B2 High-Intermediate UK 1176 Folder Collection
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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
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DC Motor, How it works?

1176 Folder Collection
raymond published on July 23, 2017
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