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July 28, 2012

Can a transformer be used to transform direct voltage and direct current? Justify your answer.

The transformer cannot be used for dc supply system (dc voltage & current). According to working principle of the transformer, it works on Faraday laws of Electromagnetic Induction. Therefore, induced emf e = (-dφ/dt). The changing rate of magnetic flux is responsible for e.m.f generation, which opposes the change of magnetic flux that is indicated by negative sign. But in case of dc supply system (d.c Voltage & Current), there is no change of magnetic flux, d φ = 0, so that induced emf at secondary side of transformer is zero. Therefore, we cannot use transformer for dc supply system.

Why are starters used for starting 3-phase induction motors?

Starters for three phase induction motors:
For starting of 3-φ induction motor, we need Y-∆ (star – delta) starter. These starters are useful for safe running of induction motor by applying 1/3 times of voltage in Y- mode for safety of induction motor. When motor gets 80% of speed we apply full voltage Vph=VL in ∆ mode; during this time sufficient back e.mf is developed to limit the input current. 

What are the Applications of Induction Motors?

Applications of Induction Motors:
The squirrel cage induction motors with single or double pole changing windings are available as follows:

(1)  Variable torque, power output α N2
Fans, centrifugal pumps.
(2) Constant –torque power output α N.
Conveyors, stokers, reciprocating, Compressors, printing presses.
(3) Inverse torque power output rating constant.
Machine Tools, lathes, boring mills, drill, planers.
The Multi-speed motors are of slip ring type used for hoist, conveyor and elevator.

July 13, 2012

The three phase system has only two types of connections viz. star connection and delta connection. Why?

In the three phase system, the three phases should be so connected that we get a system that has equal line voltages displaced 120 degrees electrical from one another and acting simultaneously in the circuit. Only star and delta connections can satisfy these conditions. Any attempt made to connect the three phase in a manner other than these two schemes will result in a system of line voltages which are neither equal nor 120 degree displaced. Such a system will not serve the function as if three phase circuits being connected.
-V. K. Mehta

July 10, 2012

What are the criteria for the classification of transmission lines as short, medium and long lines?

Classification of transmission lines (Power):
The network of transmission lines is formed by three phase AC system. This is required for Bulk power transfer from large group of generating stations to main transmission network.

Short power transmission line
Short distance power transmission lines has voltage ≤  11000 volts. These lines are required to carry power from main sub-station to local distribution area. Further power supply is distributed through pole mounted sub- station or plinth mounted sub-station.

Medium power transmission line
Medium distance power transmission line has voltage range 33-132 kV. For the economic voltages for medium lines an approximate rule is 1 kV/mile or 0.6 kV/km. kV= 5.5 √km x 0.6 + kW/100. These lines supply power directly to big organization or industry, which establishes own sub-station. 

Long power transmission line:
A long distance power transmission line has voltage range 220 kV to 750 kV. These lines are used for transfer of power from sending end to receiving end or for system interconnection for exchange of power between independently controlled networks. For longer transmission lines, higher power transmission voltages are necessary (p α v^2). Higher voltage gives lesser current, lesser I^2R losses, higher power transferability.

Give a list of the factors involved for selecting a factory drive.

Factors involved in selection of factory drives: 
  - Supply system      :  DC or AC
  - Rating                :  kW, MW
  - Drive                 :  constant speed, Variable speed
  - Special Condition  :  High starting torque, Hazardous location, Traction duty etc.
  - User’s requirement : Duty cycle, noise level, load characteristics etc
off load, load condition      
  -  Enclosures & cooling requirement.
  - Environment condition
  -  Applicable standards
  -  Cost considerations 

What are the essential differences between H.V. and L.V. switchgears?

H.V. switch gears: 
Circuit breaker: Switching during normal and  abnormal conditions, interrupt the fault currents. 

Isolators: It is disconnecting switch to disconnect the system from line parts under no load condition. 

Earth switch: This is used to discharge the voltage on the lines to earth after disconnecting them. 

Surge arrester: This is used to divert the high voltage surge to earth and maintaining continuity during normal voltage. 

Current Transformer: To step down the current for measurement, protection and control purposes. 

Potential Transformer: To step down the voltage for the purpose of protection, measurement and control. 

L.V. Switchgears: 
MCBs (Miniature Circuit Breakers): Switching OFF during abnormal conditions to interrupt the fault current. 

Fuses: A short length wire having low melting point, connected in series with circuit. In the event of fault, the circuit current rises abruptly and fuse wire melts to interrupt the circuit. 

Switches: these are used to ON /OFF  the power of  a circuit. These are used in power/ control circuits.  The switches are specified as per voltage rating, current rating, number of poles, duty cycle and fault interruption capacity. 

What do you understand by electric heating? State its advantages over other methods of heating.

Electric heating:

Electric heat is provided by use of a high temperature wire, which is resistant to electric flow with more heat being produced by more electric current being fed into the heating element, creating more resistance and more heat. This method of producing heat is called electric heating. 

Advantage of electric heating: 

  1. Cleanliness 
  2. Absence of flue gases, soot. etc
  3. Accurate control of temperature. 
  4. Low maintenance cost 
  5. Quicker operation 
  6. Higher efficiency 
  7. Uniform and economic heating capability 

Discuss the criterion for choice of voltage for transmission and distribution.

Criterion For Choice Of Voltage For Transmission And Distribution:

We know that the power  P =VI, so for the same power if voltage is increased then current will decrease. If suppose power required is 500 W and voltage is 250V then current I will be equal to 500/250 =2A. 

