BY CREANOVA
ENGINEERS…www.creanovaengineers.blogspot.in
DC MOTORS
A simple DC motor has a coil of wire that can rotate
in a magnetic field. The current in the coil is supplied via two brushes that
make moving contact with a split ring. The coil lies in a steady magnetic
field. The forces exerted on the current-carrying wires create a torque on the
coil.
The force F on a wire of length L carrying a current
i in a magnetic field B is iLB times the sine of the angle between B and i,
which would be 90° if the field were uniformly vertical. The direction of F
comes from the right hand rule*, as shown here. The two forces shown here are
equal and opposite, but they are displaced vertically, so they exert a torque.
(The forces on the other two sides of the coil act along the same line and so
exert no torque.)
* A number of different
nmemonics are used to remember the direction of the force. Some use the right
hand, some the left. For students who know vector multiplication, it is easy to
use the Lorentz force directly: F = q v X B
, whence F = i dL X B
. That is the origin of the diagram shown here.
The coil can also be considered as a magnetic
dipole, or a little electromagnet, as indicated by the arrow SN: curl the
fingers of your right hand in the direction of the current, and your thumb is
the North pole. In the sketch at right, the electromagnet formed by the coil of
the rotor is represented as a permanent magnet, and the same torque (North
attracts South) is seen to be that acting to align the central magnet.
Note the effect of the brushes on the split ring. When the plane
of the rotating coil reaches horizontal, the brushes will break contact (not
much is lost, because this is the point of zero torque anyway – the forces act
inwards). The angular momentum of the coil carries it past this break point and
the current then flows in the opposite direction, which reverses the magnetic
dipole. So, after passing the break point, the rotor continues to turn
anticlockwise and starts to align in the opposite direction. In the following
text, I shall largely use the 'torque on a magnet' picture, but be aware that
the use of brushes or of AC current can cause the poles of the electromagnet in
question to swap position when the current changes direction. The torque generated over a cycle varies with the vertical separation of the two forces. It therefore depends on the sine of the angle between the axis of the coil and field. However, because of the split ring, it is always in the same sense. The animation below shows its variation in time, and you can stop it at any stage and check the direction by applying the right hand rule.
BY CREANOVA
ENGINEERS…www.creanovaengineers.blogspot.in
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