Why we use 3-phase supply? Why are not 4 or something else?

Why we use 3-phase supply? Why are not 4 or something else?
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1 Answer

Answer :

Three phase power has several advantages over single phase power, including smaller transformer size and weight, and simplified motor construction. Four phase power, which is actually two phase power with the phases in quadrature (90° apart), doesn't really offer any advantage over three phase power but requires an extra conductor. Some experimental work has been done with 6 and 12 phase power, but these have never been put into common usage. When you consider the cost of long transmission lines from the power source, three phase power is the best alternative.
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Related questions

Why do we use 3 phase power supply for heavy loads?

Description : Why do we use 3 phase power supply for heavy loads?

Last Answer : With a single phase, the voltage drops to 0 every half-cycle. Therefore, the amount of power you can get out of the line is not constant over time; it has a very large 120-hz ripple. With ... this can be accomplished, but in every case there are drawbacks that are not present in the 3-phase system

When testing for blown fuses in a three-phase supply circuit to a motor, you should first ___________________. A. apply the megger across the tops of the line fuses B. apply an ammeter diagonally across the top of the first line fuse and the bottom of the third line fuse C. apply the voltage tester across the bottoms of the line fuses D. ensure the circuit is de-energized, and then use a continuity tester

Description : When testing for blown fuses in a three-phase supply circuit to a motor, you should first ___________________. A. apply the megger across the tops of the line fuses B. apply an ammeter diagonally across ... of the line fuses D. ensure the circuit is de-energized, and then use a continuity tester

Last Answer : Answer: D

In conversion of 3-phase to 2-phase supply with T connection, the transformation ratio of the teaser will be__ times to that of main transformer: a) 1.732 b) 1.15 c) 0.866 d) Unity

Description : In conversion of 3-phase to 2-phase supply with T connection, the transformation ratio of the teaser will be__ times to that of main transformer: a) 1.732 b) 1.15 c) 0.866 d) Unity

Last Answer : Ans: C

If a three-phase induction motor is operated under a light load and it develops an open in one of its supply lines, the motor will _____________. A. continue to run, but will vibrate and have reduced torque B. speed up due to the reduced number of poles C. run cooler due to reduced current flow D. stop

Description : If a three-phase induction motor is operated under a light load and it develops an open in one of its supply lines, the motor will _____________. A. continue to run, but will vibrate and have reduced ... speed up due to the reduced number of poles C. run cooler due to reduced current flow D. stop

Last Answer : Answer: A

Which of the listed sections of an emergency switchboard is used to supply power for alarm signals under emergency conditions? A. The generator and bus transfer section B. The 450 volt, 60 cycle, 3 phase bus C. The 120 volt, 3 phase, 60 cycle bus D. The 24 volt DC bus

Description : Which of the listed sections of an emergency switchboard is used to supply power for alarm signals under emergency conditions? A. The generator and bus transfer section B. The 450 volt, 60 cycle, 3 phase bus C. The 120 volt, 3 phase, 60 cycle bus D. The 24 volt DC bus

Last Answer : Answer: D

AC and DC generators are similar in that they _____________. A. both generate alternating voltages B. both rectify the voltage before delivery C. are constructed at the same physical size for the same kilowatt rating D. both supply three-phase power

Description : AC and DC generators are similar in that they _____________. A. both generate alternating voltages B. both rectify the voltage before delivery C. are constructed at the same physical size for the same kilowatt rating D. both supply three-phase power

Last Answer : Answer: A

In a three-phase, squirrel-cage type, induction motor the rotating magnetic field is established by the _____________. A. current induced in the rotor windings B. application of a three-phase voltage supply to the stator windings C. laminated steel core and aluminum conductors in the rotor D. interaction of the magnetic field caused by the induced current in the squirrel-cage bars with the magnetic field of the stator

Description : In a three-phase, squirrel-cage type, induction motor the rotating magnetic field is established by the _____________. A. current induced in the rotor windings B. application of a three-phase voltage ... caused by the induced current in the squirrel-cage bars with the magnetic field of the stator

