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| Electric Motor |
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| An electric motor is a device which converts electrical energy into mechanical energy. |
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| It works on the principle that when an electric current is passed through a conductor placed normally in a magnetic field a force acts on the conductor as a result of which the conductor begins to move. The direction of the force is obtained with the help of Fleming's left hand rule. |
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| The figure below shows the construction of an electric motor. The main parts of an electric motor are: |
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| D.C. Motor |
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 the armature coil ABCD mounted on an axle |
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 the commutator that is a split ring divided in two parts S1 and S2 |
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 a pair of brushes B1 and B2 |
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 a horse - shoe electromagnet |
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| The coil ABCD is wound round a soft iron and is placed in between the pole pieces of a powerful horse - shoe magnet. The coil is free to rotate about its axis. The ends of the coil A and D are connected to split parts of the ring S1 and S2 respectively. Two brushes B1 and B2, made of carbon or copper, touch the split rings S1 and S2 respectively. A dc source is connected across the brushes B1 and B2. When the coil rotates, the split rings rotate but the brushes do not move. |
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| A wheel can be mounted on the axle placed along the axis of the coil so as to drive the desired parts of the machine such as electric fan, washing machine etc. where the motor is used. |
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| The plane of the coil is horizontal and the split ring S1 touches the brush B1 while the split ring S2 touches the brush B2. The brush B1 is connected to the anode of the d.c. battery while the brush B2 is connected to the cathode. The current flows in the coil in the direction ABCD. The arms BC and DA being parallel to the magnetic field experience no force. |
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| According to Fleming's left hand rule
force 'F' acting on the arm AB, is inward and perpendicular to the plane of
the coil and the force on the arm CD is in just in the opposite direction.
The forces on the arms AB and CD being equal and opposite form an
anticlockwise couple, due to which the coil begins to rotate. It rotates in
such a way that the arm AB goes in and the arm CD comes out. |
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| When the coil reaches the vertical position, the couple becomes zero since the forces on the arms now become collinear. But due to the inertia of motion, the coil does not stop in this position. As the coil passes from the vertical the split ring S1 comes in contact with the brush B2, while the split ring S2 comes in contact with the brush B1. Now the current flows through the coil in the direction DCBA and the forces acting on the arms DC and AB of the coil again form an anticlockwise, couple due to which the coil remains rotating in the same direction. Thus, whenever the coil comes in the vertical position, the direction of the current through the coil reverses and the coil continues to rotate in the same direction. |
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| The deflecting couple on the coil is maximum when the plane of the coil is parallel to the direction of the magnetic field and the deflecting couple is minimum when the plane of the coil is perpendicular to the magnet field. |
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| The speed of rotation of the coil depends on the deflecting couple acting on the coil. Hence the speed of rotation of the coil can be increased by, |
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 increasing the number of turns of the coil |
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 increasing the strength of the current |
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 increasing the area of the coil |
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 increasing the strength of the magnetic field. |
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