INDUCTION MOTOR

Most AC motors are induction motors. Induction motors are favored due to their ruggedness and simplicity. In fact, 90% of industrial motors are induction motors.
Nikola Tesla conceived the basic principals of the polyphase induction motor in 1883, and had a half horsepower (400 watt) model by 1888. Tesla sold the manufacturing rights to George Westinghouse for $65,000.

Most large ( > 1 hp or 1 kW) industrial motors are poly-phase induction motors. By poly-phase, we mean that the stator contains multiple distinct windings per motor pole, driven by corresponding time shifted sine waves. In practice, this is two or three phases. Large industrial motors are 3-phase. While we include numerous illustrations of two-phase motors for simplicity, we must emphasize that nearly all poly-phase motors are three-phase. By induction motor, we mean that the stator windings induce a current flow in the rotor conductors, like a transformer, unlike a brushed DC commutator motor. 

you can also visit this video for more explanation click here

How does an Induction Motor Work ?

Induction motors are the most commonly used electrical machines. They are cheaper, more rugged and easier to maintain compared to other alternatives. In this video we will learn the working of a 3 phase squirrel-cage induction motor.

http://youtu.be/LtJoJBUSe28

Parts of an Induction Motor

An induction motor has 2 main parts; the Stator and Rotor. The Stator is the stationary part and the rotor is the rotating part. The Rotor sits inside the Stator. There will be a small gap between rotor and stator, known as air-gap. The value of the radial air-gap may vary from 0.5 to 2 mm. 

Construction details of a Stator

A Stator is made by stacking thin-slotted highly permeable steel lamination inside a steel or cast iron frame. The way the steel lamination are arranged inside the frame is shown in the following figure. Here only few of the steel lamination are shown. Winding passes through slots of the stator. 

Effect of 3 Phase Current Passing Through a Stator Winding

When a 3 phase AC current passes through the winding something very interesting happens. It produces a rotating magnetic field (RMF). As shown in the figure below a magnetic field is produced which is rotating in nature. RMF is an important concept in electrical machines. We will see how this is produced in the next section. 

  

The Concept of a Rotating Magnetic Field

To understand the phenomenon of a rotating magnetic field, it is much better to consider a simplified 3 phase winding with just 3 coils. A wire carrying current produces a magnetic field around it. Now for this special arrangement, the magnetic field produced by 3 phase A.C current will be as shown at a particular instant. 

 The components of A.C current will vary with time. Two more instances are shown in the following figure, where due to the variation in the A.C current, the magnetic field also varies. It is clear that the magnetic field just takes a different orientation, but its magnitude remains the same. From these 3 positions it’s clear that it is like a magnetic field of uniform strength rotating. The speed of rotation of the magnetic field is known as synchronous speed.

The Effect of RMF on a Closed Conductor

Assume you are putting a closed conductor inside such a rotating magnetic field. Since the magnetic field is fluctuating an E.M.F will be induced in the loop according to Faraday’s law. The E.M.F will produce a current through the loop. So the situation has become as if a current carrying loop is situated in a magnetic field. This will produce a magnetic force in the loop according to Lorentz law, So the loop will start to rotate.  

The Working of an Induction Motor

A similar phenomenon also happens inside an induction motor. Here instead of a simple loop, something very similar to a squirrel cage is used. A squirrel cage has got bars which are shorted by end rings. 

A 3 phase AC current passing through a Stator winding produces a rotating magnetic field. So as in the previous case, current will be induced in the bars of the squirrel cage and it will start to rotate. You can note variation of the induced current in squirrel cage bars. This is due to the rate of change of magnetic flux in one squirrel bar pair which is different from another, due to its different orientation. This variation of current in the bar will change over time. 

That's why the name induction motor is used, electricity is induced in rotor by magnetic induction rather than direct electric connection. To aid such electromagnetic induction, insulated iron core lamina are packed inside the rotor. 

Such small slices of iron layers make sure that eddy current losses are at a minimum. You can note one big advantage of 3 phase induction motors, as it is inherently self starting.

You can also note that the bars of a squirrel cage are inclined to the axis of rotation, or it has got a skew. This is to prevent torque fluctuation. If the bars were straight there would have been a small time gap for the torque in the rotor bar pair to get transferred to the next pair. This will cause torque fluctuation and vibration in the rotor. By providing a skew in the rotor bars, before the torque in one bar pair dies out, the next pair comes into action. Thus it avoids torque fluctuation. 

The Speed of Rotation of a Rotor & the Concept of Slip

You can notice here that the both the magnetic field and rotor are rotating. But at what speed will the rotor rotate?.To obtain an answer for this let's consider different cases.
Consider a case where the rotor speed is same as the magnetic field speed. The rotor experiences a magnetic field in a relative reference frame. Since both the magnetic field and the rotor are rotating at same speed, relative to the rotor, the magnetic field is stationary. The rotor will experience a constant magnetic field, so there won’t be any induced e.m.f and current. This means zero force on the rotor bars, so the rotor will gradually slow down.
But as it slows down, the rotor loops will experience a varying magnetic field, so induced current and force will rise again and the rotor will speed up.
In short, the rotor will never be able to catch up with the speed of the magnetic field. It rotates at a specific speed which is slightly less than synchronous speed. The difference in synchronous and rotor speed is known as slip.

