Salient Pole A.C. Generator Rotor Construction:
(a) Components:
A salient pole rotor consists of a central steel shaft for structural support and several poles radially extending outwards from the shaft. Here’s a breakdown of its key components:
- Shaft: Made of solid steel to ensure strength and rigidity. It transmits the mechanical torque from the prime mover (engine, turbine) to the rotor.
- Poles: These are typically constructed from laminated electrical steel to minimize eddy current losses within the rotor. Each pole has a specific polarity (north or south) and they alternate around the rotor circumference.
- Pole Shoes: Attached to the tips of the poles, these are often made of solid steel or cast iron for better mechanical strength and to concentrate the magnetic field. They may also have slots to accommodate the field windings.
- Field Windings: Copper wires wrapped around the pole shoes or embedded in slots within the pole shoes. When a DC current flows through these windings, it magnetizes the poles, creating a rotating magnetic field in the stator.
- End Rings: Two conductive rings at each end of the rotor that connect the ends of the field windings on all the poles. These rings allow for current flow through the windings in series or parallel, depending on the desired magnetic field strength configuration.
(b) Number of Poles for 50Hz at 750rpm:
The number of poles (P) required for a specific AC generator can be calculated using the following formula:
P = (120 * f) / rpm
where:
- P = number of poles
- f = frequency of the AC supply (50 Hz in this case)
- rpm = rotational speed of the rotor (750 rpm)
Plugging in the values:
P = (120 * 50 Hz) / 750 rpm = 8 poles
Therefore, a salient pole rotor with 8 poles would be required to generate a 50Hz AC supply at a rotational speed of 750 rpm.