You don’t come across DC motors face to face in your every day life. Let me give you some common applications of DC Motors.

1. Electric Train.

A kind of DC motor called the DC Series Motor is used in Electric Trains. The DC Series Motors have the property to deliver more power when they are loaded more. So the more the people get on a train, the more powerful the train becomes.

2. Elevators.

The best bidrectional motors are DC motors. They are used in elevators. Compound DC Motors are used for this application.

3. PC Fans, CD ROM Drives, Hard Drives.

All these things need motors, very miniature motors, with great precision. AC motors can never imagine any application in these places.

4. Starter Motors in Automobiles.

An automobile battery supplies DC, so a DC motor is best suited here. Also, you cannot start an engine with a small sized AC motor,

5. Electrical Machines Lab in Colleges.

Ofcourse, if the motors are not here, where else would they be?

Let me tell you the types of DC motors that are there.

1. DC Shunt Motor,

2. DC Series Motor,

3. DC Long Shunt Motor (Compound)

4. DC Short Shunt Motor (Compound)

The rotational energy that you get from any motor is usually the battle between two magnetic fields chasing each other. The DC motor has magnetic poles and an armature, to which DC electricity is fed, The Magnetic Poles are electromagnets, and when they are energized, they produce a strong magnetic field around them, and the armature which is given power with a commutator, constantly repels the poles, and therefore rotates.

The DC Shunt Motor

For example in a 2 pole DC Motor, the armature will have two seperate sets of windings, connected to a commutator at the end of the shaft that are in constant touch with carbon brushes. The brushes are static, and the commutator rotate and as the portions of the commutator touching the respective positive or negative polarity brush will energize the respective part of the armature with the respective polarity. It is usually arranged in such a way that the armature and the poles are always repelling.

The general idea of a DC Motor is, the stronger the Field Current, the stronger the magnetic field, and faster the rotation of the armature. When the armature revolves between the poles, the magnetic field of the poles induce power in the armature conductors, and some electricity is generated in the armature, which is called back emf, and it acts as a resistance for the armature. Generally an armature has resistance of less than 1 Ohm, and powering it with heavy voltages of Direct Current could result in immediate short circuits. This back emf helps us there.

When an armature is loaded on a DC Shunt Motor, the speed naturally reduces, and therefore the back emf reduces, which allows more armature current to flow. This results in more armature field, and therefore it results in torque.

When a DC Shunt Motor is overloaded, if the armature becomes too slow, the reduction of the backemf could cause the motor to burn due to heavy current flow thru the armature.

The poles and armature are excited seperately, and parallely, therefore it is called a Shunt Motor.

The DC Series Motor

A DC Series Motor has its field coil in series with the armature. Therefore any amount of power drawn by the armature will be passed thru the field. As a result you cannot start a Series DC Motor without any load attached to it. It will either run uncontrollably in full speed, or it will stop.

When you load it, its effeciency increases with load. Thats why they use it on Electric Trains and elevators.

DC Compount Motor

A compound of Series and Shunt excitation for the fields is done in a Compound DC Motor. This gives the best of both series and shunt motors. Better torque as in a series motor, while the possibility to start the motor with no load.

Above is the diagram of a long shunt motor, while in a short shunt, the shunt coil will be connected after the serial coil.

A Compount motor can be run as a shunt motor without connecting the serial coil at all. But not vice versa.

B L Theraja is an Electrical Engineering Professor, and he has written a huge 4 Volume Book called Basic Electrical Engineering, Read that and you will get the derivations, calculations, formulae, and anything that you need to become an electrical engineer.

By becoming an electircal engineer, the best you can do is to work at some power station, or at the Power Department, or as a foreman at the Electric Train Station or as a lift mechanic. But I for some reason really feel bad that I did not make a career out of Electrical Engineering. I was keen on computers when I was learning Electrical Engineering, But right now, I find my passion for Electrical Machines growing more and more than it is for computers.