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P= VIL= 240 x 100= 24Kw
6. What are the conditions to be fulfilled for the self-excitation of a dc
shunt generator?
There must be some residual magnetism.
Shunt field coils should be properly connected to the armature terminals.
(iii) Shunt field resistance should be less than the critical resistance.
Important Short Answer Questions with answers
Class/Sem : II/III
Staff Name: Mrs.,S.Panchali A.P/EEE
1. A dc motor operates at from a 240 v supply. The armature resistance is 0.2 ohm.
Determine the back emf if armature current is 50 A.
Eb= V-IaRa; V=240V; Ra= 0.2; Ia= 50
Eb= 240- (50x0.2)= 240-10= 230 V
1. Define armature reaction.
If load current starts to flow in armature conductor it will produce some
flux. This flux may distort the main flux. Due to this drop will occur in the
armature side.
7. The emf per turn of single phase 6.6 kv/440 v , 50 hz transformer is
approximately 12 v. calculate the number of turns in HV and LV windings and
calculate the net cross sectional area of the core for the maximum flux density
of 1.5 T.
Emf/Turn = 12 V; V1= 6.6 Kv; V2= 440 V; F = 50 Hz; Bm = 1.5 T
E1 = N1 * Emf/Turn
N1= 6.6 x 103 / 12 = 550 turns
N2 = 440/12 = 37 turns
A = E1/( 4.44 F N1 Bm) = 0.036 m2
2. Define Back EMF.
It is the counter EMF produced by the motor which is in opposite direction
to the supply voltage due to the rotation of armature.
Eb = ϕZN/60 (P/A)
3. Draw the speed torque characteristics of DC series motor and write the
Applications of shunt motor.
8. Define the voltage transformation ratio of a transformer.
V1= E1; V2=E2
V2/V1 = E2/E1
9. Define voltage regulation of a transformer.
The regulation of a transformer is defined as reduction in magnitude of the
terminal voltage due to load, with respect to the no load terminal voltage
% Regulation = (( V2 on no load)- (V2 when loaded))/( V2 on no load) x
10. Why the transformer rating is in KVA?
Copper loss depend on current and iron loss depend on voltage. Hence the
total loss in a transformer depend on Volt-Ampere(VA) only and not on the
power factor. That is why the rating is in KVA
11. State the condition for maximum efficiency of a transformer?
Iron Loss= Copper Loss (or) Constant Loss = Variable Loss
5. A separately excited generator has induced emf of 250 V and a full
load terminal voltage of 240 V. if the value of Ra = 0.01 Ω, find the
full load current and output of the generator. Neglect armature reaction
and brush drop.
V= 240V; Eg= 250V; Ra= 0.01
Ia=(Eg-V)/Ra = 100A = IL
12. Draw the equivalent circuit of a transformer referred to primary?
3 phase induction motor is producing rottaing MMF in the airgap which produces
some rotation where as 1 phase induction motor is producing alternating flux in
the air gap which will not produce any rotation. That’s why the 1 phase motor is
not a sekf starting motor .
13. A 3 phase 2 pole motor is to have synchronous speed of 6000 rev/min.
calculate the frequency of supply voltage.
F= N P/120 = 6000 x 2 /120 = 100 Hz
14. A stator winding supplied from a 3 phase 60 hz system is required to
Produce the magnetic flux rotating at 900 rev/min. determine the number of
P= 120 F/N = 120 X 60 / 900 = 8
15. Define slip of an induction motor. And what is the relation between speed
and frequency.
S =( Ns- Nr) / Ns
Nr =( 1-S) Ns
Fr = S F
16. Draw the torque-speed characteristics of a 3 phase induction motor.
19. Distinguish between accuracy and precision.
Accuracy refers to degree of
Precision refers to degree of agreement
closeness of the measured value to
among group of readings
the true value
Accuracy gives the maximum error Precision of a measuring system gives
that is maximum departure of the
its capability to reproduce a certain
final result from its true value
reading with a given accuracy
20. List the different types of possible errors in measurements.
gross error
systematic error
environmental error
observation error
random error
limiting error
21. What is the difference between analog and digital instruments?
Information form
As the position of
As a number
pointer against a
calibrated scale or
Human error
Does not exists
+ 0.25%
Simple in
Construction is
22. What is the need for measurements?
The need of measurement is to know the unknown magnitude.
