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Monday, 9 December 2019

Flemings Right Hand Rule for Generators

Flemings Right Hand Rule can be used to find out the direction of induced e.m.f. in a conductor undergoing generation of emf. Fig shows the method of positioning right hand with fingers at right angles to each other. Flemings Right hand rule says that keeping the first finger, second finger and the thumb of right hand at right angles to each other similar to be three axis, if the first finger points out in the direction of magnetic field, the thumb in the direction of motion of conductor, then the second finger points out the direction of induced e.m.f.

(a) Method of positing Right Hand
(b) Direction of Induced EMF
As seen from above, for an e.m.f. to be produced in a conductor there should be a flux and motion between the conductor and the flux. Here the conductor of length 'l' meter is placed in a magnetic field of Φ Wb in A square metres so that the flux density

B = Φ/A Wb/m2 or testla.

The conductor is rotated about the axis with a velocity of 'v' m/sec. According to Faradays Laws of Electromagnetic induction, the e.m.f. induced in the conductor is given by

e = Rate of change of flux linkages
= B.l.v Volts

This value is maximum since the direction of motion is at right angle to the direction of flux. If the conductor were to move parallel to the direction of flux, then the conductor does not cut the lines of force i.e.. no flux linkage. Therefore, no e.m.f. will be induced in the conductor. Hence when the conductor is made to rotate about an axis it moves from parallel to the magnetic flux to perpendicular position. Therefore, the induced e.m.f. would also vary from zero to maximum.

Consider a position between zero angle to the direction of flux to perpendicular position wherein the direction of motion at any instant makes an angle of θo with respect to the direction of flux.
The component of voltage induced perpendicular to the direction of flux at any instant is equal to v sinθ. This will be true since when the conductor moves parallel to the direction of flux θ = 0 and sin 0o = 0. When the conductor moves perpendicular to the direction of flux θ = 90°. Hence re-writing the equation of e.m.f. as

e = B.l.v sin θ volts

Where e = e.m.f. induced in the conductor (Volts)
B = Flux density (Wb/m2)
l = length of the conductor (m)
θ = Angle of incidence between the direction of flux and the direction of motion about an axis

If the coil has N number of turns, then the e.m.f. induced at any instant is given by

e = 2N.B.l.v.sin θ, Since a coil contains two sides.

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