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Answer :

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

Related questions

Description : Brushes of DC machine are made up of ?

Last Answer : Brushes of DC machine are made up of carbon.

Last Answer : Satisfactory commutation of D.C. machines requires brushes should be of proper grade and size, brushes should smoothly run in the holders, smooth, concentric commutator properly undercut.

Last Answer : The purpose of retardation test on D.C. shunt machines is to find out stray losses.

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

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

Last Answer : 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.

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

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

Last Answer : The material for commutator brushes is generally carbon.

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

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

Description : In dc machines, the armature windings are placed on the rotor because of the necessity for  (1) electromechanical energy conversion (2) generation of voltage (3) commutation (4) development of torque

Last Answer : 3  Commutation

Last Answer : Hopkinson's test on D.C. machines is conducted at full-load.

Last Answer : In D.C. machines the residual magnetism is of the order of 2 to 3 percent.

Last Answer : In D.C. machines fractional pitch winding is used to reduce the sparking.

Last Answer : For a D.C. machines laboratory following type of D.C. supply will be suitable induction motor D.C. generator set.

Description : State any two applications of single phase autotransformer.

Last Answer : Applications of single phase auto transformers: 1) Give small boost to distribution cable to correct the voltage drop. 2) As auto transformer starter for induction motors. 3) As furnace supply ... . 5) In control equipment for single phase locomotives. 6) As dimmer in lighting circuits.

Description : State any two conditions for parallel operation of 3-phase transformer.

Last Answer : Conditions for Parallel operation of 3 phase transformer 1) Voltage ratings of both the transformers must be identical. 2) Percentage or p.u. impedance should be equal in magnitude. 3) X/R ratio ... between primary & secondary voltages must be same. 6) Phase sequence on both sides must be same.

Description : Compare core type transformer and shell type transformer on the following parameters. (i) Types of winding used, (ii) Application

Last Answer : Ans:

Description : Define transformation ratio in terms of current and voltage.

Last Answer : (i) Transformation Ratio (K) is defined as the ratio of the primary current to the secondary current.  (ii) Transformation Ratio (K) is defined as the ratio of the secondary voltage to the primary voltage.

Description : Define: (i) Commercial efficiency of a transformer and  (ii) All day efficiency of a transformer.

Last Answer : (i) Commercial Efficiency : It is the ratio of output power in watt to the input power in watt. (ii) All day efficiency of a transformer: It is the ratio of output energy in kWh to the input energy in kWh in the 24 hours of the day

Description : A 50 KVA transformer has iron loss 2 kW on full load. Calculate its iron loss at 75% of full load.

Last Answer : Iron loss is dependent on voltage and independent of load. Hence iron loss at 75% of Load = Iron loss at full load = 2 kW

Description : State any four properties of Ideal transformer.

Last Answer : Properties of Ideal transformer: 1) No losses (iron and copper losses), hence no temperature rise. 2) No magnetic leakage i.e (Coefficient of coupling between primary and secondary windings is unity) 3) Zero winding ... drop i.e. E1 = V1, E2 = V2 5) Efficiency 100 %. 6) Regulation 0 %.

Description : Draw the connection diagram of long shunt differential D.C. compound generator.

Last Answer : Ans:

Last 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.

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

Description : Are we losing control over our life to machines?

Last Answer : That is up to you. You can or you can decide to not.

Description : Heat Transfer Machines

Last Answer : Heat transfer machines are just the thing you need in order to do heat transfers on your clothing or whatever surface you want to decorate. In order for you to stick designs and precious stones on porous and ... is best. This will also give you an idea of what kind of designs you can choose from.

Description : What type of motor is used for drilling machines?

Last Answer : universal motor also used in Handheld power tools, blowers, blenders, vacuum cleaners, polishers and kitchen appliances.universal motor is defined as a motor which may be operated either on direct or single phase AC supply at approximately the same speed and output

Description : The inertia constants of two groups of machines which do not swing together are M1 and M2. The equivalent inertia constant of the system is

Last Answer : The inertia constants of two groups of machines which do not swing together are M1 and M2. The equivalent inertia constant of the system is M1M2/M1+M2

Description : For protection of rotating machines against lightning surges _______ is used.  (1) lightning arrester (2) capacitor (3) combination of lightning arrester and capacitor (4) lightning conductor and arrester

Last Answer : For protection of rotating machines against lightning surges combination of lightning arrester and capacitor is used. 

Description : Laminated cores, in electrical machines, are used to reduce?

Last Answer : Laminated cores, in electrical machines, are used to reduce eddy current loss.

Description : Silicon steel is used in electrical machines because it has?

Last Answer : Silicon steel is used in electrical machines because it has low hysteresis loss.

Last Answer : The speed control of universal motor used for sewing machines is by varying the resistance.

Last Answer : Exciters of synchronous machines are d.c. shunt machines.

Description : Losses which occur in rotating electric machines and do not occur in transformers are?

Last Answer : Friction losses windage losses 

Last Answer : The armature core of a D.C. generator is usually made of silicon steel.

Last Answer : 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

Last Answer : The critical resistance of the D.C. generator is the resistance of field.

Description : If residual magnetism is present in a D.C. generator, the induced e.m.f. at zero speed will be?

Last Answer : If residual magnetism is present in a D.C. generator, the induced e.m.f. at zero speed will be zero.

Last Answer : A 220 V D.C. generator is run at full speed without any excitation. The open circuit voltage will be about 2 V.

Last Answer : In a level compounded D.C. generator, full load terminal voltage is equal to no-load terminal voltage.

Last Answer : In a D.C. generator in case the resistance of the field winding is increased, then output voltage will decrease.

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

Last Answer : A D.C. generator can be considered as rotating amplifier.

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

Last Answer : In a D.C. generator, the critical resistance refers to the resistance of field.

Last Answer : Flashing the field of D.C. generator means creating residual magnetism by a D.C. source.

Last Answer : If a D.C. generator fails to build up the probable cause could not be field resistance less than the critical resistance.

Last Answer : In D.C. generators on no-load, the air gap flux distribution in space is flat topped.

Last Answer : In a D.C. generator the magnetic neutral axis coincides with the geometrical neutral axis, when there is no load on|he generator.