Last Answer : Answer: B

Erratic operation of the devices represented in the diagrams shown in the illustration could be traced to ____________. EL-0092 A. improper contact at 'R' slip rings or 'S' connections B. a low three-phase voltage supply C. improper contact at 'S' slip rings or 'R' connections D. a high three-phase voltage supply

Description : Erratic operation of the devices represented in the diagrams shown in the illustration could be traced to ____________. EL-0092 A. improper contact at 'R' slip rings or 'S' connections B. a low three- ... . improper contact at 'S' slip rings or 'R' connections D. a high three-phase voltage supply

Last Answer : Answer: A

If a three-phase induction motor malfunctions and drops to a single-phase (one supply line open) ____________. A. the motor will continue to run if it is not heavily loaded B. more torque will be developed C. the motor will immediately stop and not be able to be restarted D. the motor will immediately stop and can only be restarted at no load

Description : If a three-phase induction motor malfunctions and drops to a single-phase (one supply line open) ____________. A. the motor will continue to run if it is not heavily loaded B. more torque will be ... be able to be restarted D. the motor will immediately stop and can only be restarted at no load

Last Answer : Answer: A

Figure 'D' of the diagram shown in the illustration has a turns ratio of four to one. If a three-phase 440 volt supply is connected to terminals 'A-B-C', what voltage should develop across terminals 'a-b-c'? EL-0084 A. 64 volts B. 110 volts C. 190 volts D. 762 volts

Description : Figure 'D' of the diagram shown in the illustration has a turns ratio of four to one. If a three-phase 440 volt supply is connected to terminals 'A-B-C', what voltage should develop across terminals 'a-b-c'? EL-0084 A. 64 volts B. 110 volts C. 190 volts D. 762 volts

Last Answer : Answer: A

Figure 'C' of the diagram shown in the illustration has a turns ratio of four to one. If a three-phase 440 volt supply is connected to terminals 'A-B-C', what voltage should develop across terminals 'a-b-c'? EL-0084 A. 64 volts B. 110 volts C. 190 volts D. 762 volts

Description : Figure 'C' of the diagram shown in the illustration has a turns ratio of four to one. If a three-phase 440 volt supply is connected to terminals 'A-B-C', what voltage should develop across terminals 'a-b-c'? EL-0084 A. 64 volts B. 110 volts C. 190 volts D. 762 volts

Last Answer : Answer: C

Figure 'B' of the diagram shown in the illustration has a turns ratio of four to one. If a three-phase 440 volt supply is connected to terminals 'A-B-C', what voltage should develop across terminals 'a-b-c'? EL-0084 A. 64 volts B. 110 volts C. 190 volts D. 762 volts

Description : Figure 'B' of the diagram shown in the illustration has a turns ratio of four to one. If a three-phase 440 volt supply is connected to terminals 'A-B-C', what voltage should develop across terminals 'a-b-c'? EL-0084 A. 64 volts B. 110 volts C. 190 volts D. 762 volts

Last Answer : Answer: B

Figure 'A' of the diagram shown in the illustration has a turns ratio of four to one. If a three-phase 440 volt supply is connected to terminals 'A-B-C', what voltage should develop across terminals 'a-b-c'? EL-0084 A. 64 volts B. 110 volts C. 190 volts D. 762 volts

Description : Figure 'A' of the diagram shown in the illustration has a turns ratio of four to one. If a three-phase 440 volt supply is connected to terminals 'A-B-C', what voltage should develop across terminals 'a-b-c'? EL-0084 A. 64 volts B. 110 volts C. 190 volts D. 762 volts

Last Answer : Answer: B

What is the supply rated o/p if two single phase transformer connect to give three phase o/p from a three phase I/p?

Description : What is the supply rated o/p if two single phase transformer connect to give three phase o/p from a three phase I/p?

Last Answer : A. Each transformer is only capable of supplying 86.6%of its o/p rating

What is the power in 3-phase supply system?

Description : What is the power in 3-phase supply system?

Last Answer : In a single-phase system power P' = VPh * IPh * cosϕ watts. In 3-ϕ system power P' = 3 * VPh * IPh * cosϕ watts. In Star connection, IL = IPh and VPh = VL/√3. Substituting the value of IPh and VPh in ... delta connected is same and power P = √3 * VL * IL * cosϕ watts. So cosϕ = P/ √3 * VL * IL .

