tda
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Electrical Machines
1. What is the function of carbon brush used in D.C generator?
The function of carbon brush is to collect current from the commutator and supply is to the external load circuit and to the field circuit.
2. Distinguish between lap winding and wave winding used in dc machine
§ Lap winding is designed for high current and low voltage machines.
§ Wave winding is designed for low current and high voltages.
3. Write the number of parallel paths in a lap and wave connected windings
In a lap wound machine, the number of parallel paths is equal to the number of poles. But in wave wound machine, the number of parallel paths is always two irrespective of number of poles.
4. Name the three things required for the generation of emf
o Presence of armature conductors
o Presence of magnetic field
o Relative motion between conductor and magnetic field
5. What are the different types of dc generator
6. What is meant by self excited and separately excited dc generator
· Self-excited generator are those whose field magnets are energized by the current produced by the generator themselves
· Separately excited generator are those whose field magnets are energized from an independent external source of dc current
7. What is the basic difference between dc generator and dc motor
Generator converts mechanical energy into electrical energy. Motor converts electrical energy into mechanical energy. But there is no constructional difference between the two.
8. Write down the emf equation of dc generator. Give the meaning of each symbol
Emf induced E=ФZNP/60A
Ф=flux per pole in Weber
Z=total number of armature conductors
N=speed of armature in rpm
P=number of poles
A=number of parallel paths
9.what is pole pitch?
The periphery of the armature is divided for a number of poles of the generator. the center to center distance between two adjacent poles is called pole pitch
It is also equal to the number of armature slots or armature conductors per pole.
10.How can the voltage in a DC generator be increased?
Increasing the main field flux and the speed of the armature can increase the voltage in a DC generator.
11.What is critical resistance of a DC shunt generator?
The value of resistance of shunt field winding beyond which the shunt generator fails to build up its voltage is known as “ critical resistance “.
12.what are the conditions to be fulfilled for a shunt generator to build up Voltage?
a) There must be some residual magnetism in the field poles.
b) The shunt field resistance should be less than critical resistance.
c) The field coils should be connected with the armature in such a way that current flowing through them should increase the EMF induced by the residual magnetism.
13.What do you mean by residual flux in DC generator.
The magnetic flux retained in the poles of the machine even without field supply is called the residual flux.
14.A DC generator fails to self excite. . List the cause for the failure for the failure.
a) Residual magnetism may not there in the poles.
b) Direction of rotation may be wrong.
c) The field resistance may be more than critical resistance.
d) There may be disconnection in the field winding.
e) Brush contact may be poor.
f) The field coils may be connected with the armature to oppose the EMF due to residual magnetism.
15.What are open circuit characteristics of DC shunt generator.
It is the relation between the field current and the emf induced in the armature
16. Why a 3-phase synchronous motor will always run at synchronous speed?
Because of the magnetic coupling between the stator poles and rotor poles the motor runs exactly at synchronous speed.
17. Mention the methods of starting of 3-phase synchronous motor.
a. A D.C motor coupled to the synchronous motor shaft.
b. A small induction motor coupled to its shaft.
c. Using damper windings as a squirrel cage induction motor.
18.What are the principal advantages of rotating field system type of construction of synchronous machines?
· Form Stationary connection between external circuit and system of conditions enable the machine to handle large amount of volt-ampere as high as 500 MVA.
· The relatively small amount of power required for field system can be easily supplied to the rotating field system via slip rings and brushes.
· More space is available in the stator part of the machine for providing more insulation to the system of conductors.
· Insulation to stationary system of conductors is not subjected to mechanical stresses due to centrifugal action.
· Stationary system of conductors can easily be braced to prevent deformation.
· It is easy to provide cooling arrangement.
19.Write down the equation for frequency of emf induced in an alternator.
F = PN / 120 Hertz
Where P = No. Of poles
N = Speed in rpm.
20.What are the advantages of salient pole type of construction used for synchronous machines?
· They allow better ventilation.
· The pole faces are so shaped radial air gap length increases from the pole center to the pole tips so that flux distribution in the air gap is sinusoidal in shape which will help to generate sinusoidal emf.
