Types Of AC Motors

Alternating Current (AC) motors are among the most widely used devices in both everyday household appliances and industrial machines. They transform electrical energy from an alternating current supply into mechanical work, forming the backbone of modern technology systems in homes, industries, and laboratories. Efficient, reliable, and versatile, AC motors can be classified into several types based on their construction and operating principles.

Basic Working Principle of AC Motors

AC motors operate primarily on the principle of electromagnetic induction. When alternating current flows through the stator windings, a rotating magnetic field is created. This changing magnetic field induces a current in the rotor, producing another magnetic field. The interaction of these fields generates a torque, which causes the rotor to turn and deliver rotational motion to connected machinery.

Main Types and Construction of AC Motors

The two broad categories of AC motors include induction motors (asynchronous) and synchronous motors. These are further divided based on phase and rotor construction. The stator—formed from laminated steel—houses the windings and generates the rotating magnetic field, while the rotor, mounted on the shaft, reacts to this field and rotates.

Below is a summary table of the key types of AC motors, highlighting their main features and applications:

Type of AC Motor	Main Feature	Application Example
Single-phase induction motor	Operates with single-phase supply, moderate starting torque	Fans, washing machines, small pumps
Three-phase induction motor	Self-starting, high efficiency, robust performance	Industrial conveyors, elevators, compressors
Synchronous motor	Runs at constant speed, needs external starting method	Power factor correction, mills
Universal motor	Can run on both AC and DC, compact size	Portable tools, kitchen appliances

Step-by-Step Approach: Example Problem

Example: A three-phase induction motor is supplied with a synchronous speed of 1500 rpm and operates with a slip of 4%. What is the actual speed of the rotor?

1. Write the formula: Actual Speed = Synchronous Speed × (1 − Slip)
2. Convert slip to decimal: 4% = 0.04
3. Actual Speed = 1500 × (1 − 0.04) = 1500 × 0.96 = 1440 rpm

This demonstrates how slip affects the operational speed of an induction motor and shows the calculation process step by step.

Key Formulas for AC Motors

Knowing key formulas is essential for solving typical exam and real-world problems. These are fundamental in AC motor analysis:

Formula Name	Equation	Description
Synchronous Speed	Ns = (120 × f) / P	f = frequency (Hz), P = number of poles
Slip	s = (Ns − N) / Ns × 100%	Difference between synchronous and actual rotor speed
Torque	T = (P × 60) / (2πN)	P = Power (W), N = Speed (rpm)

Construction Features: Stator and Rotor Details

The stator is made from a steel frame with a core of laminated silicon steel to minimize energy loss. The core features numerous slots along its periphery for coil windings. The rotor, mounted on the motor shaft, comes in two main types: squirrel-cage and phase-wound (slip-ring). Squirrel-cage rotors employ metallic bars shorted by end rings, providing simplicity and robustness.

Phase-wound rotors have windings led to slip rings, allowing external resistance to be connected during starting for better torque control. This makes them suitable for high-torque requirements and variable speed applications.

Comparing AC and DC Motors

Key differences between AC and DC motors are summarized below for clear distinction:

Feature	AC Motor	DC Motor
Current Type	Alternating Current	Direct Current
Construction	Brushless, simpler rotor design	Commutator and brushes used
Speed Control	Speed varies with frequency or load	Speed controlled by voltage variation
Typical Applications	Household, industry	Battery vehicles, toys

Practice and Next Steps

• Revise the classification and working of AC motors regularly for strong fundamentals.
• Practice essential formulas and attempt application-based questions for better retention.
• Review key construction features and differences between types of motors using tables for clarity.

Summary and Scoring Advice

• Focus on the structure and functions of the stator and rotor for diagram-based questions.
• Understand where to apply each formula, especially slip and synchronous speed.
• Relate each motor type to practical examples—single-phase motors for home appliances, three-phase for industries.
• For exam preparation, practice problems requiring calculations of speed, torque, and slip.

For deeper learning and more examples, explore related resources and question banks on Vedantu.