BASIC CONCEPTS OF ELECTRICALS

BASIC ELECTRICAL CONCEPTS

Basic electrical concepts

In each plant, the mechanical movement of different equipments is caused by an electric prime mover (motor). Electrical power is derived from either utilities or internal generators and is distributed through transformers to deliver usable voltage levels.

Electricity is found in two common forms:

• AC (alternating current)

• DC (direct current).

Electrical equipments can run on either of the AC/DC forms of electrical energies. The selection of energy source for equipment depends on its application requirements. Each energy source has its own merits and demerits.

Industrial AC voltage levels are roughly defined as LV (low voltage) and HV (high voltage) with frequency of 50–60 Hz. An electrical circuit has the following three basic components irrespective of its electrical energy form:

• Voltage (volts)

• Ampere (amps)

• Resistance (ohms).

1. Voltage is defined as the electrical potential difference that causes electrons to flow.

2. Current is defined as the flow of electrons and is measured in amperes.

3. Resistance is defined as the opposition to the flow of electrons and is measured in ohms.

All three are bound together with Ohm’s law, which gives the following relation between the three:

•  V = I × R

(a) Power

In DC circuits, power (watts) is simply a product of voltage and current.

•  P =V × I

For AC circuits, the formula holds true for purely resistive circuits; however, for the following types of AC circuits, power is not just a product of voltage and current.

Apparent power is the product of voltage and ampere, i.e., VA or kVA is known as apparent power. Apparent power is total power supplied to a circuit inclusive of the true and reactive power.

Real power or true power is the power that can be converted into work and is measured in watts

Reactive power If the circuit is of an inductive or capacitive type, then the reactive component consumes power and cannot be converted into work. This is known as reactive power and is denoted by the unit VAR.

(b) Relationship between powers

• Apparent power (VA) = V × A


• True power (Watts) = VA × cosφ


•  Reactive power (VAR) = VA × sinφ

(c) Power factor

Power factor is defined as the ratio of real power to apparent power. The maximum value it can carry is either 1 or 100(%), which would be obtained in a purely resistive circuit.

• Power factor = True power / Apparent power

Types of circuits

There are only two types of electrical circuits – series and parallel.

A series circuit is defined as a circuit in which the elements in a series carry the same current, while voltage drop across each may be different.

A parallel circuit is defined as a circuit in which the elements in parallel have the same voltage, but the currents may be different.

Transformer

A transformer is a device that transforms voltage from one level to another. Transformer working is based on mutual emf induction between two coils, which are magnetically coupled. When an AC voltage is applied to one of the windings (called as the primary), it produces alternating magnetic flux in the core made of magnetic material (usually some form of steel). The flux is produced by a small magnetizing current which flows through the winding. The alternating magnetic flux induces an electromotive force (EMF) in the secondary winding magnetically linked with the same core and appears as

a voltage across the terminals of this winding. Cold rolled grain oriented (CRGO) steel is used as the core material to provide a low reluctance, low loss flux path. The steel is in the form of varnished laminations to reduce eddy current flow and losses on account of this.

There is a very simple and straight relationship between the potential across the primary coil and the potential induced in the secondary coil. The ratio of the primary potential to the secondary potential is the ratio of the number

of turns in each and is represented as follows:

• N1/N2 = V1/V2

Current-induced 

When the transformer is loaded, then the current is inversely proportional to the voltages and is represented as follows:

•  N1/N2 = V1/V2= I2/I1

AUTOCAD Draftsman Interview Question And Answers

 

AUTOCAD is one of the commonly used software to create shapes in different views. The Draftsman interview contains common questions as every company need a guy to perform drawings. So we have collected Questions and Answers to help candidates who are appearing for interviews. Normally Manufacturing companies only hire such people and they don’t ask you deep queries in AUTOCAD but if you are applying for an software company you can expect difficult questions. So we prepared this post to help you on answering both of the Questions.

USES OF AUTOCAD

Its a Platform Developed by Autodesk Softwares to create 2D and 3D models with dimensions required. We can create our imaginary models using various tools and in companies its used to create machine drawings, spare parts drawings, tools drawings. This drawings will be used for companies to give to vendors to make an exact product as per the drawings.

Various Formats Of AUTOCAD

Normally AUTOCAD drawings will be saved in .dwg format. Some drawings will have to the .dxf format (Drawing Exchange format )which cannot be edited.

History Of AUTOCAD

The history of AUTOCAD dates back to 1982 when the first version was launched. After that around 29 versions are released so far including the last release of AUTOCAD 2015.

Questions for DRAFTSMAN Job

1,  What is the command for UNDO option –  REDO

2, Command to Zoom in 50% – Zoom 1/2x

3, Difference between DTEXT and MTEXT – Dtext – Single lined Text , Mtext – Multiline Text with more options

4, What is .ctb File – Color Settings file used with AUTOCAD

5, What is difference between associated hatching and non associated hatching – The former moves with the boundary where the later did not

6, Difference between Divide and measure command – Divide command is for dividing a line in equal dimension where measure is used to break lines as per our requirement.

7, The default workspace for AUTUCAD – My workspace

8, The key for cycle through snap points – TAB Key

9, AUTOCAD Drawing units – Architectural, Decimal, Engineering etc

10, Settings used for Polar Tracking – Increment and Additional Angles

 

11, What is the use of HOT grip – Move, Rotate, Scale

12, Steps to move a object – Point where it should be moved then use displacement option. Now select the object to be moved and then place it there.

