EEE Important Short Note for Interview-2

(1) Electromotive Force (emf)

Electromotive Force (emf): The force or electrical pressure that has the potential to cause electron flow in a closed circuit. Also called voltage, potential difference or difference of potential. Measured in volts (V).

(2) Commutator

A cylindrical arrangement of insulated metal bars connected to the armature coils of a direct-current electric motor or generator, providing a unidirectional current from the generator or a reversal of current into the coils of the motor.

(3) what is meant by armature reaction?

The effect of armature flux to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

(4) What is the Polarization index value ?

Polarization index (PI) is a ratio between insulation resistance(IR)i.e megger value for 10min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5

(5) Why Delta Star Transformers are used for Lighting Loads?

For lighting loads, neutral conductor is must and hence the secondary must be star winding and we know that, this lighting load is always unbalanced in all three phases. To minimize the current unbalance in the primary we use delta winding in the primary. So delta / star transformer is used for lighting loads or electrical power distribution system.

(6) What are the advantages of star-delta starter with induction motor?

The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times Of current of Direct online starting.

(7) Operation of Thermal power station?

The water is obtained in the boiler and the coal is burnt so that steam is obtained this steam is
allowed to flow the turbine , the turbine shaft which is coupled with the generator shaft, generator generates the
electricity.

(8) Demand Factor & Load Factor

Demand Factor: It is defined as the ratio of actual maximum demand made by the load to the rating of the connected load.

Load Factor: It is defined as the ratio of the average power to the maximum demand.

(9) Why capacitor allow ac and block dc?

Current Through a Capacitor is proportional to rate of change of Voltage (I=C*dV/dT) . But in the case of dc supply, Voltage is constant. Hence the capacitor is zero in dc system.

(10) Power factor(PF)

Power factor(PF): The ratio of active power to the apparent power in a circuit, especially those that are not purely resistive. It is also equal to the cosine of the phase angle for the circuit.

(11) Root mean square (RMS) value

Root mean square(RMS): The value of an ac sine wave that indicates its equivalent dc value for
producing heat. Also called the effective value, it is equal to 70.7% of the maximum value (peak value) for the
waveform.

(12) Resonance condition

Resonance condition: A circuit condition when the inductive reactance (XL) equals the capacitive reactance (XC). A resonant circuit is one that has been tuned to that condition and resonant frequency is that frequency that resonance occurs in a circuit and provides a maximum output for one of its circuit variables. Power factor is unity in Resonance condition.

(13) what is Ferranti Effect?

Output voltage is greater than the input voltage/Sending end Voltage or receiveing end voltage is
greater than the sending end voltage. This phenomenon is known as Ferranti Effect.

(14) What is the difference between earth and neutral?

Neutral is the return path of the conductor through which current flows back to the system, however earth is used for protection against high fault currents. When the current is very high it flows through earth and bypass the equipment thus protecting it.

(15) Float & Boost Battery Charging

We know that, in Battery Charger keeps a switch for Float & Boost Battery Charging and remarks it
“Float” & “Boost”. Float charging is used where the battery rarely gets discharged.  A typical application where float charging can be used would consist of the float charger, battery and the load in parallel.  During normal operation, the load draws the power from the charger.

Boost charging involves a high current for short period of time to charge the battery.  It is generally if the battery has been discharged heavily.  Boost charge enables the quick charging of depleted batteries.

(16) Semiconductor

Semiconductor: Materials whose conductivity falls between those of conductors and insulators are called Semiconductors. Silicon is the most common material used to build semiconductor devices.

(17) Transducer

Transducer A device used to convert input energy of one form into output energy of another.

(18) Pulse Coded Modulation(PCM)

Analog voice data must be translated into a series of binary digits before they can be transmitted (in Voice communication system). With Pulse Code Modulation (PCM), the amplitude of the analogue signal is sampled at regular intervals and translated into a binary number.

(19) Ripple

The small amount of ac voltage that still remains in the output of a dc power supply.

(20) Potentiometer

A three-terminal resistor with an adjustable center connection generally used to control voltage;
widely used for volume control in radio and television receivers. In this sense, also called a Pot.

(21) Bit & Byte

A single binary digit, can be either a 0 or 1. A group of eight binary bits, commonly used to represent digital data.

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