Implications of Using AC Supply for Field Excitation in Synchronous Machines

In the design and operation of synchronous machines, the standard practice is to apply a DC excitation to the field windings. However, there might arise situations where the application of an AC supply for field excitation is necessary. Although this practice is not typically recommended, understanding the implications is crucial for maintaining the operational integrity of the machine. This article delves into the effects of AC field excitation on synchronous machines, focusing on the consequences in detail.

Effects of AC Field Excitation

Loss of Synchronization
Synchronous machines operate under precise magnetic conditions, maintained by a constant magnetic field created through DC excitation. If an AC supply is applied to the field, the magnetic field will vary with time, leading to a loss of synchronism with the rotating magnetic field of the stator. This loss of synchronization can result in the machine operating erratically and potentially causing it to shut down.

Torque Pulsations
The alternating nature of the field creates torque pulsations, leading to mechanical vibrations. These vibrations can cause stress on the machine components, including the rotor and stator, ultimately leading to potential damage over time.

Heating
The use of AC supply in the rotor windings can cause excessive heating due to eddy currents. This excessive heating can lead to damage to the insulation, reducing the lifespan of the machine and increasing the risk of failures.

Harmonic Distortion
AC excitation can introduce harmonics into the machine's operation, affecting performance and potentially causing resonance issues in the electrical system. These harmonics can interfere with other devices connected to the same electrical network, leading to unpredictable behavior and faults.

Generator Operation Issues
If the synchronous machine is functioning as a generator, AC excitation can prevent it from generating power effectively. The power generated would not be stable or predictable, leading to inefficiencies and potential operational failures.

Loss of Power Factor Control
Synchronous generators are designed to produce a pure sinusoidal wave of a given frequency. AC excitation disrupts this design, leading to a loss of power factor control. This can result in poor performance, increased electrical losses, and inefficient power generation.

Implications in Specific Cases

Implications on Synchronous Motors
When used in synchronous motors, feeding the field winding with an AC supply leads to a rapid change in the polarity of the field poles. This change occurs at a frequency of 100 times per second in a 50 Hz supply, making it impossible for the rotor to be in magnetic locking with the stator's revolving field. As a result, the rotor will not rotate at all, rendering the motor inoperative.

Implications on Synchronous Generators
When used in synchronous generators, AC supply can lead to abnormal situations such as the development of a unidirectional voltage or a non-sinusoidal waveform of a different frequency. Such voltages are not useful, as they do not meet the requirements for consistent, sinusoidal output. This can lead to further inefficiencies and operational issues in the electrical system.

Conclusion

To summarize, using an AC supply for field excitation in synchronous machines disrupts their normal operations, leading to potential instability, mechanical damage, and inefficiencies. For optimal performance, synchronous machines should always be supplied with DC excitation current. Understanding and addressing these issues is crucial for ensuring the reliability and efficiency of these machines in industrial applications.

Keywords: AC Field Excitation, Synchronous Machine, DC Excitation, Electrical Systems, Power Generation, Motor Operations