Some Useful Electrical Equations.

Useful Electrical Equations:

· For Sinusoidal Current: Form Factor = RMS Value/Average Value = 1.11

· For Sinusoidal Current: Peak Factor = Max Value/RMS Value = 1.414

· Average Value of Sinusoidal Current (Iav) = 0.637 x Im (Im = Max.Value)

· RMS Value of Sinusoidal Current (Irms) = 0.707 x Im (Im = Max.Value)

· A.C Current = D.C Current/0.636.

· Phase Difference between Phase = 360/ No of Phase (1 Phase=230/1=360°, 2 Phase=360/2=180°)

· Short Circuit Level of Cable in KA (Isc) = (0.094 x Cable Dia in Sq.mm) /√ Short Circuit Time (Sec)

· Max.Cross Section Area of Earthing Strip (mm2) = √(Fault Current x Fault Current x Operating Time of Disconnected Device ) / K
K = Material Factor, K for Cu = 159, K for Al = 105, K for steel = 58 , K for GI = 80

· Most Economical Voltage at given Distance = 5.5 x √ ((km/1.6) + (kw/100))

· Cable Voltage Drop (%) =
(1.732 x current x (RcosǾ+jsinǾ) x 1.732 x Length (km) x 100) / (Volt(L-L) x Cable Run.

· Spacing of Conductor in Transmission Line (mm) = 500 + 18 x (P – P Volt) + (2 x (Span in Length)/50).

· Protection radius of Lighting Arrestor = √h x (2D-h) + (2D+L).
Where h= height of L.A, D-distance of equipment (20, 40, 60 Meter), L=V x t (V=1m/ms, t=Discharge Time).

· Size of Lighting Arrestor = 1.5x Phase to Earth Voltage or 1.5 x (System Voltage/1.732).

· Maximum Voltage of the System = 1.1xRated Voltage (Ex. 66KV = 1.1 × 66 = 72.6KV)

· Load Factor = Average Power/Peak Power

· If Load Factor is 1 or 100% = This is best situation for System and Consumer both.

· If Load Factor is Low (0 or 25%) = you are paying maximum amount of KWH consumption. Load Factor may be increased by switching or use of your Electrical Application.

· Demand Factor = Maximum Demand / Total Connected Load (Demand Factor <1)

· Demand factor should be applied for Group Load

· Diversity Factor =
Sum of Maximum Power Demand / Maximum Demand (Demand Factor >1)
Diversity factor should be consider for individual Load

· Plant Factor (Plant Capacity) = Average Load / Capacity of Plant

· Fusing Factor = Minimum Fusing Current / Current Rating (Fusing Factor>1).

· Voltage Variation (1 to 1.5%) = ((Average Voltage – Min Voltage) x 100)/Average Voltage
Ex: 462V, 463V, 455V, Voltage Variation= ((460 – 455) x 100)/455 = 1.1%.

· Current Variation (10%) = ((Average Current – Min Current) x 100)/Average Current
Ex: 30A,35A,30A, Current Variation = ((35-31.7) x 100)/31.7 = 10.4%

· Fault Level at TC Secondary
= TC (VA) x 100 / Transformer Secondary (V) x Impedance (%)

Motor Full Load Current = Kw /1.732 x KV x P.F x Efficiency

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Some common Interview questions (general side).

If you will take the time necessary to write out brief answers to each of the questions bellow the mentioned list, it will help you to clarify your own thinking and establish ready answers.

