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Question

how does an ac generator work ? I mean how does it change the current's direction ?

i dont get this part - according to flemings right hand rule , if the magnetic field and the force act in d same direction , then how can any other factor change the current's direction?

Answer 1

Think of the AC electrical current as a sine wave that goes to the right along the x axis; 1/2 of it above the line (positive) and the other 1/2 below the line (negative).

To make the circuit only see the positive part a device called a DIODE is used. It will only allow the current to flow in one direction and can be set to only allow the positive current to flow making the AC circuit look like DC current.

The bottom 1/2 of the wave is cut off; it pulsates very rapidly but the only current seen by the equipment is the positive current.
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DIODE:
In electronics, the word diode is described as a device that passes current in one direction much more readily than in the other. The most common function of a diode is to allow an electric current to flow in one direction (called the forward biased condition) but to block it in the opposite direction (the reverse biased condition). Thus, the diode can be thought of as an electronic version of a check valve.

Early diodes were fabricated mainly from germanium, but now are made largely from doped silicon.

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Answer 2

Consider a loop of wire rotating in a constant magnetic field.In one orientation, the right-hand rule will show current flow in one direction in the loop. After the loop has rotated 180°, though, the rule will show flow in the other direction.

Answer 3

Electric Motors and Generators

Electric Motors and Generators, group of devices used to convert mechanical energy into electrical energy, or electrical energy into mechanical energy, by electromagnetic means (see Energy). A machine that converts mechanical energy into electrical energy is called a generator, alternator, or dynamo, and a machine that converts electrical energy into mechanical energy is called a motor.

Two related physical principles underlie the operation of generators and motors. The first is the principle of electromagnetic induction discovered by the British scientist Michael Faraday in 1831. If a conductor is moved through a magnetic field, or if the strength of the magnetic field acting on a stationary conducting loop is made to vary, a current is set up or induced in the conductor (see Induction). The converse of this principle is that of electromagnetic reaction, first observed by the French physicist André Marie Ampère in 1820. If a current is passed through a conductor located in a magnetic field, the field exerts a mechanical force on it. See Magnetism.

The simplest of all dynamoelectric machines is the disk dynamo developed by Faraday. It consists of a copper disk mounted so that part of the disk, from the center to the edge, is between the poles of a horseshoe magnet. When the disk is rotated, a current is induced between the center of the disk and its edge by the action of the field of the magnet. The disk can be made to operate as a motor by applying a voltage between the edge of the disk and its center, causing the disk to rotate because of the force produced by magnetic reaction.

The magnetic field of a permanent magnet is strong enough to operate only a small practical dynamo or motor. As a result, for large machines, electromagnets are employed. Both motors and generators consist of two basic units, the field, which is the electromagnet with its coils, and the armature, the structure that supports the conductors which cut the magnetic field and carry the induced current in a generator or the exciting current in a motor. The armature is usually a laminated soft-iron core around which conducting wires are wound in coils.

II DIRECT-CURRENT (DC) GENERATORS

If an armature revolves between two stationary field poles, the current in the armature moves in one direction during half of each revolution and in the other direction during the other half. To produce a steady flow of unidirectional, or direct, current from such a device, it is necessary to provide a means of reversing the current flow outside the generator once during each revolution. In older machines this reversal is accomplished by means of a commutator, a split metal ring mounted on the shaft of the armature. The two halves of the ring are insulated from each other and serve as the terminals of the armature coil. Fixed brushes of metal or carbon are held against the commutator as it revolves, connecting the coil electrically to external wires. As the armature turns, each brush is in contact alternately with the halves of the commutator, changing position at the moment when the current in the armature coil reverses its direction. Thus there is a flow of unidirectional current in the outside circuit to which the generator is connected. DC generators are usually operated at fairly low voltages to avoid the sparking between brushes and commutator that occurs at high voltage. The highest potential commonly developed by such generators is 1500 V. In some newer machines this reversal is accomplished using power electronic devices, for example, diode rectifiers.

Modern DC generators use drum armatures that usually consist of a large number of windings set in longitudinal slits in the armature core and connected to appropriate segments of a multiple commutator. In an armature having only one loop of wire, the current produced will rise and fall depending on the part of the magnetic field through which the loop is moving. A commutator of many segments used with a drum armature always connects the external circuit to one loop of wire moving through the high-intensity area of the field, and as a result the current delivered by the armature windings is virtually constant. Fields of modern generators are usually equipped with four or more electromagnetic poles to increase the size and strength of the magnetic field. Sometimes smaller interpoles are added to compensate for distortions in the magnetic flux of the field caused by the magnetic effect of the armature.

DC generators are commonly classified according to the method used to provide field current for energizing the field magnets. A series-wound generator has its field in series with the armature, and a shunt-wound generator has the field connected in parallel with the armature. Compound-wound generators have part of their fields in series and part in parallel. Both shunt-wound and compound-wound generators have the advantage of delivering comparatively constant voltage under varying electrical loads. The series-wound generator is used principally to supply a constant current at variable voltage. A magneto is a small DC generator with a permanent-magnet field.

Answer 4

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