13, What command is used for Copying many objects – COPY

14, Various Filters – Group Filters, Property Filters, Layer standard Filters

15, Where to find the Units options – Format -> Units

16, Format for Drawing Limits – <0.00,0.00>

17, Which key will switch off the Gridlines – F7

18, Options for Visual Reference – GRID, SNAP Mode

19, To remove objects which command is used – ERASE

20, How to repeat a Command – Right click will bring a list in which first one will be the repeat command

Please share in your comments if you have experienced any questions in interview so that it will help people who will be appearing for the personal interview.

Also please correct us if we made any mistakes in the above questions.

Thank you.

RAJ SINGH Pvt I.T.I 

Recognized by - Directorate of Training & Technical Education,(Govt. of NCT of Delhi)

Affiliated with National Council of Vocational Training(NCVT)

DGE & T ministry of labour & Employment (Govt of India)

Induction Motor
Working Principle | Types of Induction Motor

One of the most common electrical motor used in most applications which is known as induction motor. This motor is also called as asynchronous motor because it runs at a speed less than its synchronous speed. Here we need to define what is synchronous speed. Synchronous speed is the speed of rotation of the magnetic field in a rotary machine and it depends upon the frequency and number poles of the machine. An induction motor always runs at a speed less than synchronous speed because the rotating magnetic field which is produced in the stator will generate flux in the rotor which will make the rotor to rotate, but due to the lagging of flux  current  in the rotor with flux  current  in the stator, the rotor will never reach to its rotating magnetic field speed i.e. the synchronous speed. There are basically two types of induction motor that depend upon the input supply - single phase induction motor and three phase induction motor. Single phase induction motor is not a self starting motor which we will discuss later and three phase induction motor is a self-starting motor.

Working Principle of Induction Motor

We need to give double excitation to make a machine to rotate.  For example if we consider a DC motor, we will give one supply to the stator and another to the rotor through brush arrangement. But in induction motor we give only one supply, so it is really interesting to know that how it works. It is very simple, from the name itself we can understand that induction process is involved. Actually when we are giving the supply to the stator winding, flux will generate in the coil due to flow of  current  in the coil. Now the rotor winding is arranged in such a way that it becomes short circuited in the rotor itself. The flux from the stator will cut the coil in the rotor and since the rotor coils are short circuited, according to Faraday's law of electromagnetic induction,  current  will start flowing in the coil of the rotor. When the  current  will flow, another flux will get generated in the rotor. Now there will be two flux, one is stator flux and another is rotor flux and the rotor flux will be lagging w.r.t to the stator flux. Due to this, the rotor will feel a torque which will make the rotor to rotate in the direction of rotating magnetic flux. So the speed of the rotor will be depending upon the ac supply and the speed can be controlled by varying the input supply. This is the working principle of an induction motor of either type – single and three phase. .

Types Induction Motor

SINGLE PHASE INDUCTION MOTOR
 Split phase induction motor
Capacitor start induction motor
Capacitor start capacitor run induction motor
Shaded pole induction motor
THREE PHASE INDUCTION MOTOR
Squirrel cage induction motor
 Slip ring induction motor
We had mentioned above that single phase induction motor is not a self starting and three phase induction motor is self starting. So what is self starting? When the machine starts running automatically without any external force to the machine, then it is called as self starting. For example we see that when we put on the switch the fan starts to rotate automatically, so it is self starting. Point to be noted that fan used in home appliances is single phase induction motor which is inherently not self starting. How? Question arises How it works? We will discuss it now.

Why is Three Phase Induction Motor Self Starting?

In three phase system, there are three single phase line with 120° phase difference. So the rotating magnetic field is having the same phase difference which will make the rotor to move. If we consider three phases a, b and c, when phase a is magnetized, the rotor will move towards the phase a winding a, in the next moment phase b will get magnetized and it will attract the rotor and then phase c. So the rotor will continue to rotate.

Working Principle of Three Phase Induction Motor

Why Single Phase Induction Motor is not Self Starting?

It will be having only one phase still it makes the rotor to rotate, so it is quite interesting. Before that we need to know why single phase induction motor is not a self starting motor and how the problem is overcome. We know that the ac supply is a sinusoidal wave and it produces pulsating magnetic field in uniformly distributed stator winding. Since pulsating magnetic field can be assumed as two oppositely rotating magnetic fields, there will be no resultant torque produced at the starting and due to this the motor does not run. After giving the supply, if the rotor is made to rotate in either direction by external force, then the motor will start to run. This problem has been solved by making the stator winding into two winding, one is main winding and another is auxiliary winding and a capacitor is fixed in series with the auxiliary winding. This will make a phase difference when  current will flow through the both coils. When there will be phase difference, the rotor will generate a starting torque and it will start to rotate. Practically we can see that the fan does not rotate when the capacitor is disconnected from the motor but if we rotate with hand it will start to rotate. So this is the reason of using capacitor in the single phase induction motor. There are several advantages of induction motor which makes this motor to have wider application. It is having good efficiency up to 97%. But the speed of the motor varies with the load given to the motor which is an disadvantage of this motor. The direction of rotation of induction motor can easily be changed by changing the sequence of three phase supply, i.e. if RYB is in forward direction, the RBY will make the motor to rotate in reverse direction. This is in the case of three phase motor but in single phase motor, the direction can be reversed by reversing the capacitor terminals in the winding.