Aux 1 Unit 15 Q8

Let’s break down the construction of a salient pole AC generator rotor:

Core Structure:

• Laminated Steel: The rotor core is built from a stack of thin silicon steel laminations, each insulated from the others to minimize eddy current losses. These laminations are typically 0.5mm to 1.5mm thick.   1. US9205487B2 – Compensation mechanism for cast rotor lamination stack height and compression pressure control – Google Patents patents.google.com
• Shaft: A central shaft made of high-strength steel runs through the core, providing the axis of rotation for the rotor.
• Spider: In large rotors, a spider, also made of steel, is used to support the laminated core and provide structural integrity.

Salient Poles:

• Projections: The defining feature of this rotor type is the presence of salient poles – distinct, projecting pole pieces that extend outward from the rotor core.
• Number of Poles: The number of poles depends on the desired generator speed and frequency. Slower-speed generators typically have more poles.
• Pole Construction:
  • Laminated Core: Each pole is also constructed from laminated silicon steel to reduce eddy current losses.
  • Pole Shoe: The outer end of each pole is often fitted with a pole shoe, which helps shape the magnetic field and improve the generator’s efficiency.

Field Windings:

• Excitation Coils: Insulated copper conductors are wound around each pole to form the field windings or excitation coils.
• DC Excitation: These coils are supplied with DC current, creating a strong magnetic field around each pole.
• Connection: The field windings are connected in series, and the ends are brought out to slip rings mounted on the shaft.

Slip Rings and Brushes:

• Electrical Connection: Slip rings provide a continuous electrical connection between the rotating field windings and the stationary external DC excitation source.   1. What’s the role of the slip ring in generators? – TutorChase www.tutorchase.com
• Carbon Brushes: Carbon brushes press against the slip rings, conducting the DC current to the field windings.   1. Carbon brush performance on slip rings – ResearchGate www.researchgate.net

Other Features:

• Damper Windings (Amortisseur Windings): These short-circuited copper bars are often embedded in the pole faces to help dampen oscillations and improve stability during transient conditions.
• Cooling: Depending on the size and design, the rotor might have internal cooling channels or fans to dissipate heat generated in the windings.

Key Points:

• Non-Uniform Air Gap: The air gap between the rotor poles and the stator is non-uniform, being smaller at the pole centers and larger between the poles. This creates a varying reluctance path that helps generate a sinusoidal output voltage.
• Suitable for Low and Medium Speeds: Salient pole rotors are typically used in generators designed for low to medium speeds, such as those driven by hydroelectric turbines or diesel engines.
• Rugged Construction: Their robust construction makes them suitable for handling high mechanical stresses and large power outputs.

In summary:

The salient pole AC generator rotor consists of a laminated steel core with projecting poles, each wound with excitation coils. The rotating magnetic field created by these poles interacts with the stator winding to generate electricity. This type of rotor is characterized by its non-uniform air gap and suitability for low to medium speed applications.  

1. [Solved] The salient pole type construction is suitable for running l – Testbook

The number of poles required for a 50 Hz supply and a rotational speed of 750 rpm is 8.  

1. [Solved] An 8-pole, 3-phase, 50 Hz induction motor is operating at 72 – Testbook

Here’s the explanation:

• Synchronous Speed: In AC motors and generators, the synchronous speed (the speed of the rotating magnetic field) is determined by the supply frequency and the number of poles. The formula is:   1. Rotating Magnetic Fields & AC Motor Operation – WorldWide Electric Corporation worldwideelectric.comNs = (120 * f) / P where:
  • Ns = Synchronous speed in revolutions per minute (RPM)   1. Technology Support – LUYANG www.luyangmotor.com
  • f = Supply frequency in Hertz (Hz)
  • P = Number of poles
• Actual Speed: The actual rotational speed of an induction motor is slightly lower than the synchronous speed due to slip. However, for this calculation, we’ll consider the synchronous speed as the desired rotational speed.   1. Induction Motors and Slip Speed — Lesson 7 – Ansys Customer Center innovationspace.ansys.com
• Calculation:
  1. We know the desired frequency (f = 50 Hz) and rotational speed (n = 750 rpm).
  2. We need to find the number of poles (P).
  3. Rearranging the formula, we get: P = (120 * f) / Ns
  4. Plugging in the values: P = (120 * 50) / 750 P = 6000 / 750 P = 8

Therefore, an 8-pole AC generator or motor is needed to achieve a rotational speed of 750 rpm when connected to a 50 Hz power supply.