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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

• 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

• Thank you

You can find DC motors in many portable home apliances

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B2 UK motor dc coil rotor torque magnetic

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