A 220/440 V, 50 Hz, 5 KVA, single phase transformer operates on 220V, 40Hz supply with secondary winding open circuited. Then (A) Both eddy current and hysteresis losses decreases. (B) Both eddy current and hysteresis losses increases. (C) Eddy current loss remains the same but hysteresis loss increases. (D) Eddy current loss increases but hysteresis loss remains the same.

Description : A 220/440 V, 50 Hz, 5 KVA, single phase transformer operates on 220V, 40Hz supply with secondary winding open circuited. Then (A) Both eddy current and hysteresis losses decreases. (B) Both ... same but hysteresis loss increases. (D) Eddy current loss increases but hysteresis loss remains the same.

Last Answer : (A) Both eddy current and hysteresis losses decreases.

What happen if we give 220 volts dc supply to the bulb or tube light?

Description : What happen if we give 220 volts dc supply to the bulb or tube light?

Last Answer : Bulbs [devices] for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element.

What Happen If We Give 220 Volts Dc Supply To D Bulb R Tube Light?

Description : What Happen If We Give 220 Volts Dc Supply To D Bulb R Tube Light?

Last Answer : Bulbs [devices] for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element.

what happens if we give 220 volts dc supply to the bulb or tube light?

Description : what happens if we give 220 volts dc supply to the bulb or tube light?

Last Answer : A:Bulbs [devices] for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current ... be so high that it may damage the bulb element. Low dc voltage will not damage the appliances

Why in DG E/F protection, we do not open class IV CB’s or supply CB’s?

Description : Why in DG E/F protection, we do not open class IV CB’s or supply CB’s?

Last Answer : Delta of aux. Transformer prevents E/F currents from grid into DG neutral.  

What happen if we give 220 volts dc supply to bulb or tube light?

Description : What happen if we give 220 volts dc supply to bulb or tube light?

Last Answer : Bulbs or devices for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element.

We use single and three phase generators not two phase generator .Why?

Description : We use single and three phase generators not two phase generator .Why?

Last Answer : In an electrical distribution system power is delivered via three wires, each operating 120 degrees from the other. This is a 3-phase system. As the voltage gets lowered to residential areas, there is ... the two coils in parallel you can create a higher output 120V system which is single phase.

Why we are using only 50 Hz for single and three phase electrical instruments?

Description : Why we are using only 50 Hz for single and three phase electrical instruments?

Last Answer : Synchronization condition: There are five conditions that must be met before the synchronization process takes place. The alternator must have equal line voltage, frequency, phase sequence, phase angle and waveform to that ... in the US and North America and 50 Hz in most of the rest of the world.

Where do we require single-phase induction motors?

Description : Where do we require single-phase induction motors?

Last Answer : Single-phase induction motors are required where (i) 3 phase supply is not available (ii) Efficiency is of lesser importance (iii) Rating is less than one H.P. (iv) Equipment is portable

Can I use 3 phase wattmeter in the single phase supply?

Description : Can I use 3 phase wattmeter in the single phase supply?

Last Answer : The three-wire, three-wattmeter connection has the advantages of indicating the power in each individual phase (not possible in the two-wattmeter connection) and phase to neutral voltages. In a single-phase system there are just two

How much power is needed to supply the losses associated with the production of the magnetic field in a 3-phase motor?

Description : How much power is needed to supply the losses associated with the production of the magnetic field in a 3-phase motor?

Last Answer : You need to conduct No load and full load losses tests todetermine the losses.

The voltage of the single phase supply to residential consumers is (a) 110 V (b) 210 V (c) 230 V (d) 400 V

Description : The voltage of the single phase supply to residential consumers is (a) 110 V (b) 210 V (c) 230 V (d) 400 V

Last Answer : (c) 230 V

The voltage of the single phase supply to residential consumers is (a) 110 V (b) 210 V (c) 230 V (d) 400 V

Description : The voltage of the single phase supply to residential consumers is (a) 110 V (b) 210 V (c) 230 V (d) 400 V

Last Answer : (c) 230 V

Explain with the neat circuit diagram only the scott connection scheme for conversion of three phase supply to two phase supply. Name one application of the same.

