Here’s a detailed explanation of AC motors, including types, working principles, construction,
characteristics, and applications.
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⚡ What is an AC Motor?
An AC (Alternating Current) motor is an electromechanical device that converts electrical energy
(AC) into mechanical energy using the interaction of a magnetic field and current-carrying
conductors.
It is widely used in household appliances, industries, transportation, fans, compressors, etc.
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🔄 Types of AC Motors
AC motors are broadly divided into two types:
1. Synchronous Motor
Rotor speed = stator magnetic field speed (synchronous speed)
Requires external means to start
Used where constant speed is needed
2. Asynchronous (Induction) Motor
Rotor speed < synchronous speed
Self-starting
Most common in industrial use
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⚙️ Construction of AC Motor
a) Stator
Stationary part
Has three-phase windings (in 3-phase motors) or a main + auxiliary winding (in single-phase
motors)
Creates a rotating magnetic field (RMF)
b) Rotor
Rotating part
Two main types:
Squirrel Cage Rotor (simple, rugged, used in most induction motors)
Wound Rotor (used in slip ring motors)
c) Housing, Bearings, Fan
Encloses the motor, supports rotation, and helps cooling
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⚡ Working Principle of AC Motors
For Induction Motors:
It works on Faraday’s Law and Lenz’s Law:
1. AC supply is given to the stator winding → produces a rotating magnetic field.
2. The magnetic field cuts the rotor conductors, inducing EMF (Faraday’s Law).
3. The induced current in the rotor produces its own magnetic field, which opposes the cause
(Lenz’s Law).
4. This interaction produces torque, causing the rotor to rotate in the direction of RMF.
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🔄 Synchronous Speed and Slip
Synchronous Speed (Ns):
N_s = \frac{120f}{P}
Where:
= frequency of supply (Hz)
= number of poles
Slip (S) (for induction motors):
S = \frac{N_s - N_r}{N_s} \times 100\%
Where = rotor speed.
Synchronous Motor: Slip = 0
Induction Motor: Slip ≠ 0
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🌀 Types of AC Motors (More Detailed)
1. Single-Phase AC Motors
Run on standard household supply (230V)
Not self-starting without an auxiliary mechanism
Types:
Split Phase
Capacitor Start / Capacitor Start-Run
Shaded Pole
2. Three-Phase AC Motors
Used in industry (415V supply in India)
Types:
Squirrel Cage Induction Motor
Slip Ring Induction Motor
Synchronous Motor
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📉 Characteristics of AC Motors
Motor Type Starting Torque Speed Control Maintenance Applications
Squirrel Cage Medium Difficult Low Fans, pumps, blowers
Slip Ring High Easy Medium Hoists, cranes
Synchronous Low (needs starter) Good Medium Constant speed apps
Single-phase Low to Medium Limited Low Domestic appliances
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🧰 Applications of AC Motors
🔹 Synchronous Motors
Used in power factor correction, constant speed machinery, robotics, etc.
🔹 Induction Motors
Squirrel Cage: Fans, blowers, washing machines, compressors
Slip Ring: Elevators, cranes, high starting torque equipment
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✅ Advantages of AC Motors
Simple and rugged construction
Low cost and maintenance
Wide range of power ratings
Used for both constant and variable speed applications
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❌ Disadvantages
Speed control (especially in squirrel cage induction motors) is complex
Single-phase motors are not self-starting
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🎯 Summary Table
Feature Synchronous Motor Induction Motor
Speed Constant Slightly less than sync.
Starting Mechanism Needs extra supply Self-starting
Applications Constant speed General industrial use
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If you'd like, I can add diagrams for:
Construction of induction motors
Rotating magnetic field
Torque-slip characteristics
Just let me know!
