Synchronous motors are synchronous machines used to convert electric power to mechanical power. Synchronous motors can be contrasted with induction motors, which must slip in order to produce torque.
The speed of the synchronous motor is determined by the number of magnetic poles and the line frequency.As the name implies, synchronous motors run in synchronism with revolving field. The speed of rotation is therefore tied to the frequency of the source. Because the frequency is fixed, the motor speed stays constant, irrespective of the load or voltage of the three-phase line.
How does the Synchronous motor works?
To understand the basic concept of synchronous motor, In Figure, which shows a two-pole synchronous motor.
The field current IF of the motor produces a steady-state magnetic field BR.
A three-phase set of voltages is applied to the stator of the machine, which produces a three-phase current flow in the windings. A three-phase set of currents in an armature winding produces a uniform rotating magnetic field BS.
Therefore, there are two magnetic fields present in the machine, and the rotor field will tend to line up with the stator field, just as two bar magnets will tend to line up if placed near to each other.
Since the stator magnetic field is rotating the rotor magnetic field (and the rotor itself) will constantly try to catch up. The larger the angle between the two magnetic fields (up to a certain maximum) the greater the torque on the rotor of the machine.
The basic principle of synchronous motor operation is that the rotor “chases” the rotating stator magnetic field around in a circle never quite catching up with it. The larger the angle between the two magnetic fields (up to a certain maximum) the greater the torque on the rotor of the machine.
The disadvantages of a synchronous motor is that it cannot be started from a standstill by applying three-phase ac power to the stator.A synchronous motor in its purest form has no starting torque. It has torque only when it is running at synchronous speed.
How to start the Synchronous motor?
Since the synchronous motor cannot start by itself, therefore the rotor is usually equipped with a squirrel-cage winding so that it can start up as an induction motor.
This figure shows a 60 Hz synchronous motor at the moment power is applied to its stator windings. The rotor of the motor is stationary and therefore the magnetic field BR is stationary. The stator magnetic field BS is starting to sweep around the motor at synchronous speed.
What are the approach to start the Synchronous motor?
Reduce the speed of the stator magnetic field
This is to a low enough value that the rotor can accelerate and lock in with it during one half-cycle of the magnetic field’s rotation. This can be done by reducing the frequency of the applied electric power.
Use an external prime mover
This is to accelerate the synchronous motor up to synchronous speed, go through the paralleling procedure, and bring the machine on the line as a generator. Then turning off or disconnecting the prime mover will make the synchronous machine a motor.
Use damper windings or amortisseur windings
Amortisseur windings are special bars laid into notches carved in the face of a synchronous motor’s rotor and then shortened out on each end by a large shorting ring.
Industrial Application of Synchronous Motor
Heavy Machinery: Used in mills, crushers, and conveyors where constant speed is crucial for performance.
Power Generation: Employed in hydroelectric power plants to synchronize with the grid and improve stability.
Mining: Utilized in hoists and other equipment where precise speed control is required.
Pumps and Compressors: Ideal for applications that require consistent speed and torque, enhancing efficiency in fluid movement.
Traction Drives: Used in electric locomotives and urban transit systems for smooth and efficient operation.
Cement and Steel Industries: Common in kilns and furnaces where steady operation contributes to product quality.
Wind Turbines: Some designs use synchronous motors for better energy conversion and grid synchronization.
Refrigeration and HVAC Systems: Provides efficient operation for large chillers and air conditioning units.
Application of Synchronous Motor
purpose it is run without mechanical load on it and over excited.
In factories having large number of induction motors or transformers operating at lagging power factor, it is used for improving power factor.
It is used to generate electric power at power station, one of the most important application of synchronous machines.
it is used to control the voltage at the end of transmission line by varying its excitation.
It is also used in rubber mills, textile mills, cement factories, air compressors, centrifugal pumps which requiring constant speed.
It is used in motor generator sets requiring constant speed.
Comparison between synchronous motor and induction motor
Synchronous Motor: The rotor rotates at the same speed as the stator’s magnetic field. The rotor can be either a permanent magnet or an electromagnet, and it requires an external DC supply for excitation.
Induction Motor: The rotor is not directly connected to the power supply; it relies on electromagnetic induction from the stator’s rotating magnetic field. The rotor always runs at a speed less than the synchronous speed (slip).
Synchronous Motor: Operates at a constant speed determined by the supply frequency, making it ideal for applications requiring precise speed control.
Induction Motor: Speed varies with load due to slip, which can complicate speed control but makes it simpler for applications with varying loads.
Synchronous Motor: Typically requires a starting method (like a pony motor or variable frequency drive) to reach synchronous speed since it can’t start on its own.
Induction Motor: Can start directly on line, making it easier to implement in many applications.
Synchronous Motor: Generally more efficient and can operate at a leading or unity power factor, which can improve the overall power factor of the system.
Induction Motor: Typically operates at a lagging power factor, especially under full load, which may require power factor correction.
Synchronous Motor: Generally more expensive due to complexity and additional components (like exciters).
Induction Motor: Usually more cost-effective, making it the choice for many industrial applications.
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