Now if voltage is increased from 250V to 500V and the power required is same then I = 500/500 =1 A 

Now, if the voltage is still increased to say 1000V for the same power 
then I = 500/1000 =0.5 A 

From the above examples, we see that if the voltage is doubled, the current will be halved and when it is quadrupled then current is reduced to 1/4th of its initial value.

Thus we conclude that if voltage is increased to n times then the current will be  reduced to 1/nth times for the same power.With the reduction of the current to 1/nth times, the conductor area will also be reduced to 1/nth times  of its original area for the same current density, Hence less material is required when the voltage is increased.

We also know that when the current passes through any conductor, there is loss of power in that particular conductor according to the relation, I^2R. As the loss is proportional to the square of the current. So if the current is reduced to ½ value , then the loss will be reduced to 1/4th its original value. Hence the efficiency of the transmission line and all others equipments associated with  the line will increase and more power will be available for use. 

When current is passing through a conductor there will be a voltage drop according to the relation V=IR. So, when the current is reduced the drop of the voltage is less in the line, of course with the same cross sectional area of the conductor. 

With the reduction of cross sectional area, considered the main advantage of transmitting electrical energy at very high voltage viz 132kV , 220kV or even 400kV.  But in case of distribution system such high voltage is dangerous, so distribution voltage is generally 400/230V. 

July 6, 2012

List out the advantages of nuclear power plants over conventional thermal power plants.

Advantage of Nuclear Power Plants:

1. Nuclear Power station reduces the demand for oil coal and gases.

2. Weight of nuclear fuel required for a given station capacity is negligible compared to that required for a conventional thermal power station. As a result the problems of transport and resulting costs are avoided. 

3. The area and volume need for a nuclear power station is less in comparison to a conventional power plant of equal capacity. 

4. In addition to producing large amount of power, a nuclear power plant produces valuable fissile material, which is extracted when the fuel is renewed.

Compare the induction motor with a transformer.

Comparison of induction motor with a transformer:
Figure: Sketch showing similarity between transformer and induction motor

The transfer of energy from stator to the rotor of an induction motor takes place entirely inductively with the help of flux mutually linking the two. Hence an induction motor is essentially a transformer with stator forming the primary and rotor forming the rotating secondary.

Induction Motor
(1) Secondary is stationary 
(1) Secondary winding is rotating
(2) Secondary is not short circuited
(2) Secondary is always short circuited
(3) No-load current is about 1% of full load 
(3) No-load current is approximately 30 to 50% current (due to low reluctance path   of full current (due to high reluctance of steel core)  of air gap)
(4) emf induced in secondary depends on   K (turns ratio)
(4) Depends on K and slip also
(5) Frequency of primary and secondary currents  are same
(5) Frequency of stator current (f) and rotor current (sf) are not the same.

July 3, 2012

List out the important advantages of HVDC transmission.

Important advantages of HVDC transmission : 
  • Economical in long distance high power transmission.
  • HVDC transmission line has only two conductors.
  • Reactive power losses do not occur in HVDC. 

State the various applications of synchronous motor.

Various applications of synchronous motor : 
  • It is capable of being operated under a wide range of power factors, both lagging and leading. Hence, it can be used for power factor correction purpose. 
  • In textile industry high speed drives are necessary with wide range of control. Variable frequency drives with synchronous motors are used for speed control. 
  • In chemical industry, rubber industry and petrochemical industry, many processes require compressor drives in which direct coupled synchronous motors supplied by frequency converters are used. 
  • An artificial fibre plant needs variable frequency synchronous motor and are supplied by static frequency converters. 
  • Synchronous motor is also useful for voltage regulation.

What are the applications of DC motor?

Applications of DC motors:

Type of motor                
Series Motor                   
For traction work, i.e electric locomotives, Rapid transit system, trolley cars etc
Cranes and hoists
Shunt motor                     
For traction work, i.e electric locomotives, Rapid transit system, trolley cars etc
Cranes and hoists
For intermittent high torque loads
For shears and punches
Elevators Conveyors Heavy planers
Rolling mills: Ice machine: Printing Presses: Air

What are the two advantages of stepper motors? Give a few applications of them .

Advantages of stepper motors:
  • They can be directly controlled by computers, micro-processors and programmable controllers.
  • Precision control of speed without using closed loop feedback. 

i) Operation control:
Such motors used for operational control in computer peripherals, textile  industries, IC fabrications and robotics etc.
ii) Incremental motion: 
Applications requiring incremental motion are typewriters, line printers, tape drivers, floppy disk drivers, numerically controlled machine tools, process-control systems and X-Y plotters.
iii) Others:
It includes commercial, military and medical applications where these motors  perform functions as mixing, cutting, striking, metering, blending and purging.  They also take part in manufacture of packed food stuffs, commercial end products and even the production of science fiction movies.

Why does a practical transformer draw some current when its secondary winding is open?

Transformer practically draws some current when its secondary winding is open due to primary circuit and this current will produce core losses. 

What is a Stepper Motor? What are the application of stepper motor?

Stepper Motor
The stepper motor is a motor which takes input pulses and then takes proportional steps to input these signals. You can use for positioning and/or speed control for most any applications.
Stepper motor with cable

The stepper motor needs a power circuit and a sequence circuit for changing phase.

  • Medical: Infusion Pump, Brad Test, X-ray, Dialysis, DNA Extraction, Pump 
  • Semiconductor: Masking, Etching, Wafer Handling, Test-equipment, Die-Bonder 
  • FA: Pick & Place, NC Machine, X-Y-Z stage 
  • OA: Printer, Fax, Scanner 
  • HA: HVAC, Cleaner Robot 
  • Other: Vending machine

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