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  • You can find DC motors in many portable home apliances

  • Automobiles and types of industrial equipments

  • In this video, we will logically understand the opperation and construction of comercials DC motors

  • Let's first start with the simplest DC motor possible

  • It looks like this

  • The stator provides a constant magnetic field

  • And the armature, witch is the rotating part, is a simple coil

  • The armature is connected to a DC power source through a pair of comutator rings

  • When the current flows through the coil, an electromagnetic force is induced on it according to the Lorentz law

  • So the coil will start to rotate

  • You'll notice that as the coil rotates, the commutator rings connected with the power source of opposite polarity

  • As it result on the left side of the coil the electricity will always flow "away"

  • And on the right side electricity will always flow "towards"

  • This ensures that the torque action is also in the same direction throughout the motion

  • So the coil will continue rotating

  • But, if you observe the torque action on the coil closely

  • you'll notice that

  • when the coil is nearly perpendicular to the magnetic flux

  • the torque action nears zero

  • As a result there will be irregular motion of the rotor

  • if you run such a DC motor

  • Here is the trick to overcome this problem:

  • Add one more loop to the rotor, with a separate comutator pair for it

  • In this arrangement when the first loop is in the vertical position

  • the second loop will be connected to the power source, so the motor force is always present in the system

  • Moreover , the more such loops, the smoother will be the motor rotation

  • In a practical motor the armature loops are fitted inside slots with highly permeable steel layers

  • This will enhance magnetic flux interaction

  • Spring loaded comutator brushes help to mantain contact with the power source

  • A permanent magnetic stator pole is used only for very small DC motors

  • Most often, an electromagnetic is used

  • The field coil of the electromagnetic is powered from the the same DC source

  • The field coils can be connected to the rotor windings in two different ways

  • Parallel or series

  • The result is two differents kinds of DC motor constructions

  • a shunt and a series motors

  • The series wound motor has good starting torque, but speed drops drastically with the load

  • The shunt motor has a low starting torque but it is able to run almost at a constant speed

  • Irrespective of the load acting on the motor

  • Unlikely the other electrical machines, DC motors exhibits a unique caracteristic:

  • The production of BACK EMF

  • A rotating loop in a magnetic field will produce an EMF according to the principle of the electic magnetic induction

  • The case of the rotating armature loops is also the same

  • An internal EMF will be induced that opposes to the applied input voltage

  • The BACK EMF reduces armature current by a large ammount

  • BACK EMF is proportional to the speed of the rotor

  • At the starting of the motor BACK EMF is too low, thus the armature current becomes too high

  • leading to the burnout of the rotor

  • Thus a proper starting mecanism that controls the applied input voltage is necessary in large DC motors

  • One of the interesting variation of the DC motor is a universal motor

  • witch is capable to run under both AC and DC power sources

  • To know more about it, please check the next video

  • Thank you

You can find DC motors in many portable home apliances

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B2 UK motor dc coil rotor torque magnetic

DC Motor, How it works?

  • 87 10
    raymond posted on 2017/07/23
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