1). What is a Synchronous Machine?
A synchronous machine is a form of alternating current machine in which the rotor rotates at the same frequency as the supply current. The rotor rotates at the same rate as the magnetic field generated by the stator. Synchronous machines function as generators & motors.
2). What is the Principle of Operation of a Synchronous Machine?
A synchronous machine works on the concept of magnetic field interaction between the stator & rotor. When the rotor’s magnetic field interacts with the stator’s revolving magnetic field, torque is generated, and the rotor rotates at the same speed.
3). What is Synchronous Speed?
Synchronous speed is the rate at which the magnetic field in the stator rotates, which corresponds to the rate at which the rotor rotates in a synchronous machine. The formula is as follows:
Ns = (120×f)/P
Where
Ns - Synchronous Speed (in RPM)
f – Supply Frequency (in Hz)
P – Machine Number of poles
4). What are the Different Types of Synchronous Machines?
Synchronous machines are classified into two different types:
Synchronous Motors
Synchronous Motors convert electrical energy into mechanical energy & operate at a constant speed.
Synchronous Generators (Alternators)
Synchronous Generators (Alternators) convert mechanical energy into electrical energy & are commonly utilized in power plants to generate electricity.
5). What is the Role of an Exciter in a Synchronous Machine?
The exciter generates the required magnetic field by applying DC excitation to the rotor windings. This magnetic field interacts with the stator’s revolving magnetic field, allowing the machine to run in synchrony. The exciter might be a separate DC generator (or) a static exciter.
6). What is the Power Factor of a Synchronous Motor?
The power factor of a synchronous motor can be changed by varying the excitation. Under-excited motors have a trailing power factor, while over-excited motors have a leading power factor. By altering the excitation, synchronous motors can also provide power factor correction.
7). What is the Difference between Synchronous and Asynchronous (Induction) Machines?
The only significant dissimilarity is that the rotor speed is same as synchronous speed of the stator magnetic field in case of synchronous machines. However in asynchronous (induction) machines, the rotor speed is always below the synchronous speed because of slip.
8). What are the Applications of Synchronous Machines?
Synchronous machines are widely used in:
- Synchronous Generators: In power stations for generating electricity.
- Synchronous Motors: In industries for driving high-precision equipment where constant speed is required, such as compressors, pumps, and conveyors.
- Power Factor Correction: In electrical systems to improve the power factor and reduce energy losses.
9). Why Do Synchronous Motors Require an Initial Starting Mechanism?
Synchronous motors do not self-start; therefore they need some form of construction to help them start. Before they can be synchronized with the AC supply, they must be at synchronous speed. At synchronous speed, the machine is capable of continuing rotation at a speed comparable to the supply frequency. Starting methods include using an external prime mover or using a damper winding.
10). What is Hunting in Synchronous Machines?
Hunting is the phenomenon in which the rotor of a synchronous machine struck to and fro around a steady state position normally due to a change in load. This in turn causes fluctuations in speed and power as far as cyclical frequencies are concerned and has an undesirable effect on the machine maintaining steadiness if it is not well regulated.
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