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Copper brushes in D.C. machine are used where low voltage and high currents are involved.

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Answer :

Brushes of D.C. machines are made of carbon.

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Brushes of DC machine are made up of ?

Answer :

Brushes of DC machine are made up of carbon.

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The number of brushes in a commutator depends on amount of current to be collected.

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In lap winding, the number of brushes is always same as the number of poles.

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The rapid wear of brushes takes place due to abrasion from dust, excessive spring pressure, rough commutator bars, high mica insulation between com-mutation bars.

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Armature coil is short circuited by brushes when it lies along neutral axis.

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The material for commutator brushes is generally carbon.

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Description :

In a DC motor, it is important to place the brushes along ______ A) Magnetic Neutral Axis B) Normal Geometric Axis C) Nominal Geometric Axis D) None of these

Answer :

In a DC motor, it is important to place the brushes along Magnetic Neutral Axis

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72 views 1 answer
Description :

Select insulating materials for following parts : (i) Insulation between heating element and base plate of electric iron. (ii) Insulation used over copper or aluminium conductor used for making coils. (iii) Transformer bushings. (iv) Insulation between transmission line and pole.

Answer :

Parts Insulating Materials  Insulation between heating element and base plate of electric iron. Mica Insulation used over copper or aluminium conductor used for making coils.  ... Transformer bushings Porcelain  Insulation between transmission line and pole. Porcelain

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Description :

Why is aluminium used instead of copper for overhead cables?

Answer :

Because copper reacts with atmosphere variation and it's cost High as compared to aluminium and aluminium is good property and less cost so aluminium is used

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explain skin effect in copper interconnects. what are the various ways of reducing this effect?

Answer :

Skin effect generally observed in ac conductor in transmission network, in skin the current flowing through outer layer of conductor is more than iner area of conductor thus we can say the ... for reducing this effect we can use boundel conductor or used hollow conductor for power transmission

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Description :

At full -load of a transformer, the iron loss and copper loss are 3000 W and 4000 W respectively. Then total loss at maximum efficiency is :  (A) 7000 W (B) 6000 W (C) 8000 W (D) 4000 W

Answer :

At full -load of a transformer, the iron loss and copper loss are 3000 W and 4000 W respectively. Then total loss at maximum efficiency is : 6000 W

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Description :

Which is a semiconductor: copper, silicon, or neon?

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Copper is a metal. Silicon is a semiconductor. Neon is inert gas.

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Description :

Compare copper and aluminium.

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Copper Aluminium Resistivity is less Resistivity is more More weight Low weight Mechanically strong Mechanically weaker Specific Gravity high Specific Gravity Low ... Tensile strength is half of that of copper Used for winding Used for overhead lines

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Description :

A 40 kVA transformer has a core loss of 400 W and a full load copper loss of 800 W. The proportion of full load at maximum efficiency is

Answer :

A 40 kVA transformer has a core loss of 400 W and a full load copper loss of 800 W. The proportion of full load at maximum efficiency is 0.707

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Two copper conductors have equal length. The cross-sectional area of one conductor is four times that of the other. If the conductor having smaller cross-sectional area has a resistance of 40 ohms the resistance of other conductor will be?

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Two copper conductors have equal length. The cross-sectional area of one conductor is four times that of the other. If the conductor having smaller cross-sectional area has a resistance of 40 ohms the resistance of other conductor will be 10 ohms.

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Current velocity through a copper conductor is

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The ground wire should not be smaller than No 10 copper.

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Auto-transformer makes effective saving on copper and copper losses, when its transformation ratio is?

Answer :

Auto-transformer makes effective saving on copper and copper losses, when its transformation ratio is approximately equal to 1.

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How do magnets and copper wire make electricity?

Answer :

According  to  Faradays  law , a changing  magnetic  field  on  a  conductor  induses  current  in  the  conductor  through   mutual  induction.

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Answer :

If brushes of a D.C. generator are moved in order to bring these brushes in magnetic neutral axis, there will be crossmagnetisation as well as demagnetising.

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Description :

Answer :

In D.C. generators, the brushes on commutator remain in contact with conductors which lie under interpolar region.