· Due the variable reluctance, the machine develops additional reluctance power, which is independent of excitation.
21Why do cylindrical rotor alternators operate with steam turbines?
Steam turbines are found to operate at fairly good efficiency only at high speeds. The high-speed operation of rotor tends to increase mechanical losses, so the rotors should have smooth external surface. Hence smooth cylindrical type rotors with less diameter and large axial length are used for synchronous generators driven by steam turbines with either 2 or 4 poles.
22.Which type of synchronous generators are used in Hydroelectric plants and why?
As the speed of operation is low, for hydro turbines used in hydroelectric plants, salient pole type synchronous generators are used. These allow better ventilation and also have other advantages over smooth cylindrical type rotor.
23.What is the relation between electrical degree and mechanical degree?
Electrical degree θe and mechanical degree are related to one another by the number of poles P, the electrical machine has, as given by the following equation.
θe = (P/2) θm
24. What is the meaning of electrical degree?
Electrical degree is used to account the angle between two points in rotating electrical machines. Since all electrical machines operate with the help of magnetic fields, the electrical degree is accounted with reference to the polarity of magnetic fields. 180 electrical degrees is accounted as the angle between adjacent North and South poles
25. Why short-pitch winding is preferred over full pitch winding?
Advantages: -
· Waveform of the emf can be approximately made to a sine wave and distorting harmonics can be reduced or totally eliminated.
· Conductor material, copper is saved in the back and front-end connections due to less coil span.
· Fractional slot winding with fractional number of slots/phase can be used which in turn reduces the tooth ripples.
· Mechanical strength of the coil is increased.
·
26. Write down the formula for distribution factor.
Kd = sin (mβ/2) or Kdn = sin (mnβ/2)
msin(β/2) msin(nβ/2)
where
m - number of slots/pole/phase
β - angle between adjacent slots in electrical degree
n - order of harmonics.
27. Define winding factor.
The winding factor Kw is defined as the ratio of phasor addition of emf induced in all the coils belonging to each phase winding of their arithmetic addition.
28. Why are alternators rated in kVA and not in kW?
The continuous power rating of any machine is generally defined as the power the machine or apparatus can deliver for a continuous period so that the losses incurred in the machine gives rise to a steady temperature rise not exceeding the limit prescribed by the insulation class.
Apart from the constant loss the variable loss incurred in alternators is the copper loss, occurring in the 3-phase winding, which depends on I2R, the square of the current delivered by the generator. As the current is directly related to apparent power delivered by the generator, the alternators have only their apparent power in VA/kVA/MVA as their power rating
29. What are the causes of changes in voltage of alternators when loaded?
· Voltage variation due to the resistance of the winding R.
· Voltage variation due to the leakage reactance of the winding X1.
· Voltage variation due to the armature reaction.
30. What is meant by armature reaction in alternators?
The interaction between flux set up by the current carrying armature conductors and the main field flux is defined as the armature reaction.
31. What do you mean by synchronous reactance?
It is the sum of the leakage reactance X1 and armature reactance Xa
Xs = X1 + Xa
32. What is synchronous impedance?
The complex addition of resistance R and synchronous reactance jXs is synchronous impedance Zs.
Zs = (R+jXs) = Zs∟θ
Where θ = tan –1(Xs/R)
|Zs| = √(R2+jXs2)
33. What is meant by load angle of an alternator?
The phase angle introduced between the induced emf phasor E and terminal voltage phasor V during the load condition of an alternator is called load angle. The load angle increases with increase in load. It is positive during generator operation and negative during motor operation.
34. Define the term voltage regulation of alternator.
It is defined as the change in terminal voltage from no load-to-load condition expressed as a function or terminal voltage at load condition, the speed and excitation conditions remaining same.
% Regulation = (E-V)/V x 100
35. What is the necessity for predetermination of voltage regulation?
Most of the alternators are manufactured with large power rating and large voltage ratings. Conduction load test is not possible for such alternators. Hence other indirect methods of testing are used and the performance can be predetermined at any desired load currents and power factors.
