ENERGY CONVERSION DEVICE
1.(a) what is energy conversion ?
ans- An energy conversion device is defined as the energy convert one form of energy into another.the development of energy conversion device through time can be used as a gauge in the history of cililization.
e.g-electric heater, battery , electric motor and generator.
(b)what are the working principle of dc generator ?
ans- The coil to be rotating in clock-wise direction .as the coil assume successive position in the field, the flux linked with it change.hence an e.m.f. is induced in it. which proportional to rate of change of flux linkage.when it is in position,i,then flux linked with the coil is maximum but rate of change of flux linked is minimum.
(c) why transformer with losses but no magnetic leakage ?
ans- we will consider two cases :-
(1) when such a transformer is on no load and
(2) when it is loaded
when no load condition the transformer is put on load, there is iron loss in the core and copper loss in the windings .
when the secondary is loaded,the secondary current is set up.
(D) what are the different types of 3-phase transformer connection ?
ans- there are various method available for transforming 3-phase voltages to
higher or lower 3-phase voltages i.e.for handling a considerrable amount of power.the most common connection are Y-Y,Δ-Δ,Y-Δ,Δ-Y,V-V and T-T.
(e) What causes heating of armature ?
ans – The following are the resons due to which the armature gets heated.
(i) eddy current.
(ii) moisture which almost short circuits the armature.
(iii) unequal strength of magnetic poles.
(iv) operation above rated voltage and below normal speed.
(f) what is lap winding ?
Ans- To increase the voltage of the machine to a suitable value the number of conductors are connected in series parallel configuration.the conductor devided into p groups. Where p is the no of poles. The conductors under each pole are connected in series. A// P groups are joined parallally. This type of winding is used for low voltage and high current machines.
A=The number of parallel paths
P=Number of poles
e= e.m.f induced in each conductor.
(g) An 8-pole lap-connected armature has 40 slots with 12 conductors per slot generates a voltage of 500V. determine the speed at which it is running if the flux per pole is 50mwb ?
Ans- lap wound,A=p=8
Total number of conductor=(number of slots) x conductor per slot
Ф = 50mwb = 50×10-3 wb
Eg= ФzN/60XP/A ( lap wound A=P)
= N= 60XEg/ФZ = 60X 500/50×10-3x480
(h) what is the Advantages and Disadvantages of parallel connection ?
Ans- Advantages – it is more economical because power loss is less.
Disadvantages – it provides two speeds only.
(i)calculate the pitch factor and distribution factor of 3-phase balanced winding having a coil span of 8 slot with 72 stator slots and 8 poles ?
Ans- coil span = 8 slots
72 slots and 8 poles
No. of slots/pole = 72/8 =9
P = 8poles. α =( 180/9) x 1 = 200
Pitch factor ke = cos α/2 = cos 100 = 0.984
Distribution factor kd = sin mp/2/m sin β/2 =0.959
(j) Why thedeveloped torque in case of a single-phase induction motor is zero at starting ?Explain by drawing its torque speed characteristic ?
Ans- The developed torque in case of a 1-Ф 1.M is zero at starting because when 1Ф supply is fred to
1 Ф stator winding of the motor it produces a uniaxial alternating flux which can not produce rotation.
torque speed characteristic :
According to double field theory resultant alternating flux.
composed of two synchronously rotating component flux in opposite directions. let Tf
their corresponding to
Both the torque are numerically equal but oppositely directed and at starting the
resultant torque = 0.
resultant torque =0
(2)(a) Draw the diagram and explain motor principle ?
An Electric motor is a machine converts electric energy into mechanical energy. its action is based on the principle that when a current -carrying conductor is placed in a magnetic field,it experiences a mechanical force whose direction is given by fleming’s left-hand rule and whose magnitude is given by F=Bll newton.0
Constructionally, there is no basic difference between a D.C. generator and a D.C motor. in fact the same d.c. machine can bbe used interchangeably as a generator or as a motor. D.C. motor are also like generators,shunt-wound or series-wound or compound-wound.
a part of multipolar d.c. motor is shown . when i its field magnets are excited and its armature conductors are supplied with current from the supply mains,they experience a force tending to rotate the armature.armature conductor under N-pole are assumed to carry current downwards and those under S-pole, to carry current upwards. By applying Fleming’s left-hand rule, the direction of the force on each conductor can be found. it is shown by small arrows placed above each conductor.each conductor experiences a force F which tends to rotate the armature in anticlockwise direction. these forces collectively produce a driving torque which sets the armature rotating.
it should be noted that the function of a commutator in the motor is the same as in a generator. By reversing current in each conductor as it passes from one pole to another, it helps to develop a continuous and unidirectional torque.