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In a DC shunt generator working on load, the brushes are moved forward in the direction of rotation, as a result of this, commutation will   (a) Improve but terminal voltage will fall   (b) Worsen and terminal voltage will fall   (c) Improve and terminal voltage will rise   (d) worsen and terminal voltage will rise 

Answer :

In a DC shunt generator working on load, the brushes are moved forward in the direction of rotation, as a result of this, commutation will Improve but terminal voltage will fall 

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Answer :

Eddy currents are induced in the pole shoes of a D.C. machine due to relative rotation between field and armature.

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Iron losses in a D.C. machine are independent of variations in load.

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In a D.C. machine stray loss is the sum of iron loss and mechanical loss.

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To avoid formation of grooves in the commutator of a D.C. machine the brushes of opposite polarity should track each other.

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In case of D.C. machine winding, number of commutator segments is equal to number of armature coils.

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Answer :

In a four-pole D.C. machine alternate poles are north and south.

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The number of brushes in a 6-pole double-layer lap wound dc machine is (A) 8 (B) 6 (C) 4 (D) 2 

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Answer :

Open circuited armature coil of a D.C. machine is identified by the scarring of the commutator segment to which open circuited coil is connected and indicated by a spark completely around the commutator.

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Armature reaction of an unsaturated D.C. machine is crossmagnetising.

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If  equalising bar is connected to the 2 DC generators  the current will be equal in 2 generators irrespective of parallel connection so that we connect equaliser bar

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Compensating windings are used in D.C. generators to neutralise the cross-magnetising effect of the armature reaction.

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Description :

Carbon brushes are used in electric motors to:  (1) Brush off carbon deposit on the commutator (2) Provide a path for flow of current (3) Prevent overheating of armature windings (4) Prevent sparking during commutation

Answer :

Carbon brushes are used in electric motors to: Provide a path for flow of current.

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Answer :

In any rotating machine that part which houses the conductors and in which e.m.f. induced is to be utilised is called armature.

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Answer :

In D.C. generators, lap winding is used for low voltage, high current.

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Electrical contact materials used in switches, brushes and relays must possess   (A) high thermal conductivity and high melting point  (B) Low thermal conductivity and low melting point   (C) High thermal conductivity and low melting point  (D) Low thermal conductivity and high melting point.

Answer :

Electrical contact materials used in switches, brushes and relays must possess high thermal conductivity and high melting point 

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Answer :

In a D.C. generator the ripples in the direct e.m.f. generated are reduced by using carbon brushes of superior quality.

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In case of D.C. machines, mechanical losses are primary function of speed.

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Answer :

In D.C. generators the pole shoes are fastened to the pole core by counter sunk screws.

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To prevent damage from lightning, lightning conductors are used on tall structures. The lightning conductor (1) should be made of a good conductor but can be of any shape (2) should be in the form of a vertical rod (3) can be of any shape (4) should be made of a good conductor like copper with sharp-pointed edges

Answer :

should be made of a good conductor like copper with sharp-pointed edges

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HTML is used to create (A) machine language program (B) high level program (C) web page (D) web server

Answer :

(C) web page

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The CPU of a system having 1 MIPS execution rate needs 4 machine cycles on an average for executing an instruction. The fifty percent of the cycles use memory bus. A memory read/write employs one machine cycle. For execution of the programs, the system utilizes 90 percent of the CPU time. For block data transfer, an IO device is ... transfer rate if programmed IO data transfer technique is used? (A) 500 Kbytes/sec (B) 2.2 Mbytes/sec (C) 125 Kbytes/sec (D) 250 Kbytes/sec

Answer :

(D) 250 Kbytes/sec

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Which of the following protocols is used by email server to maintain a central repository that can be accessed from any machine? (A) POP3 (B) IMAP (C) SMTP (D) DMSP

Answer :

(B) IMAP

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The bearings used to support the rotor shafts are generally ball bearings.

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D.C. generators are connected to the busbars or disconnected from them only under the floating condition to avoid sudden loading of the primemover, to avoid mechanicaljerk to the shaft, to avoid burning of switch contacts.

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D.C. generators are installed near the load centres to reduce line losses.

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Answer :

D.C. generators are normally designed for maximum efficiency around full-load.

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