Understanding Stationary Armature in Alternators

Alternators, also known as synchronous generators, are electrical machines that convert mechanical energy into AC electrical energy. They typically have two main parts: the stator (the stationary part) and the rotor (the rotating part).

In modern, large-capacity alternators, the armature winding (where the AC voltage is generated) is placed on the stator, and the field winding (which produces the magnetic field) is placed on the rotor. This configuration is known as a stationary armature or rotating field type alternator.

Advantages of Stationary Armature Alternators

Placing the armature on the stator offers several significant advantages compared to a design where the armature rotates and the field is stationary:

• High voltage/current handling: The armature winding generates the main output voltage and current. When it's stationary (on the stator), it's much easier to insulate for high voltages and to handle the heavy conductors needed for large currents. The connections to the external circuit are also simpler and more reliable, without needing slip rings to collect the high-power output.
• Reduced sparking issues: Collecting high-voltage, high-current AC power from a rotating part using slip rings and brushes would lead to significant sparking and wear. With the armature stationary, this high-power collection via slip rings is avoided. Slip rings are only needed for the low-power DC excitation supply to the rotating field winding.
• Easier insulation: The stator core and windings are accessible, making it easier to inspect, maintain, and apply robust insulation for the high generated voltage.
• Requires only two slip rings for DC excitation: The rotor carries the DC field winding. Supplying DC power to this rotating winding only requires two slip rings and brushes, which handle relatively low voltage and current compared to the armature output. Insulating these DC slip rings is also simpler.
• Robust rotor construction: The rotor primarily carries the field winding and is subject to centrifugal forces. A rotating field design allows for a strong, robust rotor structure, which is crucial at high speeds.

Analyzing the Given Statements

Let's evaluate each statement based on the advantages discussed:

• Statement 1: It needs only two slip rings on DC side. This is a correct statement and a significant advantage of the stationary armature design. The slip rings are used to supply DC excitation to the rotating field winding on the rotor, and only two are needed.
• Statement 2: The stator weight is less compared to rotor weight. This statement is generally NOT true, especially in large alternators. The stator houses the heavy armature winding (which carries high current and requires substantial copper and iron) and the large stator core. The rotor carries the field winding and field poles. In most cases, the stator, due to its size and the amount of copper and iron it contains for the armature circuit, is significantly heavier than the rotor. Therefore, this statement does NOT represent an advantage; it's often the opposite of the reality.
• Statement 3: Sparking at brushes is completely avoided. While not "completely avoided" because brushes are still used on the DC slip rings, the statement refers to avoiding the significant sparking issues associated with collecting high-voltage, high-current AC power from a rotating armature via slip rings. Compared to that scenario, sparking is drastically reduced and less problematic. This is considered an advantage in the context of high-power generation.
• Statement 4: It is easy to insulate sliprings which are on DC. This is a correct statement and an advantage. The slip rings handle the DC excitation voltage, which is typically much lower than the AC voltage generated in the armature. Insulating for lower DC voltage on the slip rings is much easier and more reliable than insulating for high AC voltage and current if the armature were rotating.

Conclusion

Based on the analysis, statements 1, 3 (in context), and 4 describe advantages of having a stationary armature. Statement 2, "The stator weight is less compared to rotor weight," is typically inaccurate for large alternators and is therefore NOT an advantage.