36. Why is the synchronous impedance method of estimating voltage regulation is considered as pessimistic method?
Compared to other methods, the value of voltage regulation obtained by this method is always higher than the actual value and therefore is called pessimistic method.
37. Why is the MMF method of estimating the voltage regulation is considered as the optimization method?
Compared to EMF method, MMF method involves more number of complex calculation steps. Further the OCC is referred twice and SCC is referred once while predetermining the voltage regulation for each load condition. Reference of OCC takes core saturation effect. As this method requires more effort, the final result is very close to the actual value. Hence this method is called the optimistic method.
38.What does hunting of synchronous motor mean?
When the load applied to the synchronous motor is suddenly increased or decreased, the rotor oscillates about its synchronous position with respect to the stator field. This action is called hunting.
39. What could be the reasons if a 3-phase synchronous motor fails to start?
It is usually due to the following reasons
a. Voltage may be too low.
b. Too much starting load.
c. Open circuit in one phase or short circuit.
d. Field excitation may be excessive
40. What is synchronous condenser?
An over-excited synchronous motor used for the improvement of power factor is called as synchronous condenser because, like a capacitor it takes a leading current.
41. Write the applications of synchronous motor.
a. Used for power factor improvement in sub-stations and in industries.
b. Used in industries for power applications.
c. Used for constant speed drives such as motor-generator set, pumps and compressors.
42. What is an inverted 'V' curve?
For a constant load, if the power factor is plotted against various values of field exciting current, the curve formed is known as inverted 'V' curve.
43. How the synchronous motor is made self-starting?
By providing damper windings in the pole faces, it will start and run like a squirrel cage induction motor.
44. A synchronous motor starts as usual but fails to develop its full torque. What could it be due to?
a. Exciter voltage may be too low.
b. Field spool may be reversed.
c. There may be either open-circuit or short-circuit in the field.
45.What are the two types of 3-phase induction motor?
a. Squirrel cage induction motor.
b. Slip ring induction motor.
46.Write the two extra features of slip ring induction motors.
a. Rotor is having 3-phase winding.
b. Extra resistance can be added in the rotor circuit for improving in the power factor with the help of three slip rings.
47.Can we add extra resistance in series with squirrel cage rotor? State the reason?
We cannot add extra resistance in series with the rotor because all the copper bars of the rotor are short circuited in both the sides by copper end rings to have a closed circuit.
48.Why an induction motor is called rotating transformer?
The rotor receives electrical power in exactly the same way as the secondary of a two winding transformer receiving its power from primary. That is why an induction motor can be called as a rotating transformer i.e., in which primary winding is stationary but the secondary is free to rotate.
49.Why an induction motor will never run at its synchronous speed?
If it runs at synchronous speed then there would be no related speed between the two, hence no rotor emf, no rotor current so no rotor torques to maintain rotation. That is why the rotor runs at its synchronous speed.
50.What are slip rings?
The slip rings are made of copper alloys and are fixed around the shaft insulating it. Through these slip rings and brushes the rotor winding can be connected to external circuits.
51.State the difference between slip ring rotor and cage rotor of an induction motor?
Slip ring rotor has 3-phase windings. Three ends of which are stared and the other three ends are brought up and connected to 3 slip rings mounted in the shaft. Extra resistance can be added in the rotor circuit. Squirrel cage rotor has short-circuited copper bars. Extra resistance can’t be added as slip ring rotor.
52.Write an expression for the slip of an induction motor.
Percentage slip = (Ns -Nr) / Ns * 100.
53.Define slip of induction motor?
Slip is descriptive of the way in which the rotor slips back from synchronism. It is usually expressed as percentage of synchronous speed.
54.What is cogging of an induction motor?
When the number of stator and rotor teeth’s is equal they have a tendency to align themselves exactly to minimum reluctance position. Thus the rotor may refuse to accelerate. This phenomenon is known as cogging.
55.What are the advantages of cage motor?
a) Since the rotor has very low resistance, the copper loss is low and efficiency is high
b) On the account of simple construction of rotor, it is mechanically robust.
c) Initial cost is less.
d) Maintenance cost is less.
e) Simple stating arrangement
56.Give the condition for maximum torque for 3-phase induction motor?