 

DC MOTOR

INTRODUCTION

Almost every mechanical movement that we see today is accomplished by an electric motor. An electric motor takes electrical energy and produces mechanical energy. Electric motors come in various ratings and sizes. Some applications of large electric motors include elevators, rolling mills and electric trains. Some applications of small electric motors are robots, automobiles and power tools. Electric motors are categorized into two types: DC (Direct Current) motors and AC (Alternating Current) motors. The function of both AC and DC motors is same i.e. to convert electrical energy to mechanical energy.

The basic difference between these two is the power supply which is an AC source for AC motors and a DC source like a battery for DC motors. Both AC and DC electric motors consist of a stator which is a stationary part and a rotor which is a rotating part or armature of the motor. The principle of working of an electric motor is based on the interaction of magnetic field produced by the stator and the electric current flowing in the rotor in order to produce rotational speed and torque.

There are different kinds of DC motors and they all work on the same principle. A DC motor is an electromechanical actuator used for producing continuous movement with controllable speed of rotation. DC motors are ideal for use in applications where speed control and servo type control or positioning is required.

A simple DC motor is shown below.

Working Principle of DC Motor

An electromechanical energy conversion device will take electrical energy at the input and produces a mechanical energy at the output side. There are three electrical machines that are extensively used for this task: a DC motor, an induction or asynchronous motor and a synchronous motor. Induction motor and synchronous motors are AC motors. In all the motors, the electrical energy is converted into mechanical when the magnetic flux linking a coil is changed.

An electric motor takes electrical energy as input and converts into mechanical energy.
When the electrical energy is applied to a conductor which is placed perpendicular to the direction of the magnetic field, the result of the interaction between the electric current flowing through the conductor and the magnetic field is a force. This force pushes the conductor in the direction perpendicular to both current and the magnetic field, hence, the force is mechanical in nature.
The value of the force can be calculated if the density of the magnetic field B, length of the conductor L and the current flowing in the conductor I are known.
The force exerted on the conductor is given by

F = B×I×L Newtons

The direction of the motion of the conductor can be determined with the help of Fleming’s Left Hand Rule.
Flemming Left Hand Rule is applicable to all electric motors.
The figure representing Flemming Left Hand Rule is shown below.

When a conductor which is carrying current is placed in a magnetic field, a force acts on the conductor that is perpendicular to both the directions of magnetic field and the current.
According to Fleming’s Left Hand Rule, the left hand thumb represents the direction of the force, the index finger represents the direction of the magnetic field and the middle finger represents the direction of the current.
A DC motor consists of two sets of coils called armature winding and field winding. Field winding is used to produce the magnetic field. A set of permanent magnets can also be used for this purpose. If field windings are used, it is an electromagnet. The field winding is the fixed part of the motor or a stator. The armature winding is rotor part of the motor. The rotor is placed inside of stator. The rotor or the armature is connected to the external circuit through a mechanical commutator.
Generally, Ferro magnetic materials are used to make both stator and rotor which are separated by air gap. The coil windings inside the stator are made of series or parallel connections of number of coils. The Copper windings are generally employed for both armature and field windings.
The principle of operation of a DC motor is explained below.

Consider a coil placed in a magnetic field with a flux density of B Tesla. When the coil is supplied with direct current by connecting it to a DC supply, a current I flows through the length of the coil. The electric current in the coil interacts with the magnetic field and the result is exertion of a force on the coil according to the Lorenz force equation. The force is proportional to the strength of the magnetic field and the current in the conductor.
The same principle is used in DC motor and it consists of several coils that are wound on the armature and all the coils experience the same force. The result of this force is the rotation of the armature. The rotation of the conductor in the magnetic field will result in torque. The magnetic flux linking with the conductor is different at different positions of the coil in the magnetic field and these causes to induce an emf in the coil according to the Faradays laws of electromagnetic induction. This emf is referred to as back emf. The direction of this emf is opposite to the supply voltage which is responsible for current to flow in the conductor. Hence the total amount of current flowing in the armature is proportional to the difference between the supply voltage and the back emf.

DC CIRCUITS BASICS

INTRODUCTION-

• A circuit that can be AC or DC is the combination of active elements (power supply sources) and passive elements (resistors, capacitors and inductors). Thus, the circuit theory or analysis helps to understand the circuit behavior or characteristics by finding out the voltages and currents in various elements in a circuit by using different techniques. So let us discuss in brief about basic concepts of electricity before we could deal with DC circuit theory in later articles.

Basic Concept of Electricity

• According to the atomic theory, every material is made up from the atoms. This atom consists of centrally charged nucleus with a surrounded electrons based on Niels Bohr atom model. The nucleus consists of neutrons and positively charged protons. Electrons are negatively charged particles and rotate around the nucleus. This atom has an equal number of protons and electrons and a great force of attraction exist between these opposite charges results the electrons to track the nucleus.
  Bohr’s model gives the distribution of electrons in each shell of an atom. The most importantly the valence shell which is an outermost cell from the nucleus consists of eight electrons and never more than that. These electrons are at furthermost distance from the nucleus so some extra energy is required to make these electrons free. These electrons flow gives the electricity. But number of electrons in the outermost valence shell decides the electricity flow because the energy of the shell is shared by the electrons in it. Each electron has one eight of the shell’s energy if that valence shell has eight electrons.
  