1.How would you describe yourself? Introduce yourself?
2. What are your hobbies?
3.How did you choose this company?
4.What are your long-range career goals?
5.What experience have you had in this type of work?
6.Which accomplishments have given you the most satisfaction?
7.Why did you decide to go to this particular discipline/field/subject /profession?
8.How did you spend your vacation while in university?
9.Are you willing to travel (or move)?
10.Why should we hire you?
11.What things are the most important to you in a job?
12.Who is your favorite person?
13.Tell us an interesting story/memorable event of your life?
14. In what type of position are you most interested?
15. Why do you think you might like to work for our Company?
16. What courses did you like best? Least? Why?
17. What do you know about our company?
18. What qualifications do you have that make you feel that you will be successful in your field?
19. What extracurricular offices have you held?
20. What are your ideas on salary? How much money do you hope to earn at age 30? 40?
21. How do you feel about your family?
22. Do you prefer any specific geographic location? Why?
23. What personal characteristics are necessary for success in your chosen Field?
24. What is your fatherÕs occupation?
25. Are you looking for a permanent or temporary job?
26. Do you prefer working with others or by yourself?
27. What kinds of boss do you prefer?
28. Can you take instructions without feeling upset?
29. How did previous employers treat you?
30. Can you get recommendations from previous employers?
31. Do you like routine work?
32. Are you willing to go where the company sends you?
33. What is your greatest strengths and weakness?
34. What jobs have you enjoyed the most? The least? Why?
35. Would you prefer a large or a small company? Why?
36. Are you interested in research?
37. To what extent do you smoke?

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Interview Questions For Electrical Engineering

Given the highly intelligent and competitive nature of electrical engineering, expect a tough interview process. Although many of the questions will test your technical skills, it is also important to be prepared to respond to inquiries about other aspects of your character that will impact your probability of success as an employee.

Interview Questions For Electrical Engineering

The following list presents non-technical interview questions for electrical engineering you should be prepared to discuss during the interview process (along with a few other helpful hints):

INTERVIEW PREPARATION
Engineering is a meticulous profession that requires a highly detail-oriented person to succeed. The first chance you get to demonstrate these skills is in your preparation for the interview:

• Prepare a curriculum vitae and/or resume that highlights your education and experience
• Reacquaint yourself with the details and highlights of major projects you’ve worked on
• Put together a list of questions you may want to ask the interviewer
• Collect letters of recommendation
• Gather references

COMMON INTERVIEW QUESTIONS FOR ELECTRICAL ENGINEERING
Here are some common questions regarding education and training, personality, career choice, experience, and interests:

Education and training background

• What were your favorite engineering classes? Why?
• How would you describe your educational experience?
• Are you happy with your academic performance?
• What would you change about your academic experience?
• Do you have a license?

Personality traits

• How would you describe yourself as an employee?
• What are your strengths and weaknesses?
• How would go about correcting your weaknesses?
• How would your co-workers describe you?
• How would your friends describe you?
• What type of personality types do you work best with?
• Are you comfortable working in a team?
• Why are you looking for a new job? (if you’re leaving one)

Career choice

• Why did you choose engineering?
• Why did you specialize in electrical engineering?
• What do you like most about electrical engineering? Least?
• What do you think you can contribute to the electrical engineering profession?
• Do you think you’ll still be interested in electrical engineering in ten years? Why?
• What have you done in the last year to improve your knowledge?

Project experience

• Describe your favorite project you’ve worked on.
• What was your role?
• Describe a technical problem you encountered and how you solved it.
• What quality controls did you employ?
• What did you do best while working on this project?
• What could you improve on?
• What did you learn from this project?
• What design tools are you familiar with?
• What is your favorite part of the project lifecycle?
• Do you have any technical writing experience?
• What has been your greatest achievement in engineering so far? 

Future and other interests

• What are your goals as they relate to electrical engineering?
• Do you have any interest/experience in management?
• Have you considered getting an MBA?
• Describe your volunteer or community service activities.
• What non-engineering organizations did/do you belong to?
• What personal goals do you have for you or your family?
• What do you like to do in your free time?