(b) Calculate the regulation of a transformer in which ohmic drop is 1% and reactance drop 5% of the voltage at the following power factors.
(i) 0.8 lagging
(ii) 0.8 leading
percentage resistive drop =(I2R02/V2)*100=1
Percentage reactive drop= (I2X02/V2)*100=5
(i) when power factor =0.8 lagging=0.6
Voltage regulation= (I2RO2COSФ+I2X02sinФ)/v2*100
(ii) When power factor cosФ=0.8(leading);sinФ=-0.6
Voltage regulation=(I2 R02 /V2)*100COSФ+(I2X02/v)100sinФ
(3)Explain the characteristics of D.C. shunt generator ?
Ans- The circuit diagram for a shunt generator to draw NO-load characteristics.show in fig. the manner in which a self excited generator manages to excite its own field and build a D.C. voltage across the armature is described to with referenc .
At rated speed before closing the switch, the initial value of voltage E indicate voltage due to residual magnetism. the building up of voltage is shown in fig.
when switch is closed, the residual voltage Er in the armature also across the field circuit and drives a currene I1 in the field circuit.
Where field circuit of the generator in increase in MMF result.the residual in increase the induced voltage across the field circuit causes a current increase the MMF . which produce field circuit.
(4) What are the constructional features of transformer?
Ans – The transformer consists of basically
(a) A magnetic core (magnetic circuit) over which the coil are wound.
(b) Two or more coil wound around the core know as windings.
(a) core :- The core is generally made from high permeability silicon steel stampings each about (0.35 to .5mm) thickness.Each stamping has thin insulation on both sides to restrict the flow of eddy current . The laminations are firmly clamped together to provide a continuous magnetic circuit with minimum airgap to prevent vibration and noise.
The amount of eddy current loss is more if a solid core is used. High resistivity CRGO (Cold Rolled Grain Oriented) steel laminations are used to reduced the eddy current loss. The orientation of the cold rolled grain oriented steel should be in the direction of flux path.
Constructionally the transformers are of two general types.
(i) Core type
(ii) Shell type
In this type of transformer the winding surround a considerable path of the core as shown in fig. the core is normally constructed using L type of stampings as .the magnetic circuit consists of two vertical legs or limbs with two Horizontal sections,called Yokes.the primary and secondary winding are on the opposite legs. But construction these are always interleaved to reduced leakage flux as shown fig. the primary and half of secondary winding has been placed side by side and or concentrically on each limb.the low voltage winding.
(ii) Shell type :-
In this type core surrounds a considerable path of the winding. The magnetic core is built up of E and I stamping as shown in fig.
And have three limbs. The central limb carries both primary and secondary winding and the magnetic circuit has two parallel paths through the core. The low voltage (L.V) winding is wound next to the core and high voltage (H.V) winding is done on it.A simple diagram of a shell type transformer is shown in fig. the practical diagram is shown.
This is generally used for low capacity transformers.
(5) Explain the EMF equation of D.C gnnerator ?
Ans – Let P= Total no. of poles
Z = Total no. of conductor
N = Speed of armature in rpm
ɸ = Flux per pole
A = No. of parallel path = 2 (wave winding)
= p (Lap winding)
When ‘1’ conductor of armature rotates for 1 revolution, the total flux cut by the armature conductor is ɸP .
The time taken for 1 revolution t = 60/N sec.
So the emf generatated per conductor
E=dɸ/dt = ɸp/60/N = ɸpn/60
As armature conductor are placed is series,parallel form, the total emf generated per parallel path of the armature = ɸpN/60 × Z/A
Eg = ɸPNZ/60 A
(6) Explain Ward-leonard voltage control method with Adv & Disadvantages?
Ans – diagram
The motor is connected with 3-ɸ AC supply which is a induction motor or a synchronous motor.
The generator ‘g’ & the d.c motor m are connected to each othed & forms a motor generator set.
The speed of the motor m can be control by varying the voltage across the generator.which will vary the voltage across the armature of motor & hence for different voltage we will get different speed.
The field winding of motor m is supplied from a constant d.c source & the field winding of generator is connected to a rheostat so that we can varie the field current of it& due to which the generatorated emf of the generator is change.
There is a reversible switch connected in series with the field winding which can reverse the field current of the generator.& also the direction of rotation of armature is reversed.
The motor & generator set always run in the same direction.
Advantage :- Here a wide range of speed control can be obtain & the speed can be reversed in both the direction.
This method is useful for speed control of large motor when d.c. supply is not available.
The losses in the motor generator set is high.if the motor is operating light load conduction.