Synchronous Motors vs Induction Motors

A Comprehensive Comparison of Two Essential Types of Electric Motors

In the world of electric motors, two types stand out for their unique characteristics and applications: Synchronous Motors and Induction Motors. While both are used to convert electrical energy into mechanical energy, they differ significantly in their design, functionality, and usage. In this article, we’ll delve into the differences between Synchronous Motors and Induction Motors, helping you understand which one is best suited for your specific needs.

Synchronous Motors: Precision and Efficiency

Synchronous Motors are a type of AC motor that operates at synchronous speed — meaning the rotor rotates in exact synchrony with the rotating magnetic field of the stator. This constant speed makes synchronous motors ideal for applications requiring precision, high efficiency, and power factor correction.

Key Construction Features:

• Stator Windings (Armature Windings)
  Just like in induction motors, the stator is wound with 3-phase AC coils. These create the rotating magnetic field that drives the motor. Importantly, in most synchronous motors, the stator is also referred to as the armature because it’s where the main AC power interacts.

• Rotor Windings (Field Windings)
  The rotor contains DC-excited windings, supplied either through slip rings or a brushless exciter. These windings generate a steady magnetic field that locks onto the stator’s rotating field, ensuring the rotor rotates at the same speed as the stator field.

• Exciter (Auxiliary DC Source)
  A small auxiliary generator called an exciter provides the DC power to the rotor windings. Some modern motors use brushless excitation systems, improving efficiency and reducing maintenance.

Clarification:
The armature winding is often misunderstood to be on the rotor. In synchronous motors used as motors (not generators), the armature (AC winding) is on the stator, while the rotor carries the DC field winding.

Induction Motors: Robustness and Versatility

Induction Motors, on the other hand, are the most widely used type of AC motor. They work on the principle of electromagnetic induction, where the stator’s magnetic field induces a current in the rotor, causing it to rotate. Induction Motors are known for their ruggedness, simplicity, and low maintenance costs. They are available in a range of designs, including squirrel-cage and wound-rotor types, making them suitable for various applications.

What Happens when a DC and AC supply is applied to synchronous motor

When the Rotor Takes DC Supply (in a Synchronous Motor):

When DC power is supplied to the rotor windings, it creates a steady magnetic field (north and south poles) on the rotor. This magnetic field does not rotate on its own but becomes a fixed magnetic source. When the motor is started and brought close to synchronous speed (typically using an external motor starter or damper winding), this DC-excited rotor locks into synchronism with the rotating magnetic field produced by the stator. From that point on, the rotor and stator field rotate together at the same speed, enabling stable and constant-speed operation.

When the Stator Takes AC Supply (in a Synchronous Motor):

Supplying 3-phase AC power to the stator windings generates a rotating magnetic field at synchronous speed. This rotating field sweeps around the stator and creates a rotating magnetic flux. This flux interacts with the DC-excited magnetic field on the rotor. Once synchronism is achieved, the rotor is “pulled” along by the rotating stator field and continues turning at exactly the same speed — this is why the motor is called “synchronous.”

What Happens When AC Supply is Fed to an Induction Motor:

When three-phase AC supply is fed to the stator windings of an induction motor, it produces a rotating magnetic field that sweeps around the stator at a speed called synchronous speed. This changing magnetic field cuts across the rotor conductors, which are usually short-circuited bars (in squirrel cage motors).

According to Faraday’s Law of Electromagnetic Induction, this motion induces a current in the rotor conductors. The induced current in the rotor then produces its own magnetic field. The interaction between the stator’s rotating magnetic field and the rotor’s induced magnetic field produces a torque that causes the rotor to turn.

However, the rotor can never reach synchronous speed because there must be relative motion between the stator field and the rotor for induction to occur. This small difference in speed is called “slip.” The rotor typically rotates at about 95–98% of the synchronous speed, depending on load conditions.

Key Construction Features:

• Stator Windings
  Like synchronous motors, induction motors also use a 3-phase stator winding to generate a rotating magnetic field.

• Rotor (No Windings in Squirrel Cage Type)
  Most induction motors use a squirrel cage rotor. This rotor has conductive bars (usually aluminium or copper) short-circuited by end rings — no coil windings or electrical connections are required on the rotor.

• Induced Current
  The rotating field of the stator induces current in the rotor bars. This current interacts with the magnetic field, generating torque and causing the rotor to turn. The rotor speed always lags behind the stator field, hence the term “asynchronous.”

Key Differences Between Synchronous and Induction Motors

Choosing the Right Motor

When deciding between a Synchronous Motor and an Induction Motor, consider the specific requirements of your application. Synchronous Motors are ideal for:

• High-precision applications, such as robotics and CNC machines
• High-power applications, such as industrial drives and generators
• Applications requiring high efficiency and power factor correction

Induction Motors, on the other hand, are suitable for:

• General-purpose applications, such as pumps, fans, and conveyor belts
• Applications where simplicity and ruggedness are essential
• Applications with variable speed requirements

Synchronous Motors and Induction Motors have distinct characteristics that make them suited for different applications. By understanding the differences between these two types of motors, you can make an informed decision when selecting a motor for your specific needs. Whether you require precision, efficiency, or robustness, there’s a motor out there that’s right for you.