# Electrical Engineering Interview Questions

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#### UPF and LPF Wattmeters

How is UPF Wattmeter different from LPF Wattmeter?

#### Siva

• Apr 20th, 2017

LPF Watt meters are specially designed to measure the power at low power factor (inductive circuits).

UPF type Watt meters are not so accurate in measuring power at low factor (resistive circuits).

#### sharath

• Oct 28th, 2011

LPF watt meter is used for measuring power in highly inductive circuits & UPF watt meter is used for measure power in resistance circuit in which the power factor ranges from 0.5 to unity.

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#### Half Controlled and Full Controlled Rectifier

What is the difference between half controlled and full controlled rectifier? Why we go for full controlled while half controlled can control voltage?

#### k.r.kartheeswari

• Apr 10th, 2017

In Half controlled rectifiers, the controlled output will be in one half cycle, so the efficiency is less. But in full controlled rectifies, the controlled output will be in both the half cycle since both the sides having SCRs. So efficiency also doubled compared to half controlled.

• Feb 4th, 2016

Diode is uncontrolled and SCR is fully controlled element. In SCRs we can turn ON and OFF by gate pulse or by the other means. Whenever the diode is used in rectifier, the rectification is done by the...

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#### ravindra kushwah

• Mar 30th, 2017

The transformer works on the principal of mutual inductance for which current in one coil must change uniformly. if DC supply is given, the current will not change due to constant supply and transform...

#### Ash

• Feb 17th, 2017

Instead if DC if you apply Pulse DC will it be ok

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#### DC Pulse when applied to Transformer

When DC is applied, Transformer goes in to saturation, What happens if DC Pulses are applied, will this be safe? Will it act as transformer?

• Mar 29th, 2017

If we apply DC to the transformer, the nature of DC is constant magnitude so constant flux generate in primary winding there is no change in flux so EMF will not induce excepting when switch ON and switch OFF. If DC is continually supplied to the primary it will burn.

#### Ravi

• Mar 7th, 2017

When DC applied to Transformers the induced emf is zero in primary because of steady current, which leads to heavy current flow coz only coil resistance is connected directly to the source. Which dama...

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### Editorial / Best Answer

• Dec 23rd, 2006

100 kw is the full load rating of the generator..... ..that means it can supply 100 kw without exceeding in specified temperature limits .that doesnt mean that it wont be able to supply a load more than 100 kw,it can but not for a longer period of time.after sometime the generator will burn out.if a load of 50 kw is connected to generator the generator will generate only 50 kw& not more than that.the generator will generate power only according to the requirement.

#### AJAI VAIDYANATHAN H

• Mar 21st, 2017

The generator is designed to generate power at 50Hz frequency. If you are going to load your 100Kw generator with only 50Kw, then the input should be a little more than 50KW based on the generator eff...

#### cfanez

• Jul 14th, 2011

Generator operating at 50Kw of rated value of 100kW will work however,it not appropriate operating a generator below optimal capacity. It is better to load generator at about 80% of its rated capacity...

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#### Load Increases when Power Generation is Constant

What happens if load increases when the generation is constant?

#### Ravi

• Mar 7th, 2017

The excess load draws more current from source. since the the power is constant in source side, voltage on load side drops and losses are more and power factor reduced.

#### Rizwan

• Jan 22nd, 2017

If load increases and generation is constant then power factor decreases.

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#### How does a generator generate reactive power

Does it absorb reactive power when ever it is needed ?

#### Amit kalyan

• Mar 5th, 2017

If there is excessive reactive reverse power the turbine trips.

• Feb 4th, 2016

It denotes the apparent power by VA/kVA/MVA. The active power is thus VIcos(phi) and reactive power is VIsin(phi). So the reactive power is nothing but apparent power sin component. The machine automa...

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#### DC-DC Flyback Convertor

What is the flyback convertor?

#### Harita

• Feb 18th, 2017

Flyback converter is a buck boost converter used in the SMPS (Switch Mode Power Supply, small size and large frequency)

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#### Measure Reactive Power

Can we measure reactive power by using only a single wattmeter? If Yes, How?

#### Vamsi Krishna

• Feb 15th, 2017

Why single wattmeter is enough to measure 3 phase reactive power? Explain it elaborately.

#### merna eshak

• Nov 11th, 2015

Of course we can do it! by just one watt meter.

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#### Constant Frequency Grid

Why frequency should remain constant in Grid? If it varies then what will happen to other generators?

