The machines classified as AC machine and DC machine. In AC machine, the induction machine and synchronous machine are widely used.
In this article, we will discuss the synchronous machine.
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Construction of Synchronous Machine
The synchronous machine has two important parts; stator and rotor.
The stator is a stationary part of a machine. It carries stator winding. The voltage generated in the stator winding. If the synchronous machine used as an alternator, the output is taken from the stator.
The rotor is a rotating part of a machine. The rotor connected with the shaft. If the synchronous machine used as a motor, the output is taken from the rotor.
It is a stationary part of a motor that contains a stator frame, stator winding, and a stator core.
The stator frame made up of cast iron. It is the outer body that protects the inner part of the motor.
The stator core made up of silicon steel. The core is laminated to reduce the eddy current loss and hysteresis loss. It provides a low magnetic path to the stator current and holds the stator winding.
The stator has slots, in which the stator winding placed. Generally, copper used for the winding. The winding is star connected. The winding of each phase distributed over several poles. The number of poles of the stator is equal to the number of poles in the rotor.
The rotor is a rotating part of a machine. There are two types of the rotor; salient pole type and cylinder rotor type.
The salient pole machine also known as the projecting pole rotor. This type of motor used in low and medium-speed engine driven alternators.
On the circumference of the rotor, the large number of salient or projecting poles placed with a fixed position.
It has a large diameter and short axial length compare to the cylindrical synchronous motor. Poles made up of cast irons and silicon steel and concentrated windings placed on the poles.
Poles and pole shoes laminated to reduce eddy current loss. To prevent hunting or damp out, the damper bars used in the poles faces.
This condition may occur in case of a sudden change in load conditions. Damper bars made up of copper and short-circuited at both ends by heavy copper rings. The maximum speed of this motor is 1500 rpm.
This type of medium-speed used in hydropower plants. So, this also known as hydro-alternator or hydro-generators.
This motor also known as a non-salient pole motor. This type of motor used in a very high-speed turbo alternator.
The rotor is cylindrical. Number of rotor poles are less compare to the cylindrical motor. The rotor has only 2-4 poles.
It has large axial length and small diameter compare to salient pole synchronous motor. It made up of silicon steel and the slots made on the circumstances of the rotor to receive the field winding. The maximum speed of the cylindrical rotor is 3000 rpm.
This type of alternator used in a thermal power plant or gas plant, where the high speed of the turbine used.
Applications of Synchronous Machine
- It used in powerhouses and sub-stations in parallel to the bus bars to improve the power factor. For this purpose, it is run without a mechanical load on it and powerhouses.
- A large number of induction motors or transformers operating at lagging power factor, it used for improving a power factor.
- Generate electric power at power plants.
- Control the voltage at the end of the transmission line by varying its excitation (DC supply).
- It used in motor-generator sets requiring constant speed.
- It also used in rubber mills, textile mills, cement factories, air compressors, centrifugal pumps which requiring constant speed and Pulp beaters and grinders, steel and metal rolling mills, rock, and ore crushers generally coupled or geared to the motor.
- Synchronous motors can constructed with wider air gaps than induction motors which makes these motors mechanically more stable.
- In this motor, easy to insulate armature winding for the high voltage for which alternator designed.
- The ability to control the power factor is one of the major advantages of the synchronous motor. An over-excited synchronous motor have a leading power factor. It can operated in parallel to induction motors and other lagging power factor loads thereby improving the system power factor.
- In the synchronous motor, the speed remains constant irrespective of the loads. This characteristic helps in industrial drives where constant speed required irrespective of the load it is driving.
- In synchronous motors, electromagnetic power varies linearly with the voltage.
- Synchronous motors usually operate with higher efficiencies (more than 90%) especially in low speed and unity power factor applications compared to induction motors.
- It is also useful when the motor is required to drive another alternator to supply at a different frequency as in frequency changes.
- It cannot used for variable speed jobs as there is no possibility of speed adjustment.
- It requires DC excitation which must supplied from external sources.
- Synchronous motor cannot be started under loaded conditions. Its starting torque is zero.
- It is not self-starting. Special methods adopted to make it self-starting.
- Additional damper winding is necessary.
- Collector rings and brushes required.
- It has a tendency to hunt.
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