17. Why synchronous motor is called so.
The synchronous motor rotate in synchronous speed (i.e Ns =120 F /P) or in
constant speed that’s why this motor is called as synchronous motor.
18. Why the 3 phase induction motor is a self-starting motor and why not in 1 phase?
23. List the main functional elements used in most of the measurement systems.
(i) primary sensing unit
(ii) variable conversion element
(iii) data presentation element
24. How the transducers are classified.
Transducers are classified
on the basis of transduction form used
as primary and secondary transducers
as active and passive transducers
as analog and digital transducers
as transducers and inverse transducers.
25. What are the advantages of digital instruments over analog instruments?
(i) high accuracy
(ii) high precision
(iii) better resolution
(iv) no parallax and observational error
(v) very high input resistance et…
26. How are the digital voltmeter classified.
(i) integrating
(a) V/F converter type
(b) potentiometric type
(c) dual slope integrating type
(ii) Non integrating type
(a) potentiometer type
servo type
null balance type
successive approximation type
(b) Ramp type
linear type
staircase type
27. Define resolution and sensitivity of DVM.
Resolution R= 1/ 10n ; n- number of full digits in a digital display
Sensitivity S = (Fs)min x R ; Fs min – Full scale value of minimum range
28. What is sampling oscilloscope and list a few applications of CRO.
Sampling oscilloscope:
Above the range of 50-300 MHz, sampling techniques have to be employed to
obtain suitable display and CRO employing such sampling methods are called
sampling oscilloscopes.
Applications of CRO
It is capable of measuring
rise & fall time.
29. What is AC bridges and what are the 2 conditions must be satisfied to make an
bridge balance.
These type of bridges are used to measure the impedances consisting of
inductances and capacitances. Here the excitation voltage is AC.
maxwel bridge
hay bridge
schering bridge
Anderson bridge
Owen bridge
Wien bridge
Z1Zx = Z2 Z3
Θ1 + θx = θ2 + θ3
30. What is kelvin and kelvin double bridge?
Wheatstone bridge is not suitable for measurement of very low resistance. Kelvin
bridge is modification of W Bridge and is used to measure vales of resistance
below 1 ohm. In kelvin double bridge it include second set of ratio arms. This
circuit is mainly used for measuring very low resistance from 1 to 0.00001 ohm.
31. Why cannot induction motor run at synchronous speed?
Induction motor always run at a speed of less than synchronous speed. If the rotor
catch the MMF then ther will not be any relative velocity in turn the EMF produced is
also zero and due to this there is no torque to produce further rotation
32. What do you meant by step down transformer?
If number of turns in secondary winding is less than the number of turns in
primary winding ( N2 < N1) then the transformer is said to be a step down
33. Draw the equivalent circuit of induction motor with respect to primary?
34. Mention some applications of synchronous motor?
For constant speed applications.
Power factor improvement
35. What are the types of induction motor?
(i) 3 phase induction motor.
(a) squirrel cage
(b) slip ring or wound rotor
(ii) 1 phase induction motor.
36. Why should an alternator always runs at synchronous speed?
Alternator is meant for producing three phase voltage at constant
frequency. As the frequency of generated emf changes then it can’t feed to
the transmission line. So to produce the emf at constant frequency it must
rotate at synchronous speed.
37. Write down the emf equation of alternator?
RMS value of EMF/Phase = 4.44 KpKd F ϕ T volts
Where KP – pitch factor (or) coil span factor = cos (α/2)
Kd – Distribution factor = (sin mβ/2) / (m sin β/2)
F – frequency of induced EMF in Hz.
