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A simple AC generator

 A simple AC generator

Being able to generate a voltage by moving a conductor through a magnetic field is extremely useful as it provides us with an easy way of generating electricity.

 

Unfortunately, moving a wire at a constant linear velocity through a uniform magnetic field presents us with a practical problem simply because the mechanical power that can be derived from an aircraft engine is available in rotary (rather than linear) form!

 

The solution to this problem is that of using the rotary power available from the engine (via a suitable gearbox and transmission) to rotate a conductor shaped into the form of a loop.

 

The loop is made to rotate inside a permanent magnetic field with opposite poles (N and S) on either side of the loop.

 

There now remains the problem of making contact with the loop as it rotates inside the magnetic field but this can be overcome by means of a pair of carbon brushes and copper slip-rings.

 

The brushes are spring-loaded and held against the rotating slip-rings so that, at any time, there is a path for current to flow from the loop to the load to which it is connected.

 

The opposite sides of the loop consist of conductors that move through the field. At 0° (with the loop vertical) the opposite sides of the loop will be moving in the same direction as the lines of flux.

 

 At that instant, the angle, θ, at which the field is cut is 0° and since the sine of 0° is 0 the generated voltage (from E [1] Blv sin θ ) will consequently also be zero.

 

If the loop has rotated to a position which is 90° (position B) the two conductors will effectively be moving at right angles to the field.

 

 At that instant, the generated e.m.f. will take a maximum value (since the sine of 90° is 1).

 

At 180° from the starting position the generated e.m.f. will have fallen back to zero since, once again, the conductors are moving along the flux lines (but in the direction opposite to that at 0 °).

 At 270° the conductors will once again be moving in a direction that is perpendicular to the flux lines (but in the direction opposite to that at 90°).

 

At this point, a maximum generated e.m.f.

will once again be produced. It is, however, important

to note that the e.m.f. generated at this instant will

be of opposite polarity to that which was generated

at 90°. The reason for this is simply that the relative

direction of motion (between the conductors and fl ux

lines) has effectively been reversed.

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