a rotor. Basically there are two types of 3 phase inducton motors - 1. Squirrel cage induction motor and 2. Phase Wound induction motor (slip-ring induction motor). Both types have similar constructed stator, but they differ in construction of rotor. This is explained further below -
Stator construction
The stator of a 3-phase induction motor is a stationary part made of laminated steel stampings with slots. These slots house the three-phase winding, which is connected to a three-phase AC supply. The number of poles in the stator winding is determined by the required motor speed. A higher speed requires a lower number of poles, and vice versa.
When the stator windings are energized by a three-phase supply, they produce a rotating magnetic field that revolves at a constant speed called the synchronous speed (Ns). This speed is calculated using the formula: Ns = (120f) / P where f is the frequency of the supply and P is the number of poles. This rotating magnetic field induces a current in the rotor windings, according to Faraday's law of mutual induction, causing the rotor to spin.
Rotor Construction
The rotor is the rotating part of the motor. As mentioned earlier, there are two primary types of rotors: the squirrel cage rotor and the phase-wound rotor.
Squirrel Cage Rotor
This is the most common type, used in over 90% of induction motors due to its simple and robust construction. The squirrel cage rotor consists of a laminated cylindrical core with parallel, slightly skewed slots. These slots contain heavy bars of copper, aluminum, or alloys that act as rotor conductors.
The rotor bars are short-circuited at both ends by short-circuiting end rings, giving the assembly the appearance of a squirrel cage. The slight skewing of the rotor bars provides several advantages:
- Reduces Magnetic Locking: It minimizes the tendency of the rotor teeth to align with the stator teeth due to magnetic attraction.
- Improves Starting Torque: It enhances the effective transformation ratio between the stator and the rotor.
- Increases Rotor Resistance: The increased conductor length boosts the rotor resistance, which improves starting performance.
Since the rotor bars are permanently short-circuited, it is not possible to add any external resistance to the rotor circuit.
Also Read: Starting Methods of three phase induction motors
Phase-Wound Rotor (or Slip-Ring Rotor)
The phase-wound rotor is wound with a **three-phase, double-layer, distributed winding**. The number of poles in the rotor is kept the same as the number of poles in the stator. The three terminals of the rotor winding are internally connected in a star configuration, and the remaining three terminals are brought out to three insulated slip rings mounted on the motor shaft. Carbon brushes rest on these slip rings and are connected to an external star-connected rheostat. This external resistance is a key feature of the phase-wound motor, allowing for the adjustment of starting torque and speed.
Once the motor reaches its rated speed, the slip rings are automatically short-circuited by a metal collar, and the brushes are lifted to reduce frictional losses.
How Do 3 Phase Induction Motors Work?
Step-by-Step Working Principle
- Rotating Magnetic Field Creation: Three-phase stator windings create a rotating magnetic field at synchronous speed
- Electromagnetic Induction: Rotating field cuts rotor conductors, inducing EMF and currents
- Torque Generation: Interaction between rotor current and rotating field produces torque
- Motor Rotation: Rotor follows the rotating field but at slightly lower speed (slip)
Key Performance Parameters
(where f = frequency, P = poles)
Slip Calculation: S = (Ns - N) / Ns × 100%
- Typical Slip Range: 2-5% at full load
Advantages of 3-Phase Induction Motors
Three-phase induction motors are a popular choice for industrial applications due to their numerous benefits:
- Simple and Rugged Construction: They are built to last, with a durable design that makes them virtually indestructible.
- High Reliability and Low Cost: Their simple design translates to high reliability and lower manufacturing costs.
- High Efficiency: They operate with high efficiency and a good power factor, which is crucial for energy savings.
- Minimal Maintenance: They require very little maintenance, reducing downtime and operational costs.
- Self-Starting Capability: Unlike single-phase motors, a 3-phase induction motor is self-starting, eliminating the need for an external starter or specialized starting arrangements.
Disadvantages of 3-Phase Induction Motors
Despite their many advantages, they do have a few drawbacks:
- Speed-Load Dependency: The motor's speed decreases as the load increases, similar to a DC shunt motor.
- Efficiency Reduction with Speed Control: Controlling the speed of 3 phase induction motor often involves sacrificing some efficiency, which can be a limiting factor in applications requiring variable speeds.
Industrial Applications of 3 Phase Induction Motors
Manufacturing and Industrial
- Conveyor belts and material handling
- Industrial pumps and compressors
- Machine tools and manufacturing equipment
- Textile machinery
HVAC and Building Systems
- Centrifugal fans and blowers
- Air conditioning compressors
- Circulation pumps
- Ventilation systems
Power and Energy
- Water treatment plants
- Power generation auxiliaries
- Oil and gas processing equipment
- Mining operations
FAQ on 3 Phase Induction Motors
- Why is a 3 phase induction motor self-starting?
- Because the 3-phase stator winding produces a rotating magnetic field, which induces current in the rotor and starts rotation automatically.
- What is the difference between squirrel cage and wound rotor motors?
- Squirrel cage motors have permanently short-circuited rotor bars and are maintenance-free, while wound rotor motors have accessible rotor windings through slip rings, allowing external resistance control for variable starting characteristics.
- What is the efficiency of a 3 phase induction motor?
- Typically between 85% – 95%, depending on size and load.
- Can induction motor speed be controlled?
- Yes, but speed control often reduces efficiency. Methods include rotor resistance control, VFDs (Variable Frequency Drives), and pole-changing techniques.