Two Chapter 7

Electric Generators: Alternating Current and Direct Current

• By the the mid 19^th century, the theory had been developed for producing a device that would be capable of producing a continuous electric current.
• The device required a magnetic field and conductor to be continually in motion with respect to each other.
• An external force would sustain the motion.
• Generator - A device which converts the mechanical energy of motion into electrical energy

Alternating Current Generator

• Rotation is the most convenient form of mechanical energy for a generator.
• Rotation can be driven by:
  • wind mill
  • water - wheel
  • turbine driven by falling water or expanding steam
  • etc.
• A coil of wire rotated in the jaws of a magnet by one of the above methods would form a simple electric generator.

** RULE:

With your right hand, point your fingers in the direction of motion of the conductor. Twist your arm so your fingers can curl into the direction of the magnetic field. Your thumb points in the direction of the conventional current.

CONSIDER THE YZ PART OF THE LOOP

• Given the direction of rotation, and applying the rule above, the current must be from Z to Y (Similarly for XW, current must be from X to W).
• After the coil moves 1/2 rotation, the YZ side is in XW position and vise versa. Therefore the current reverses direction.
• We therefore see that the current reverses direction every 1/2 turn.
• Alternating Current Generator - a generator in which the induced current changes its direction every 1/2 cycle.
• Field Magnet - The permanent U-shaped magnet in a generator
• Slip Rings - Rings made of a conductor that attach to the coils of wire which are rotated in the magnetic field of a generator.
• Contact Brushes - pieces of carbon that slide on the slip rings to carry current from a generator to a load.
• The maximum current in the above AC generator occurs when the plane of the coil is parallel to the magnetic field (in the diagram, the coil is as shown).
• The minimum current occurs when the plane of the coil is perpendicular to the magnetic field. (in the diagram, the coil would be perpendicular to what is shown).
• Lenz's law can also be used to explain the direction of current flow in the coil. This is illustrated below

LENZ'S LAW

CASE 1

Lenz's law tells us that the induced current opposes the motion (rotation). Therefore, the left side must be a South pole so attraction occurs to oppose the rotation. Using the right hand rule, current is as shown.

CASE 2

Lenz's law tells us that the induced current sets up a field which opposes the motion (rotation). Therefore the left side must be a N-pole so like poles will repel to oppose the motion. This means that current still leaves via the bottom.

CASE 3

The left side of the armature must be a S-pole to oppose the motion. Therefore the hand rule gives the direction of the current. The current leaves the top of the generator which means that the current has reversed direction.

CASE 4

The left side must be a N-pole. Therefore the current still leaves via the top of the generator and has the same direction as in case 3.

• Note that as the cycle continues, current changes direction between stages 2 and 3 as well as between 4 and 1.
• Current is not reversed between stages 1 and 2 as well as between 3 and 4.

Direct Current Generator

• Notice that the double slip ring in the DC generator is replaced by a single split ring. The plane of the splits is perpendicular to the plane of the coil.
• Commutator - The split ring in the DC generator.
• As with the AC generator, once the YZ side of the coil reaches the top, the direction of the current in the coil reverses.
• Because the plane of the splits is perpendicular to the plane of the coil, at the time the current in the coil switches direction, (when the plane of the coil is vertical) the brushes switch sides on the commutator. The electrons continue to flow out of the right side brush and into the left side brush with respect to the generator. This is because 2 now connects with the left side brush and 1 connects with the right side brush.
• Direct Current Generator - the type of generator that causes current to flow in one direction only in the external circuit.
• Current from an AC generator is graphically represented by

• Current from a DC benerator is graphically represented by

• As with AC generators, DC generators produce maximum current when the plane of the coil is parallel to the magnetic field and minimum current when the plane of the coil is perpendicular to the magnetic field.
• Lenz's law can also be used to explain the direction of current in the coil of wire.

LENZ'S LAW APPLICATION

CASE 1

The motion of the left end of the armature is away from the N-pole of the permanent magnet. Lenz's law states that this motion is opposed, therefore the left end of the armature becomes a S-pole, so it will be attracted to the N-pole of the permanent magnet . The right hand rule shows that the current is as shown. It enters from the top through A and leaves through B.

CASE 2

The right side is rotating towards the S-pole of the field magnet. The induced current will oppose this motion, therefore the right end of the armature is a S-pole, which repels the S-pole of the field magnet. The right hand rules tells us the direction of the current is as shown. The current must continue to enter through A and leave by B.

CASE 3

The right side of the armature moves away from the S-pole of the field magnet, which induces a current which opposes this motion. The right side of the armature must be a N-pole and therefore the current must be as shown (right hand rule). The top brush now connects with B and current therefore flows out of A (out of the bottom as usual) and into B (from the top as usual). There is no reversal of current.

CASE 4

• To Produce maximum electrical energy and efficiency we must
  • increase the number of turns on the coil
  • use a soft iron core for the coil
  • increase the speed of rotation
  • increase the strength of the field magnet
• Strong field magnets are often electromagnets.
• The rise and fall of DC current can be eliminated by winding several separate coils of wire on the armature each at a different angle.
• Each separate coil is connected to a separate section of a multi-segmented commutator.
• This way brushes connect with a coil that is always near maximum current production.
• Out put from such a generator would look like this

January 17, 2014