  
   Hence great amount of external energy is required to make the electrons free so that the electricity is produced. Generally the materials which are not having free electrons in the outermost cell are called as insulators. Typically insulators have five to seven valence electrons in its valance shell. In other hand materials with one valence electron requires a little energy to free the electrons, so that the current is produced and the materials are called as conductors. Typically conductors have two or three valence electrons. These good conductors include silver, copper, aluminum, gold, etc. In prior to this, materials with four valence electrons that have both conductor and insulator properties called as semiconductors.
  
  As from above atomic theory, the flow of electrons gives the electricity. We know that like charges repel whereas unlike charges attract. The separation the charges makes negative charges to accumulate at one terminal and positive charges to other terminal with the application of source. The current starts to flow when the path is made between these two charges. The unit of the charge is Coulomb and it has a charge of 6.25 X 10¹⁸ electrons. The external force or voltage applied causes the charge to move and the rate at which the charge flow is decided by the amount of voltage applied.
  

  Introduction to Simple DC Circuit and Its Parameters

  • We know that the electricity is of two types, Alternating Current (AC) and Direct Current (DC). A circuit that deals with AC is referred to as AC circuit and a circuit with DC source is termed as DC circuit.  As of now we only discuss about DC circuit and its theory. The DC source allows the electricity or current to flow with an unvarying polarity that doesn’t change with time. A simple DC circuit is given in below figure to make the reader get aware of DC circuit components and its parameters.
  
  
  

 The above DC circuit consists of the voltage source and resistance with a specific current flow. So let us know about these parameters in brief.

Electric Voltage

The potential difference between two points or voltage in an electric circuit is the amount of energy required to move a unit charge between two points. It is measured in Volts and indicated with a letter V as shown in below figure. This voltage can be either positive or negative and expressed mostly with prefixes like KV, mV, uV, etc. to denote sub-multiples of the voltage. Batteries and generators are the most commonly used DC voltage sources which can produce the DC voltage from 1V to 24V DC for functioning of general electronic circuits.

Electric Current

It is the flow of electrons or electric charge. It is measured in Amperes or simply Amps, and denoted by the letter ‘I’ or lower case i. This electric current can be direct or alternating. The Direct Current (DC) flows in a unidirectional way and generally it is produced by batteries, solar cells, thermocouples, etc. In case of AC, electric charge movement periodically changes as we can observe in case of sine wave.

 Generally in circuits the direction of current flow is indicated with a letter I or lower case I with an arrow associated with it. But this direction actually indicates the conventional current flow rather than actual electron current flow.

Difference Between Conventional and Electron Current Flow

Electrons flow from negative terminal to positive terminal is referred as electron current flow, whereas from positive terminal to the negative terminal is referred as conventional current flow as shown in figure.

The electrons have always been repelled by the negative charge where the terminal is connected to the negative terminal of the battery and are attracted at positive terminal due to the positive charge. Hence the electrons flow from negative terminal to positive terminal is referred as electron current flow. But conventional method of assuming current flow is from positive to negative so this is referred as conventional current flow. Conventional current is indicated on many circuit diagrams and actual electron flow current is indicated in the case of describing the individual current flow. 

The conventional current flow is due to the positive charge carriers. The conventional current is measured in the opposite direction of actual electron current flow, which is due to the negative charge carriers (Electrons) therefore, conventional current is always positive. It is also measured in Amps. 

The difference of conventional and actual electron flow does not effect on any computational results and real time behavior. Most of the analyzing concepts of DC circuit results are independent of the direction of current flow. However, the conventional current is the standard and mostly follows.

Resistance

The resistance of a conducting material opposes the flow of electrons. It is measured in ohms and denoted by the Greek symbol Ω.  Depends on the resistor value in a circuit voltage applied to the circuit is decided. Thus, resistance can be defined as the voltage required for a circuit for making 1 ampere current flow. This also referred as Ohm’s law and written as R = V/I. That means if a circuit requires 200V to produce 2A current then the resistance should be 100 ohms. The resistance value is always positive. Resistors can be fixed or variable resistors as shown in figure.

Electric power (P) and Energy

 The power is termed as the work done in a given amount of time. In electrical circuits, power is exactly equal to the product of voltage and current. Since the voltage is the work per unit charge and current is the rate at which electrons move in a circuit. The Power is measured in watts (W) and its formula is

P = I x V

According to Ohms law,

R = V/I

V= IR

Substituting in the above equation,

P = (IR) R

P = I²R

Or also, by substituting I = V/R, we can get

P= V x (V/R)

P= V²/R

Electrical Energy

The rate at which electrical power consumed is generally referred as electrical energy. The energy is measured in watt-seconds as the power measured in watts and time in seconds. Often it is measured in kilowatt-hours as we can observe in our home electricity meter.

Electrical energy = power × time