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