Description : Explain with the neat circuit diagram only the scott connection scheme for conversion of three phase supply to two phase supply. Name one application of the same.

Last Answer : Three-phase to Two-phase Transformation (Scott Connection of Transformers): Working: i) Scott connection can be used for three-phase to two-phase conversion using two single-phase transformers ... a 3-phase system with a two-phase system with the flow of power in either direction. 

Define the terms related to a.c. supply with waveform : i) Instantaneous value ii) Time period iii) Amplitude iv) Phase difference.

Description : Define the terms related to a.c. supply with waveform : i) Instantaneous value ii) Time period iii) Amplitude iv) Phase difference. 

Last Answer : i) Instantaneous value: The instantaneous value is the value of an alternating quantity (it may ac voltage or ac current or ac power) at a particular instant of time in the cycle .  OR  ... difference: Phase difference is the difference in phase angle between two sinusoids or phasors. 

Define phase sequence of three phase supply with waveforms.

Description : Define phase sequence of three phase supply with waveforms. What is normal phase sequence? Which 3 standard colours are used to denote the phase sequence?

Last Answer : It  is  sunnosoidal  sequence.

A single phase transformer is designed to operate from 50 Hz, 1100 V supply and to have an output voltage

Description : A single phase transformer is designed to operate from 50 Hz, 1100 V supply and to have an output voltage of 240 V at no load. The core has mean area of 30 cm2 and the flux density has not to exceed 1.3T. The number of secondary turns will be (a) 1270 (b) 1017 (c) 768 (d) 277  

Last Answer : E2=e2=4.44*flux*N2*f=240 V Flux=Flux density* Area Area=30 cm2  =30*10-4 m2 Flux= 1.3*30*10-4  Wb=39*10-4 Wb E=240= 4.44*39*10-4 *N2*50      N2=277.

A 3-phase induction motor is running at slip ‘s’. If its two supply leads are interchanged, then the operating

Description : A 3-phase induction motor is running at slip ‘s’. If its two supply leads are interchanged, then the operating slip at that instant will be:

Last Answer : A 3-phase induction motor is running at slip ‘s’. If its two supply leads are interchanged, then the operating slip at that instant will be: (2-s)

A solar cell of 350 V is feeding power to an ac supply of 440 V, 50 Hz through a 3-phase fully controlled bridge

Description : A solar cell of 350 V is feeding power to an ac supply of 440 V, 50 Hz through a 3-phase fully controlled bridge converter. A large inductance is connected in the dc circuit to maintain the dc current at 20 A ... thyristor will be reverse biased for a period of  (A) 125o (B) 120o (C) 60o (D) 55o

Last Answer : A solar cell of 350 V is feeding power to an ac supply of 440 V, 50 Hz through a 3-phase fully controlled bridge converter. A large inductance is connected in the dc circuit to maintain the ... the solar cell resistance is 0.5 Ohm ,then each thyristor will be reverse biased for a period of 125o

No-load test on a 3-phase induction motor was conducted at different supply voltage and a plot of input

Description : No-load test on a 3-phase induction motor was conducted at different supply voltage and a plot of input power versus voltage was drawn. This curve was extrapolated to intersect the y-axis. The intersection point ... ) Core loss (B) Stator copper loss (C) Stray load loss (D) Friction and windage loss

Last Answer : No-load test on a 3-phase induction motor was conducted at different supply voltage and a plot of input power versus voltage was drawn. This curve was extrapolated to intersect the y-axis. The intersection point yields Friction and windage loss

Total instantaneous power supplied by a 3-phase ac supply to a balanced R-L load is

Description : Total instantaneous power supplied by a 3-phase ac supply to a balanced R-L load is

Last Answer : Total instantaneous power supplied by a 3-phase ac supply to a balanced R-L load is constant

A 3-phase induction motor under locked rotor condition and with stator supply, induces

Description : A 3-phase induction motor under locked rotor condition and with stator supply, induces (A) small voltage at slip frequency (B) large voltage at rotor frequency (C) zero voltage at zero frequency (D) large voltage at slip frequency

Last Answer : A 3-phase induction motor under locked rotor condition and with stator supply, induces (A) small voltage at slip frequency (B) large voltage at rotor frequency (C) zero voltage at zero frequency (D) large voltage at slip frequency

The main application of delta/zig-zag connected 3-phase transformer is to supply

Description : The main application of delta/zig-zag connected 3-phase transformer is to supply (A) balanced loads (B) unbalanced loads (C) furnace loads (D) single-phase loads

Last Answer : The main application of delta/zig-zag connected 3-phase transformer is to supply unbalanced loads

Three equal impedances are first connected in delta across a 3 -phase balanced supply. If the same...