The rotor resistance and rotor reactance should be equal for developing maximum torque i.e. R2 = s X2 where s is the slip.
57. What is reason for inserting additional resistance in rotor circuit of a slip ring induction motor?
Introduction of additional resistance in the rotor circuit will increase the starting torque as well as running torque. Also it limits the starting current.
58. List out the methods of speed control of cage type 3-phase induction motor?
a) By changing supply frequency
b) By changing the number of poles
c) By operating two motors in cascade
59.Mention different types of speed control of slip ring induction motor?
a) By changing supply frequency
b) By changing the number of stator poles
c) By rotor rheostat control
d) By operating two motors in cascade
60.What are the advantages of 3-phase induction motor?
a) It was very simple and extremely rugged, almost unbreakable construction
b) Its cost is very low and it is very reliable
c) It has been suffiently high efficiency .No brushes are needed and hence frictional losses are reduced
d) It requires minimum of maintenance.
61. What is meant by crawling of induction motor?
Squirrel cage type, sometimes exhibit a tendency to run stably at speeds as low as 1/7 the of their synchronous speed this phenomenon is known as crawling
62. State the application of an induction generator?
a) Used in windmill for generating electric power.
b) Used in regenerative breaking places like traction.
63.Mention two types of core construction of transformer
Core type and shell type.
64.What are the main parts of a transformer? What type of material is used for the core?
Laminated core and primary and secondary windings are the main parts. The core is built up of thin soft iron or high-grade silicon steel laminations to provide a path of low reluctance to the magnetic flux.
65.Write down the emf equation of a single-phase transformer.
Emf induced in the primary E1=4.44fÆN1 volts, where F = frequency of supply main, Æ=flux linking both the primary and the secondary windings, N1 = number of turns in the primary, N2 = number of turns in the secondary.
66.State the principle of operation of a transformer
Transformer operates on the principle of mutual induction between inductively coupled coils. When AC source is connected to one coil, flux is produced in the core, which links both the coils. As per Faraday’s Laws of Electromagnetic Induction, EMF is induced in the secondary coil also. If the external circuit is closed the power is supplied.
67.Why is the rating of a transformer given in KVA?
Copper loss of a transformer depends on current and iron loss on voltage. Hence total transformer loss depends on volt ampere (VA) and both on the phase angle between voltage and current, i.e., it is independent of load power factor. That is why the rating of a transformer is in KVA.
68.How do you reduce hysteresis loss in a transformer?
Hysteresis loss can be reduced by selecting suitable core material. Silicon steel is having less Stein Metz hysteresis coefficient.
69.The efficiency of a transformer is always higher than that of rotating electrical machines. Why?
In rotating machines, there are mechanical losses (frictional and windage losses) due to the rotating parts. As there is no rotating part in a transformer, efficiency of transformer is always higher than rotating electric machines.
70.What are the advantages of OC and SC tests of a transformer over the load test?
The performance of a transformer can be calculated on the basis of its equivalent circuit, which contains R0, X0, R01 and X01 (or R02 and X02 referred to secondary) These constants or parameters can be easily determined by the OC and SC tests. These tests are very economical and convenient because they furnish the required information without actually loading the transformer.
71.What is the condition for obtaining maximum efficiency of a transformer?
The efficiency will be maximum when the variable losses are equal to the constant (iron) losses.
72.State the conditions under which OC and SC tests are conducted in a transformer.
Since no load current, I0 is very small. Therefore, pressure coils of the wattmeter and the voltmeter should be connected such that the current taken by them should not flow through the current coil of the wattmeter.
The transformer should be energized with rated voltage when conducting OC test.
For conducting SC test, the secondary winding should be short-circuited and a very low voltage should be given for the circulation of full-load current.
73.Define regulation of a transformer.
Voltage regulation of a transformer is defined as the change in secondary voltage from no-load to the load concerned. It is always expressed as a percentage of secondary voltage.
74.What do you understand by all-day efficiency?