GENERAL GUIDELINES FOR ANSWERING INTERVIEW QUESTIONS

• Use examples – The best way deliver an answer that will impress your interviewer is by backing it up with real-life examples.
• Quote people who know you – If a co-worker once said, “You are the hardest worker I know,” use that statement in an answer.
• Be clear – Organize your answers so they are easy to follow.
• Be yourself – Employers want to know the real you to make sure you can work well together.
• Be honest – The last think you want is an employer to find out that they were lied to. They will respect your honesty.
• Be excited and enthusiastic – You will likely work daily with the people you meet during the interview, so show them that it will be a rewarding experience.
• Show interest – Show the interviewer that you are serious about this job.
• Show respect – Anyone you meet during the interview may have an impact on whether or not you get the job.
• Be curious – The more you know about the job, the better you will be able to determine if you would be happy there.

QUESTIONS YOU MAY WANT TO ASK THE INTERVIEWER

• What is the typical size and scope of the projects you work on?
• Do you specialize in a particular industry or geography?
• What makes you different from other firms?
• How is your firm changing?  Is it growing?
• How do you market your company?
• What are your goals for the company?
• What role would I play in this company?
• What would your goals be for me?

Post Credit: Careerealism 

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What is the Role of Capacitor in AC and DC Circuit?

Role of Capacitor in AC Circuits: 

In an AC circuit, capacitor reverses its charges as the current alternates and produces a lagging voltage (in other words, capacitor provides leading current in AC circuits and networks) 

Role and Performance of Capacitor in DC Circuit: 

In a DC Circuit, the capacitor once charged with the applied voltage acts as an open switch.

Rule of Capacitor in AC and DC Circuit

What is the Role of Capacitor in AC and DC Circuit?

Let’s explain in detail, but we will go back to the basics of capacitor first to discuss the matter.

What is Capacitor?

The capacitor is a two terminal electrical device used to store electrical energy in the form of electric field between the two plates. It is also known as a condenser and the SI unit of its capacitance measure is Farad “F”, where Farad is a large unit of capacitance, so they are using microfarads (µF) or nanofarads (nF) nowadays.

How Capacitor Works?

Working and Construction of a capacitor:

Whenever voltage is applied across its terminals, (Also known as charging of a capacitor) current start to flow and continue to travel until the voltage across both the negative and positive (Anode and Cathode) plates become equal to the voltage of the source (Applied Voltage). These two plates are separated by a dielectric material (such as mice, paper, glass, etc. which are insulators), which is used to increase the capacitance of the capacitor.

When we connect a charged capacitor across a small load, it starts to supply the voltage (Stored energy) to that load until the capacitor fully discharges.

Capacitor comes in different shapes and their value is measured in farad (F). Capacitors are used in both AC and DC systems (We will discuss it below).

Capacitance (C):

Capacitance is the amount of electric charge moved in the condenser (Capacitor), when one volt power source is attached across its terminal.

Mathematically,                 

Capacitance Equation:

C=Q/V

Where,

       C=Capacitance in Farads (F)

       Q=Electrical Charge in Coulombs

       V=Voltage in Volts

We will not go in detail because our basic purpose of this discussion is to explain the role and application/uses of capacitors in AC and DC systems. To understand this basic concept, we have to understand the basic types of capacitor related to our topic (as there are many types of capacitor and we will discuss capacitor types latter in another post because it is not related to the question).

Polar and Non-Polar Capacitor:

Non Polar Capacitor: (Used in both AC and DC Systems)

The Non Polar capacitors can be used in both AC and DC systems. They can be connected to the power supply in any direction and their capacitance does not effect by the reversal of polarity.

Polar Capacitor: (Only used in DC Circuits and Systems)

This type of capacitor is sensitive about their polarity and can be only used in DC systems and networks. Polar Capacitors don’t work in the AC system, because of the reversal of polarity after each half cycle in AC supply.

Types of Capacitors: Polar and Non Polar Capacitors with Symbols

Role of Capacitors in AC Circuits and System:

The capacitor has lots of applications in AC systems and we will discuss few uses of capacitor in AC networks below:

Transformer less power supply:

Capacitors are used in transformer less power supplies. In such circuits, the capacitor is connected in series with the load because we know that the capacitor and inductor in pure form does not consume power. They just take power in one cycle and deliver it back in the other cycle to the load. In this case, it is used to reduce the voltage with less power wastage.

