Induction Machine and Synchronous Machine Working Principle
Definition
An induction machine is an AC machine in which the rotor current is produced by electromagnetic induction due to the relative motion between the stator’s rotating magnetic field and the rotor conductors. It operates with a non-zero slip between the rotor speed and synchronous speed.
A synchronous machine is an AC machine in which the rotor rotates at the same speed as the stator’s rotating magnetic field, known as synchronous speed. It uses external excitation or permanent magnets to produce rotor magnetic flux and therefore maintains constant speed under steady-state operation.
Main Content
1. Rotating Magnetic Field and Its Role
- When a three-phase AC supply is given to the stator windings, it produces a rotating magnetic field. This field rotates at synchronous speed, which depends on supply frequency and number of poles.
- The rotating magnetic field is the fundamental concept behind both induction and synchronous machines because it acts as the moving magnetic influence that interacts with the rotor to produce torque.
In a three-phase stator, the windings are spaced 120 electrical degrees apart. When balanced three-phase currents flow through them, each winding produces a sinusoidal magnetic flux. The combined effect is a magnetic field of nearly constant magnitude that rotates continuously around the stator. The synchronous speed is given by:
where:
- = synchronous speed in rpm
- = supply frequency in Hz
- = number of poles
This rotating magnetic field is essential because:
- In an induction machine, it cuts the rotor conductors and induces emf.
- In a synchronous machine, the rotor magnetic field locks with it and rotates in step.
For example, if a 50 Hz, 4-pole supply is used, the synchronous speed is:
This means the rotating magnetic field travels at 1500 rpm.
2. Induction Machine Working Principle
- The induction machine works on electromagnetic induction: rotor emf is induced because of relative motion between the rotating stator field and the rotor.
- Torque is produced only when there is slip, meaning the rotor speed is slightly less than synchronous speed in motor operation.
In an induction motor, the stator is connected to a three-phase AC supply. The rotating magnetic field produced by the stator sweeps past the rotor conductors. Since the rotor is initially stationary or moving slower than the field, the field cuts the rotor bars and induces an emf according to Faraday’s law. Because the rotor winding is closed, current flows in the rotor. This rotor current interacts with the stator field to produce electromagnetic torque.
A key point is that the rotor cannot reach synchronous speed in normal induction motor operation. If rotor speed became equal to synchronous speed, there would be no relative motion, no induced emf, no rotor current, and therefore no torque. Hence, the rotor must always lag behind the rotating field.
Slip is defined as:
where:
- = slip
- = synchronous speed
- = rotor speed
The larger the slip, the larger the induced rotor emf and rotor current, especially during starting. At starting, slip is 1 because rotor speed is zero. This is why induction motors draw high starting current and develop starting torque.
Example: If rpm and rotor speed rpm,
This 4% slip is typical for many induction motors at full load.
3. Synchronous Machine Working Principle
- The synchronous machine works on the principle of magnetic locking between the stator’s rotating magnetic field and the rotor field.
- Rotor speed remains exactly equal to synchronous speed, and the machine requires excitation to establish the rotor magnetic field.
In a synchronous motor, the stator is supplied with three-phase AC and produces a rotating magnetic field. The rotor is given DC excitation through slip rings and brushes, or it may use a permanent magnet rotor in some modern designs. The rotor field interacts with the stator field, and once the rotor is brought close to synchronous speed, the rotor poles lock with the rotating stator poles. This locking action makes the rotor rotate at exactly synchronous speed.
Unlike the induction machine, there is no continuous slip in steady-state operation. The speed remains constant as long as the load is within the machine’s pull-out limit. If the load increases, the rotor develops a larger torque angle but still continues at synchronous speed until overload causes loss of synchronism.
In a synchronous generator, the principle is similar but used in reverse. A mechanical prime mover rotates the rotor at synchronous speed, and the rotating magnetic field induces AC voltage in the stator windings. Thus, the same machine principle applies in both motor and generator modes.
The torque developed depends on the angle between stator and rotor magnetic fields, called the load angle or torque angle. The greater the load angle, the higher the torque, up to a maximum limit. Beyond this limit, the machine loses synchronism.
Example: In a 50 Hz, 4-pole synchronous machine, the speed is fixed at 1500 rpm. Even if load changes, the speed remains 1500 rpm in steady state, unlike an induction motor.
Working / Process
1. Apply the three-phase AC supply to the stator
- In both machines, the stator is fed by a three-phase AC source.
- This creates a rotating magnetic field that revolves at synchronous speed.
- The direction of rotation depends on phase sequence.
2. Establish interaction between stator field and rotor
- In an induction machine, the rotating field cuts rotor conductors and induces emf, causing rotor current and torque.
- In a synchronous machine, the rotor is excited by DC or permanent magnets, and its magnetic field interacts directly with the stator rotating field.
3. Develop torque and achieve steady operation
- In an induction motor, the rotor accelerates and settles at a speed slightly below synchronous speed so that slip remains present.
- In a synchronous motor, the rotor locks into synchronism and runs exactly at synchronous speed.
- Under load changes, induction motor speed changes slightly, while synchronous motor speed remains constant until overload.
Advantages / Applications
- Induction machines are rugged, simple in construction, economical, and require less maintenance because they do not need external rotor excitation in normal operation.
- Synchronous machines provide constant speed operation, high efficiency, and power factor correction capability, making them suitable for precision and large power applications.
- Induction motors are widely used in pumps, fans, compressors, conveyors, machine tools, and domestic appliances, while synchronous machines are used in power plants, large industrial drives, alternators, and systems needing stable speed.
Summary
- The induction machine operates by electromagnetic induction and needs slip to produce torque.
- The synchronous machine operates by magnetic locking between stator and rotor fields and runs at synchronous speed.
- Both machines depend on the rotating magnetic field produced by three-phase AC supply.
- Induction machines are common for general-purpose driving, while synchronous machines are preferred where constant speed and high efficiency are important.