The principle behind the working of a motor and revising the classifications

Electric motors are everywhere! In your house, almost every mechanical movement that you see around you is caused by an AC or DC electric motor. By understanding how a motor works you can learn a lot about magnets, electromagnets and electricity in general. An electric motor is all about magnets and magnetism. A motor uses magnets to create motion. If you have ever played with magnets you know the law of magnets: Opposites poles attract and like poles repel. So if you have two bar magnets with their ends marked "north" and "south," then the north end of one magnet will attract the south end of the other. On the other hand, the north end of one magnet will repel the north end of the other. Inside an electric motor, these attracting and repelling forces create rotational motion. The armature (or rotor) is an electromagnet. The motor below has copper wound around a soft iron core. The field magnet is still a permanent magnet, only this time there are two semi-circular magnets fitted inside a steel casing. In some larger motors and generators the field magnet could also be an electromagnet. In smaller motors it usually isn't to save the electricity that would otherwise be needed to make magnetism and also to reduce complexity. Actually, these days there are quite a few large motors using magnets to drive cars and the like. Electric motors have proved themselves to be among the greatest inventions of all time. Let's pull some apart and find out how they work.

The basic parts of the motor:

Electricity, magnetism, and movement: 

The basic idea of an electric motor is really simple: you put electricity into it at one end and an axle(metal rod) rotates at the other end giving you the power to drive a machine of some kind. 

Fleming's Left-Hand Rule  

You can figure out the direction in which the wire will jump using a handy mnemonic (memory aid) called Fleming's Left-Hand Rule (sometimes called the Motor Rule). Hold out the thumb, first finger, and second finger of your left hand so all three are at right angles. If you point the second finger in the direction of the current (which flows from the positive to the negative terminal of the battery), and the First finger in the direction of the Field (which flows from the North to the South pole of the magnet), your thumb will show the direction in which the wire Moves. First finger = Field Second finger = Current Thumb = Motion Basic parts of electric motor: 

The motor we have dismantled above is a simple electric toy motor. If you take apart a small electric motor, you will find that it contains the same pieces described above: two small permanent magnets inside casing, two brushes held in a housing, and an electromagnet made by winding wire around pieces of shaped metal (laminations) on a steel shaft, known as an armature or rotor. Almost always the rotor will have three poles or more. 

TYPES OF ELECTRIC MOTORS 

DC Motors:

A DC motor is designed to run on DC electric power. DC motors are in wide use, particularly in niche applications like cars and small appliances. 

 

Types of DC Motors: 

1. Permanent magnet motors : 

Here, permanent magnets instead of armature windings are mounted on the rotor. Since the magnetic field produced on the rotor is limited in strength and isn't controllable, permanent magnet motors are typically small and produce little horsepower. 

2. Series motors: 

Series motors connect the field windings in series with the armature. Series motors lack good speed regulation, but are well-suited for high-torque loads like power tools and automobile starters because of their high torque production and compact size. 

3. Shunt motors: 

Shunt motors use high-resistance field windings connected in parallel with the armature. Varying the field resistance changes the motor speed. Shunt motors are prone to armature reaction, a distortion and weakening of the flux generated by the poles that results in commutation problems evidenced by sparking at the brushes. Installing additional poles, called interpoles, on the stator between the main poles wired in series with the armature reduces armature reaction.

4. Compound motors:

Here, the concept of the series and shunt designs are combined. The Figure above shows one way of wiring a compound motor with interpoles. The blue lines indicate the shunt field, the red lines designate the series field, and the green lines show the interpole windings in series with the armature. After more than a century, DC motors are still in widespread use, and thanks to niche applications that show no signs of disappearing, they'll be around for many years to come.  

AC Motors: 

A.C. Motors are playing a very vital role in everyday life, right from pumping water to overhead tank to modern robots. The main factor which lead to the adoption & wide usage in various fields is its flexibility and its huge variety which can be matched with almost any kind of demand. To know what are the different types of A.C.Motors available, to match it perfectly with the demand, it is highly essential to know about the different classifications of A.C. Motors. 

Types of AC Motors:

Classification Based On Principle Of Operation:

(a) Synchronous Motors.

    1. Plain

    2. Super

(b) Asynchronous Motors.

    1. Induction Motors:

        (a) Squirrel Cage

        (b) Slip-Ring (external resistance).

    

    2. Commutator Motors:

    (a) Series

    (b) Compensated

    (c) Shunt

    (d) Repulsion

    (e) Repulsion-start induction

    (f) Repulsion induction 

Classification Based On Type Of Current:

    1. Single Phase

    2. Three Phase

Classification Based On Speed Of Operation:

    1. Constant Speed.

    2. Variable Speed.

    3. Adjustable Speed.

Classification Based On Structural Features:

    1. Open

    2. Enclosed

    3. Semi-enclosed

    4. Ventilated

    5. Pipe-ventilated

    6. Riveted frame-eye etc..

Overview of A.C.Motors:

1) Synchronous Motors & it's Uses:

These motors have the rotor(which is connected to the load) rotating at the same speed as the speed of rotation of the stator current. In other words, we can say these motors don't have slip with respect to the stator current. They are sometimes used no to drive the load but instead act as "synchronous condenser", to improve the power factor of the local grid to which it is connected to. These kind of motors are used even in high precision positioning devices like modern robots. They can also act as stepper motors.

2) Asynchronous Motors & it's Uses:

The most common form of motor which is used in everyday life from pumping water up the overhead tank to power plant boiler feed pumps, these kind of motors rule. These motors are very flexible to use and matches the load demand almost for everything. The most widely used Induction. Motors are very important for many industries due to their load bearing capacity and flexibility. These motors, unlike synchronous motors, slip when compared to the stator current field. They are generally used for various types of pumps, compressors and acts as prime movers for many machineries.

3) Single & Three Phase Motors and their Uses:

The A.C.Motors can find their usage in 2 forms based on their power supply.The single phase motors are generally found their use in low power requirements/domestic appliances like ceiling fans, mixer grinders, portable power tools etc. The three phase motors are generally found for high power requirements like power drives for compressors, hydraulic pumps, air conditioning compressors, irrigation pumps and many more. 

 

4) Constant, Variable & Adjustable Speed Motors:

As already said, A.C.Motors are highly flexible in many ways including their speed control. There are motors which should be run at a constant speed for the  air compressors. Certain cooling water pumps driven by ac motors can be run at two or three speeds by just switching the number of poles used. If the number of poles are changed then the speed also changes. These serve best for sea water cooling pumps in marine engine room applications & many power plants. The speed of the motors can also be varied continuously by some electronic arrangements thus this can be suited for certain applications like a ship's cargo pump, whose discharge rate has to lowered as per the terminals requirement.

5) Varied Structure Motors:

These types of motors have different outer cage arrangements, depending upon the usage or any special industrial requirement. For motors used in gas and oil terminals, the casing must be of intrinsically safe, thus it may either have a enclosed casing or a pipe ventilated arrangement such that the sparks produced inside the motor does not cause a fire outside it. Also many motors are totally enclosed as it may be open to weather like those used in hydro-electric power plants.