#### Ashu the Sharp

• Feb 2nd, 2017

Because Power will vary and rest of generators will loose sync

#### Jay-quellin

• Feb 8th, 2016

It synchronizes the generators, which would run out-of-phase if they dont all run at the same frequency. Power would not be constant. Imagine a digital and analog clock. The analog clock will keep tra...

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#### Variable Frequency Passed through Transformer

1) What does active, reactive and apparent means practically?
2) What happens when variable frequency is passed through transformer?

#### tariq faridi

• Jan 25th, 2017

If variable frequency is given to transformer then the EMF generated is variable due to which load varies a lot with change in voltage hence it will damage the windings of transformer...

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#### How will you differentiate the transformer

How will you differentiate the transformer is LT or HT transformer whether belongs to Voltage range or MVA ratings. Explain?

#### B Vinod kumar

• Jan 21st, 2017

Depending upon the rating of transformer we can say, is that transformer belongs to LT or HT and we have to talk about transformer in VA only not in voltage of watts because, it is load less ie., There will be no power factor.

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#### Hashim

• Jan 19th, 2017

What happens on generator connected with grid if grid frequency increases or decreases

#### expertnayak

• Jun 12th, 2009

Pole slipping typically occurs under severe fault conditions which cause a transient torque on the generator which exceeds the ability of the field to hold the generator rotor synchronised to the stat...

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#### Electric Voltage AC

for 3 phase AC supply Frequency= 50Hz, V =440 between 2 phase; then V = ? between phase and netural.

#### Anoar Hossain

• Jan 15th, 2017

254V

`Code1v1nn`

#### Srinivasan

• Nov 15th, 2016

230

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### Editorial / Best Answer

KaranSorout

• Member Since Jul-2006 | Jul 13th, 2006

The power ratio between Star to Delta is 1:3

Explanation:

LetÂ’s suppose Vs be the supply voltage per phase.

So the line voltage of the supply will be Ã–3Vs.

Now assume any type of load; for simplicity IÂ’m assuming it a only resistive load.

And let it be Â‘RÂ’ per phase.

For Delta connected load:

Calculation for per phase power; PD= I2R

Where I Ã  load current (per phase)

And,

I = Ã–3Vs/R       {as line voltage of the supply is directly applied to the phase of the delta load}

So,

Pd = (Ã–3Vs/R )2R = 3Vs2/R  watts per phase.

For 3 phases:

P3D = 3Pd = 3*3Vs2/R = 9Vs2/R watts.

Now for Star connected load:

PS = I2R = (Vs/R)2R = Vs2/R watts

For 3 phases: P3S = 3PS = 3 Vs2/R watts

Conclusion:

P3S / P3D  =  3Vs2/R / 9Vs2/R = 1/3

It may be better concluded by drowing figures. If any variations, can connect to me through karan.sorout@gmail.com

#### Pankaj sawale

• Jan 3rd, 2017

The explanation must be 9Vs^2/R/27Vs^2/R and then it become 1/3

#### Aritra Rakshit

• Mar 23rd, 2016

Load is not same. Because if you connect same impedance in star and again in delta. Equivalent impedance will change per line.

For Delta:
z=z1||(z2+z3)

For Star:
z=z1+z2

so, ultimately power ratio remains unity.

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#### DC Voltage Frequency

What is the frequency of DC Voltage?

#### Mahima solanki

• Dec 25th, 2016

If we talk about pulsating or AC signals then it means some value of frequency but for DC it is 0 hz because the pulsating DC is when passed through filter it becomes pure DC and here we are talking about pure DC.. So its 0

#### Nagendra Swaero

• Nov 22nd, 2016

We know that the frequency is inversely proportional to time period. DC forms a straight line, so there is no cycles and no time period. So there is no frequency.

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#### Derating factors for Cables Calculation

The cable's actual current rating will usually be lower than the standard (un-derated) value quoted by manufacturerâ€™s brochures. How to find the Derating factors for depth of laying, grouping of cables, etc.

#### Mohhamedibrahim

• Dec 21st, 2016

Derating factors
1. Native soil of thermal resistivity
2. If Cable circuit to cable circuit crossing.
3. Wrong sheath bonding.

#### shinu.arora

• Jun 17th, 2010

There are many factors who can derate the cable rating.1. Temperature.2. Group Laying of cable.RegardsShobhit Gupta

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