Split phase induction motors:

The capacitors are also used in induction motor to split a single phase supply into a two phase supply to produce a revolving magnetic field in the rotor to catch that field. This type of capacitor is mostly used in household water pumps, Fans, air conditioner and many devices which need at least two phases to work.

Power Factor Correction and Improvement:

There are lots of advantages of power factor improvement. In a three phase power systems, capacitor bank is used to supply reactive power to the load and hence improve the power factor of the system. Capacitor bank is installed after a precise calculation. Basically, it delivers the reactive power which was previously traveled from the power system, hence it reduces the losses and improves the efficiency of the system.

Role of Capacitors in DC Circuits and system:

Power conditioning:

In DC systems, capacitor is used as a filter (mostly). Its most common use is converting AC to DC power supply in rectification (such as bridge rectifier). When AC power is converted into fluctuating (with ripples i.e. not a steady state with the help of rectifier circuits) DC power (Pulsating DC) in order to smooth and filter out these ripples and fluctuation, DC Polar capacitor is used. Its value is calculated precisely and depends on the system voltage and the demand load current.

Decoupling Capacitor:

Decoupling capacitor is used, where we have to decouple the two electronics circuits. In other words, the noise generated by one circuit is grounded by decoupling capacitor and it does not affect the performance of other circuit.

Coupling Capacitor:

As we know that Capacitor blocks DC and allows AC to flow through it (we will discuss it in the next session that how does it happens). So it is used to separate AC and DC signals (also used in the filter circuits for the same purpose). Its value is calculated in such a way that its reactance is minimized on the basis of frequency, which we want to pass through it. Coupling Capacitor is also used in filters (ripple remover circuits like RC filters) to separate AC and DC signal and removes the ripples from pulsating DC supply voltage to convert it into pure AC voltage after rectification.

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What is the Difference Between Neutral, Ground and Earth?

The Main difference between Neutral, Ground and Earth?

‪

To understand the difference between Neutral, Ground and Earth, we must understand the need of these things first.

Neutral

Neutral is return path for an AC circuit which is supposed to carry current in normal condition. This current may be because of many reasons, primarily because of phase current imbalance and some time because 3rd and 5th harmonics also.

There may be others reasons too but the magnitude of this current is in fraction of phase current and in few case it can be even double of phase current. So Neutral wire is always assumed to be charged (in active circuit). This neutral wire is given to ground (by grounding) to make the second terminal of neutral wire at zero potential.

Earth or Ground

Earth or Ground is for safety concerns against leakage or residual currents on the system via least resistance path. While phase and neutral is connected to main power wiring, earth may be connected to body of equipment or to any system which in normal condition doesn’t carry current but in case of some insulation failure, is supposed to carry some minor current.
This current is not directly coming from live or phase wire, but is from secondary links which was not in touch with live system in normal condition. This current is usually much lesser than main line current or phase current and mostly is in order of mA. But this leakage current is good enough to kill someone or may risk fire. Such current are being provided a low resistance path and sent to earth via earth wire.
Because of the difference in application we never mix grounding of neutral and earth. However both are made grounded (of-course the process may be different). If both will be mixed then the earth wire which is not supposed to carry any current in normal condition , may have some charges across and will become hazardous.

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What is the difference between a battery and a capacitor?

While batteries and capacitors have similarities, there are several key differences. Like -

• Electrical energy in a Capacitor is stored in an electric field, where a Battery stores its electrical energy in a chemical form. 

• When a battery is discharging it, can be slower than a capacitor's ability to discharge because there is a latency associated with the chemical reaction to transfer the chemical energy into electrical energy. A capacitor is storing the electrical energy directly on the plates so discharging rate for capacitors are directly related to the conduction capabilities of the capacitor plates. 