Φ – flux/ pole in Webbers
T – number of coils (or) turns / phase
38. Write down the EMF equation of transformer.
RMS value of EMF = 4.44 F ϕ N (or)
= 4.44 F N Bm A
Where F- frequency; ϕ – flux ; N- number of turns; Bm – maximum flux
density and A- Area of cross section of the core.
There are two types of induction motor
squirrelcage induction motor
wound rotor or slip ring induction motor.
42. Explain the working principle of induction motor.
Three phase supply produces stator current in the winding which in turn
produces rotating magnetic field. Rotating magnetic field produces emf if
the mmf cuts the stationary conductor. Due to this rotor current will
produce and this current produces the rotor field. The interaction of stator
and rotor fields develops torque.
43. Write the torque equation of an 3 phase induction motor under running
and Starting condition.
Trunning = (KsE22R2)/ (R22 + sX22)
Tstarting = (KE22R2)/ (R22 + X22)
45. What are the losses in a 3 phase induction motor?
(i) Core loss (or) Iron Loss ( Constant loss)
(ii) Copper loss
Mechanical loss ( friction loss + Windage loss) (Constant loss)
46. Explain the no load current in a 1 phase transformer.
The current which flows in the primary when secondary is in open.(I 0)
I0 = is comprises two components Iw and Iµ. Iw = I 0 cos ϕ0 and
Iµ = I0 sin ϕ0 ; I0 = √𝐼𝑊 2 + 𝐼𝜇 2
47. Relate the slip power ( PCU), Air gap power ( P2) and Power developed in
the rotor( Pm).
1: (1-S): S = P2 : Pm : Pcu
39. Define voltage regulation of alternator.
The percentage regulation is defined as the ratio of change in terminal
voltage from full load to no load rated terminal voltage.
% Reg = (E0 – V ) / V *100
where E0 – No load terminal voltage and V- full load rated terminal voltage.
40. Why the core is laminated?
To minimize the eddy current loss.
44. Draw the power flow diagram of a 3 phase induction motor.
41. Write the classifications of an 3 phase induction motor.
Dc motor is a self regulating machine, back EMF which causes a
changes in the load or armature current to produce the required torque
or torque developed by the motor to satisfy the load requirements
52. What are the different methods of excitation of generator?
Self and separate excitation
In self excitation it is classified as series, shunt and compound
53. What is a dc compound generator?
A generator has both series and shunt field windings is called
compound generator. It is classified as flat compound, over compound
and under compound.
54. What are the functions of interpoles?
To compensate the drop in main field flux and to provide better
55. What are the advantages of digital voltmeter?
48. Define shaft torque.
Motor develop a torque called armature torque or gross torque(Tm)
which is not maintained in the shaft. Due to some frictions the torque
is lost(Tf). The remaining torque is available in the shaft(Tsh)
Tsh= Tm-Tf
49 . Why series motor cannot be started without any load.
Speed of a dc motor is expressed as N= Eb/ϕ, initially the value of
flux is zero (i.e) field current or armature current is zero. Therefore
the speed becomes infinite. To avoid this series motor should be
started with some load.
50 Mention the applications of series and compound motor.
Series Motor applications: cranes, hoist load, traction etc..
Compound motor applications: driving a heavy machine tools, shear,
punching machines.
51. What is the significance of back emf?
high accurate reading can be taken.
better resolution.
High input impedance.
Reading speed is very high.
Digital output can be directly recorded.
56. List out the general characteristics of DVM.
input range.
Absolute accuracy.
Input resistance
Input capacitance
57. What is Q meter.
It is an instrument designed to measure some of the electrical properties of coils
and capacitors. The basic principle is series resonance. Q = XL/R
58. What are the various sources of errors in Q meter?
due to distributed capacitance.
due to residual inductance
Due to shunt resistor.
Due to conductance of Q voltmeter.
59. List the applications of Q meter.
to measure the Q of a coil.
to measure the inductance and capacitance.
To measure the distributed capacitance of a coil.
To measure q and power factor of a dielectric material.
To measure the mutual inductance of coupled circuits. Etc…