The ordinary or commercial efficiency of a transformer is given by the ration between output (in Watts) and the input (in Watts) but the performance of a distribution transformer is determined by the all-day efficiency. It is defined as the total energy output (KWH) in 24 hrs a day to the total energy input in KWH for the same 24 hours of the day.
75.List the merits of an autotransformer.
Continuously varying voltage can be obtained.
Requires less copper and is more efficient.
Voltage regulation is superior.
76.What do you mean by no-load current of a transformer?
It is the input drawn by a transformer from the supply mains when its secondary windings are kept opened (or) no-load is connected to secondary.
77.Why the iron losses in a transformer are independent of the load current?
The hysteresis and eddy-current losses depend upon the maximum flux density in the core and the frequency. Since from no-load to full-load the flux linking with the core and the supply frequency remains constant, these losses remain constant, i.e., iron loss is independent of load current.
78.Name the two windings of a single-phase induction motor.
I. Running winding
ii. Starting winding.
79.What are the various methods available for making a single-phase motor self-starting?
I. By splitting the single phase
ii. By providing shading coil in the poles.
80.What is the function of capacitor in a single-phase induction motor?
I. To make the phase difference between the starting and running winding.
ii. To improve the power factor and to get more torque.
81.Give the names of three different types of single-phase motor.
I. Split phase motor
ii. Shaded pole motor.
iii. Single phase series motor.
iv. Repulsion motor.
82.What is the use of shading ring in a pole motor?
The shading coil causes the flux in the shaded portion to lag behind the flux in unshaded portion of pole. This gives in effect a rotation of flux across the pole face and under the influence of this moving flux a stating torque is developed.
83. State any four use of single-phase induction motor.
Fans, Wet grinders, Vacuum cleaners, small pumps, compressors, drills
84.State the advantages of capacitor start run motor over capacitor start motor.
Running torque is more; Power factor during running is more.
85.What is Universal motor?
A Universal motor is defined as a motor, which may be operated either on direct current or single-phase ac supply.
86.state some application of universal motor.
Used for sewing machines, table fans, Vaccum cleaners, hair driers, blowers etc
87.Explain why single-phase induction motor is not self-starting one.
When the motor is fed from a single phase supply its stator winding produces an alternating or pulsating flux, which develops no torque.
88.What type of motor is used for ceiling fan?
Capacitor start and capacitor run single-phase motor is used for ceiling fans.
89.what is the type of induction motor used in wet grinders?
Capacitor start capacitor run single-phase induction motor.
90what kind of motor is used in mixie?
Single-phase ac series motor is used in mixie.
91 what is the application of shaded pole induction motor?
Because of its small starting torque, it is generally used for small fans, toys, instruments, hair driers, ventilators, electric clock etc.
92. In which direction does a shaded pole motor run?
The rotor starts rotation in the direction from unshaded part to the shaded part.
93.why single-phase induction motor has low power factor?
The current through the running winding lags behind the supply voltage by a very large angle. Therefore power factor is very low.
94.Diffrentiate between “capacitor start “and “capacitor start capacitor run “induction motor?
In capacitor start motor, capacitor is connected in series with the starting winding. But it will be disconnected fro m the supply, when the motor picks up its speed. But in capacitor start capacitor run motor the above starting winding and capacitor are not disconnected, but always connected in the supply .so it has high starting and running torque.
95.What are the constant losses that occur in transformer.
i.Hysteresis loss
ii.Eddy current loss
96.Can the voltage regulation of atransformer of atransformer be negative,If so
under what condition.
For a leading power factor load the voltage regulation of atransformer will
be negative in value
97.What are the losses of ashunt machine assumed as constant.
Core losses,Mechanical losses and shunt field copper loss are assumed as
constant in shunt machine.
98.What is the condition for maximum efficiency of a D.C machine
Efficiency will be maximum when variable losses are equal to constant losses
99.What do you mean by residual EMF in agenerator.
The EMF induced in the armature conductor only due to the residual flux in the
field poles is known as residual EMF
100.Which metal is used for making commutator.
Commutator is used of high conductivity hard drawn copper.
