IE243 ELECTRICAL MACHINES–I [Cr. Hrs = 3+1] [Marks: 100+50]

By

Asif Ahmed Memon

• The simplest possible rotating dc machine is shown in Figure. It consists of a single loop of wire rotating about a fixed axis. The rotating part of this machine is called the rotor, and the stationary part is called the stator.

• The magnetic field for the machine is supplied by the magnetic north and south poles shown on the stator in Figure

• Notice that the loop of rotor wire lies in a slot carved in a ferromagnetic core. the iron rotor, together with the curved shape of the pole faces, provides a constant-width air gap between the rotor and stator.

• The reluctance of air is much much higher than the reluctance of the iron in the machine.

• To minimize the reluctance of the flux path through the machine, the magnetic flux must take the shortest possible path through the air between the pole face and the rotor surface.

• Since the magnetic flux must take the shortest path through the air, it is perpendicular to the rotor surface everywhere under the pole faces.

• Also, since the air gap is of uniform width, the reluctance is the same everywhere under the pole faces.

• The uniform reluctance means that the magnetic flux density is constant everywhere under the pole faces.

The Voltage Induced in a Rotating Loop

The Voltage Induced in a Rotating Loop

The Voltage Induced in a Rotating Loop

•To determine the total voltage eind on theloop, examine each segment of the loopseparately and sum all the resulting voltages.•The voltage on each segment is given byEquation

The Voltage Induced in a Rotating LoopSegment ab In this segment, the velocity of the wire is tangential to the path of rotation. The magnetic field B points out perpendicular to the rotor surface everywhere under the pole face and is. zero beyond the edges of the pole face. Under the pole face, velocity v is perpendicular to B , and the quantity v x B points into the page. Therefore, the induced voltage on the segment is

The Voltage Induced in a Rotating Loop

Segment bc. In this segment, the quantity v x B is either into or out of the page, while length l is in the plane of the page, so v x B is perpendicular to l. Therefore the voltage in segment be will be zero:

The Voltage Induced in a Rotating Loop

Segment cd. In this segment, the velocity ofthe wire is tangential to the path of rotation.The magnetic field B points in perpendicularto the rotor surface everywhere under thepole face and is. zero beyond the edges ofthe pole face. Under the pole face, velocity vis perpendicular to B , and the quantity v x 8points out of the page. Therefore, theinduced voltage on the segment is

The Voltage Induced in a Rotating Loop

Segment da. Just as in segment bc, v x B is perpendicular to I. Therefore the voltage in this segment will be zero too:

The Voltage Induced in a Rotating Loop

The Voltage Induced in a Rotating Loop

The Voltage Induced in a Rotating Loop

http://www.ece.umn.edu/users/riaz/animations/dcmachine.html

https://www.youtube.com/watch?v=d_aTC0iKO68&list=TLPC-jiNlNkGg

https://www.youtube.com/watch?v=Gzca2CbZ6EM