Asynchronous vs. Synchronous Motors: Construction, Function & Use Cases

Asynchronous (Induction) and Synchronous motors are the two main types of AC motors used in industrial, commercial, and household applications. Both share a similar stator-rotor construction, but they differ significantly in operating principles, efficiency, and application areas.

This article explores the key differences between these motors, their advantages, and where each type is best suited.

🔹 What Are Asynchronous and Synchronous Motors?

Both motor types convert electrical energy into mechanical motion using electromagnetic fields. However, their rotational speed and interaction with the stator's magnetic field set them apart.

✔️ Asynchronous (Induction) Motor

📌 Rotor speed is lower than the stator’s rotating magnetic field (RMF).
📌 Operates based on electromagnetic induction without external excitation.
📌 Commonly used in fans, pumps, conveyors, and compressors.

✔️ Synchronous Motor

📌 Rotor speed is equal to the RMF speed (zero slip).
📌 Requires external DC excitation for its rotor field.
📌 Used in high-precision applications like industrial drives and power factor correction.

📌 Simply put, asynchronous motors work without an external power source for the rotor, while synchronous motors require one.

1️⃣ Key Structural Differences

Both motor types consist of:
✔ Stator – A stationary part containing copper windings that generate a rotating magnetic field.
✔ Rotor – A rotating component interacting with the stator's field to produce motion.

However, their rotor designs and operation differ significantly.

✔️ Asynchronous Motor Rotor Types

1️⃣ Squirrel Cage Rotor
✔ Made of conductive bars shorted at both ends
✔ Simple, reliable, and widely used in most AC motors

2️⃣ Wound Rotor (Slip Ring Rotor)
✔ Uses external resistors to control torque and speed
✔ Found in industrial applications needing controlled startups

📌 Most asynchronous motors use the squirrel cage rotor because of its simplicity and durability.

✔️ Synchronous Motor Rotor Types

1️⃣ Electromagnetic Rotor
✔ Requires external DC excitation to generate its magnetic field
✔ Uses slip rings and brushes to supply current

2️⃣ Permanent Magnet Rotor
✔ Uses pre-installed magnets instead of electromagnets
✔ Found in modern servo motors, industrial automation, and wind turbines

📌 Synchronous motors need external excitation to function, unlike induction motors that operate on electromagnetic induction.

2️⃣ Working Principle: How Each Motor Operates

✔️ How an Asynchronous (Induction) Motor Works

1️⃣ When AC voltage is applied to the stator, it generates a rotating magnetic field (RMF).
2️⃣ This RMF induces current in the rotor, creating its own magnetic field.
3️⃣ The rotor experiences a force due to electromagnetic interaction, causing it to rotate.
4️⃣ Rotor speed is always lower than the RMF speed due to slip, which is necessary for induction to occur.

📌 Induction motors are self-starting and widely used due to their simplicity.

✔️ How a Synchronous Motor Works

1️⃣ The stator generates a rotating magnetic field like in an induction motor.
2️⃣ The rotor is powered by DC excitation from an external source, creating its own steady magnetic field.
3️⃣ The rotor locks onto the RMF and spins at exactly the same speed, resulting in zero slip.
4️⃣ Synchronous motors require an external starter mechanism (e.g., damper windings or a separate motor) to reach synchronous speed.

📌 Synchronous motors offer precise speed control but require additional components to operate.

3️⃣ Efficiency & Performance Comparison

📌 Asynchronous motors are cheaper and easier to maintain, while synchronous motors offer precise speed control and power factor correction.

4️⃣ Applications: Where Each Motor is Used

✔️ Where Asynchronous Motors Are Used

✅ Fans, Pumps, & Compressors – Low cost, reliable operation
✅ Conveyor Belts & Elevators – Rugged design for continuous use
✅ HVAC Systems – Efficient air circulation motors
✅ Household Appliances – Washing machines, refrigerators, and air conditioners

📌 Induction motors are ideal for applications requiring durability and minimal maintenance.

✔️ Where Synchronous Motors Are Used

✅ Industrial Automation & Robotics – Precise speed control
✅ Power Plants & Grid Stabilization – Used for power factor correction
✅ Mills, Crushers, & Large Industrial Equipment – High torque applications
✅ Electric Vehicles & Wind Turbines – Permanent magnet synchronous motors (PMSM)

📌 Synchronous motors are best for high-precision, high-efficiency applications.

5️⃣ Key Takeaways: Choosing the Right Motor

✔️ When to Choose an Asynchronous Motor

✔ If cost-effectiveness and simplicity are priorities
✔ If variable speed isn’t critical
✔ If low maintenance and durability are needed

📌 Perfect for general-purpose industrial and household applications.

✔️ When to Choose a Synchronous Motor

✔ If precise speed control is required
✔ If energy efficiency and power factor correction matter
✔ If used in heavy industrial loads or renewable energy systems

📌 Ideal for high-performance industrial applications.

6️⃣ Conclusion: Which Motor is Better?

Both asynchronous and synchronous motors have their strengths and weaknesses.

✅ For general industrial applications, induction motors remain the go-to choice due to their simplicity, affordability, and reliability.

✅ For specialized applications requiring exact speed control, synchronous motors offer superior performance.

🚀 As industries move toward energy efficiency, synchronous motors (especially permanent magnet types) are gaining traction in modern automation and electric vehicle applications.