• A capacitor is able to discharge and charge faster than a battery because of this energy storage method also. But unlike a battery that can turn its electrical current on and off, once a capacitor is connected to an outside circuit it will discharge as fast as it can until all the charge is drained. 

• Capacitors are much larger than a battery that stores equivalent charge. 

• The battery runs for longer time, but a capacitor discharges almost instantaneously. 

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Why Power Plant Capacity Rated in MW and not in MVA?

For the following reasons, a Power plant capacity rating may be expressed in MW instead of MVA:

In a Generating station, the prime mover (Turbine) generates only and only Active Power. That’s why we rated a power plant capacity in MW instead of MVA. It's mean no matter how large your generator is, but it depends on the capacity of the  engine (Prime mover/Turbine) I.e. a 50MW turbine connected to a 90MVA alternator in a power plant will generate only 50MW at full load. In short, a power plant rating is specified in terms of prime mover /Turbine (Turbine rating may be seen by nameplate rating which is in MW or Horsepower (HP) not in MVA) and not by the alternator set coupled to it.

Another thing is that, electric power company charges their consumer for kVA while they generate kW (or MW) at the power station (Power plant).They penalize their consumer for low Power factor because they are not responsible for low power factor and kVA but you. Moreover, in power plant, power factor is 1 therefore MW is equal to MVA …… (MW = MVA x P.f).

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Why Motor rated in kW instead of kVA?

Why Motor rated in kW/Horsepower instead of kVA?

We know that Transformer rating may be expressed in kVA as well as Generator andAlternator rated in kVA Designer doesn’t know the actual consumer power factor while manufacturing transformers and generators i.e. the P.F (Power factor) of Transformer and Generator/Alternator depends on the nature of connected load such as resistive load, capacitive load, and inductive load as Motors, etc. But Motor has fixed Power factor, i.e. motor has defined power factor and the rating has been mentioned in KW on Motor nameplate data table. That’s why we are rated Motor in kW or HP (kilowatts/ Horsepower) instead of kVA.

In addition, Motor is a device which converts Electrical power into Mechanical power. In this case, the load is not electrical, but mechanical (Motor’s Output) and we take into the account only active power which has to be converted into mechanical load. Moreover, the motor power factor does not depend on the load and it works on any P.F because of its design.

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Why AC rated in Tons, Not in kW or KVA? A Guide about Air-conditioning system and Refrigeration.

Why AC rated in Tons, Not in KW?

AC (Air-conditions and Refrigeration are always rated in Tons.

Air conditioners are always rated in Tons capacity instead of kW because Air conditioners are designed on the basis of quantity of heat removal from room, hall or specific area. Quantity of heat is termed in Tons means if an air conditioner is able to remove 1000 kilocalories of heat or 4120 kilojoules or 12000 BTU of heat in an hour that AC rated as 1 Ton of AC because 1000 Kilocalories or 4120 kilojoules or 12000 BTU equal to one Ton of heat. Also, this is the same case for freezer and refrigerator i.e. refrigeration system.

Good to know:

BTU = British thermal unit. A measurement of heat, specifically, the amount of heat needed to raise the temperature of a pound of water by 1°F.

Definition of Ton

A Ton of refrigeration (RT) is approximately equivalent to 12,000 BTU/h or 3,516.8528 W or 4.7142Hp.

A Ton of refrigeration (RT) is a unit of power used to describe the heat-extraction capacity of air conditioning and refrigeration equipments. It is defined as the heat of fusion absorbed by melting 1 short ton of pure ice at 0 °C (32 °F) in 24 hours.

How many kW and HP are there in 1 Ton?

1 Ton = 3.5168525 kW = 4.714Hp

Explanation

1 Ton = 12,000 BTU/h

1 Watt = 3.412141633 BTU/h

1 Ton = 12,000 / 3.412141633 = 3,516.8528 Watts = 3.5168528 kW.

1 Ton = 3,516.8528 Watts = 3.516 kW.

Also

1 Ton = 3,516.8528W / 746 = 4.7142798928 Hp →→→ (1 Hp = 746 Watts)

1 Ton = 4.714 Hp

How to convert Ton to Kw and vice versa?