Description : Three equal impedances are first connected in delta across a 3 -phase balanced supply. If the same impedances are connected in star across the same supply then,

Last Answer : A

If a 220/440 V, 50 Hz, 5 KVA, single-phase transformer operates on 220V, 40Hz supply with secondary winding...

Description : If a 220/440 V, 50 Hz, 5 KVA, single-phase transformer operates on 220V, 40Hz supply with secondary winding open circuited: (A) both the eddy current and the hysteresis losses will decrease ... losses will increase (D) eddy current losses will increase but the hysteresis losses will remain the same

Last Answer : If a 220/440 V, 50 Hz, 5 KVA, single-phase transformer operates on 220V, 40Hz supply with secondary winding open circuited: both the eddy current and the hysteresis losses will decrease

A separately excited dc motor is required to be controlled, from a 3-phase supply, for its operation in the first...

Description : A separately excited dc motor is required to be controlled, from a 3-phase supply, for its operation in the first quadrant only. The most preferred converter would be  (a) 3-phase fully controlled ... converter with free wheel diode (c) 3-phase dual converter (d) 3-phase half wave converter 

Last Answer : A

In a three-phase supply floating neutral is undesirable because it way give rise to?

Description : In a three-phase supply floating neutral is undesirable because it way give rise to?

Last Answer : In a three-phase supply floating neutral is undesirable because it way give rise to unequal line voltages across the load.

A capacitor-start single phase induction motor is switched on to supply with its capacitor replaced by...

Description : A capacitor-start single phase induction motor is switched on to supply with its capacitor replaced by an inductor of equivalent reactance value. It will?

Last Answer : A capacitor-start single phase induction motor is switched on to supply with its capacitor replaced by an inductor of equivalent reactance value. It will not start at all.

Which of the following statements represents the FIRST precaution to be taken prior to working on any installed electrical component? A. Wear rubber gloves and boots. B. Use only approved nonconducting tools. C. Ground the case of the machine before beginning any repairs. D. Open the supply circuits and tag the switches.

Description : Which of the following statements represents the FIRST precaution to be taken prior to working on any installed electrical component? A. Wear rubber gloves and boots. B. Use only approved nonconducting ... the machine before beginning any repairs. D. Open the supply circuits and tag the switches.

Last Answer : Answer: D

Which of the following statements represents an application of a silicon controlled rectifier? A. To provide DC power for a main propulsion motor. B. For use as a voltage reference diode. C. For sensing flame in an automated burner. D. To eliminate power supply hum.

Description : Which of the following statements represents an application of a silicon controlled rectifier? A. To provide DC power for a main propulsion motor. B. For use as a voltage reference diode. C. For sensing flame in an automated burner. D. To eliminate power supply hum.

Last Answer : Answer: A

The rating of the Transformer __ when the supply frequency is increased: a) Remains unchanged b) Increases c) Decreases d) None of the above

Description : The rating of the Transformer __ when the supply frequency is increased: a) Remains unchanged b) Increases c) Decreases d) None of the above

Last Answer : Ans: B

In accordance with Coast Guard Regulations (46 CFR), a steering gear feeder circuit from the emergency switchboard or alternate power supply must have the capacity for continuous operation of the rudder 15° side to side in not more than 60 seconds for a A. 1/4 hour B. 1/2 hour C. 1 hour D. 2 hours

Description : In accordance with Coast Guard Regulations (46 CFR), a steering gear feeder circuit from the emergency switchboard or alternate power supply must have the capacity for continuous operation of the rudder 15° side to side in not more than 60 seconds for a A. 1/4 hour B. 1/2 hour C. 1 hour D. 2 hours

Last Answer : Answer: B