One RT(Refrigeration Ton) = 3.5168528 kW…

1 RT = 3.5168528 kW

1 kW = 0.284345 RT(Refrigeration Ton)

1 kW = 0.28434517 RT

So,

The power P in kW = Power P in RT (Refrigeration Ton) times 3.5168528….

P(kW) = P(RT) × 3.5168528

Example

Convert 3 Ton AC into kW i.e. Convert 3 RT to kW.

Solution:

P(kW) = 3 RT × 3.5168528

P(kW) = 10.55 kW

3 Ton AC = 10.55 kW

How much Current in Ampere will a 2 Tons AC draw in Single Phase & Three Phase System?

Suppose, There are 230V and Power factor = Cosθ = 0.95 in Single Phase AC system…

1 Ton = 3,516.8528 Watts = 3.516 kW.

2 Ton = 2 x 3.516 kW = 7.032kW = 7032W

Power in a Single Phase AC System

P = VxI Cosθ and current…

I = P / (V x Cosθ)….. Where Cosθ = Power factor

I = 7032W / (230V x .95)

I = 32.18 A

Therefore, a 2 Ton AC (Air-condition in Single Phase AC system will take 31.18 Ampere Current

Andin Three Phase System

Suppose, There are 440V and Power factor = Cosθ = 0.85 in Three Phase AC system…

Power in a Three Phase AC System

P =√3 x VLxIL Cosθ and current….

I = P /( √3xVxCosθ)

I = 7032W / (1.732 x 440V x .85) Where Cosθ = Power factor and √3 = 1.732

I = 10.855 A

Therefore, a 2 Ton AC (Air-condition in Three Phase AC system will take 10.855 Ampere Current

Good to Know: This is just calculation based on Electrical formulas. In real, Air conditioner current depends a lot on operating conditions such as ambient temperature, refrigerant pressure, Energy Efficiency Ratio (EER) etc. for instance, if EER is 6, then input power for 2 Tons Air conditioner is 24000BTU/ 6 = 4000 watts.. 

If this is a 230 volt system, then air conditioner load current would be = 4000/(230x.95) = 18.5 A

For More detail…Check the Air conditioner Name plate rating.

Another similar rating is Coefficient of power (COP) which is the output power in watts divided by input power, so with a COP = 1.8, for instance, input power for 2 Tons Air conditioner  is 7032W / 1.8 = 3906 watts. Now you can find current by using the above method which is equal to 18A approx.

How many 2 Ton A.C (Air conditioner) can I run on a 25 kVA Generator?

2 Ton = 2 x 3.516 kW = 7.032kW = 7032W

The Efficiency of Utility Power Generator is 90% approximately.

Efficiency of Generator = 25kVA x (90/100) = 22.5kVA

Now the Number of 2 Ton AC (Air conditioners) which you can run on a 25 kVA Generator smoothly..

22.5kVA / 7032W

= 3

So you can run Three Air conditioners of 2 Tons each on a 25kVA Generator.

What is the suitable rating of MCB for 2 Ton and 1 Ton AC (Air conditioner) and why?

As we have calculated the load current for 2 Ton AC Air conditioner…

Calculated Current for 2 Ton A.C = I = 32.18 A

Now 40A Class “C” MCB (miniature circuit breaker) would be suitable for 2 Ton AC (air-condition) because in starting time it takes more current of the full load current

And 20 A Class “C” MCB would be better for 1 Ton AC (air-condition)

Good to Know:

Class “’C’ Type MCBs

Class “C” Type MCBs are suitable for installations with high inrush of current at the starting switching time. in other words, equipment and devices having inductive loads such as air-conditioners, induction motors, fluorescent lamps, transformers etc.

A general AC (Airconditioner ) Name plate rating Data:

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