Assignment-5
UNIT-4(a):D.C.
Generators
1. (A)
Explain in detail, the principle of a single loop DC generator.
(OR)
Explain the principle of
operation of DC generator.
(B). A 4-pole
generator having wave wound armature winding has 48 slots, each slot contains
20 conductors. What will be the voltage generated in the machine when driven at
1500 r.p.m? Assume the flux per pole is 7.0 mWb.
2. (A).
Explain briefly various types of generators.
(Or)
Explain different types of excitations in D.C. Generators
(Or)
Explain how will you classify DC generators in detail and also
explain the types of DC generators
(B) Write the applications of D.C generators.
(C). A 6 – pole, Lap wound armature has 840 conductors and flux
per pole of 0.018 Wb. Calculate
the emf generated when the machine is running at 600 rpm.
3. (A) Draw
a detail sketch of a DC machine and identify the different parts. Briefly
explain the function of each major part.
(or)
List out the main parts of a DC machine and explain with help of
neat sketch.
(or)
Explain the
construction of D.C.Generator
(B) A 4 – pole DC shunt generator with lap connected armature supplies a
load of 100 A at 200 V. The armature resistance 0.1 ohms and the shunt field
resistance is 80 ohms find the i) Total armature current ii) Current per
armature path iii) Emf generated.
4.
(A). Explain the losses that occur in a DC machine.(or) explain the total
losses in D.C. Generator (or)Explain the total losses in D.C. Motor and explain
how to reduce them
(B) A 4 pole D.C. shunt generator
with wave wound armature has 45 slots each having 15 conductors. The armature
resistance is 0.5 Ω and the shunt field resistance is 200 Ω.
The flux per pole is 30 m Wb. If a load of 15 Ω resistance is
connected across the
Armature terminals. Calculate the voltage across the load when
the generator is driven
at 1200 r.p.m. (ans 781.9)
5. (A). Derive from
first principle an expression for the induced EMF in the armature of the dc
Machine. (Or) Derive the emf equation of D.C. Generator.
(B) A dynamo has a rated armature
current at 250A.what is the current per path of the armature if the armature
winding is lap or wave connected? The machine has 12 poles.
6. (A) Discuss in detail the process of voltage build up in
a dc generator and explain about causes for failure of voltage build up process
in d.c generator.
(B) A separately exicted D.C generator ,when running at
1000RPM supplied 200A at 125V.what will be the load current when the speed
drops to 800RPM if “If” is unchanged? Given that the armature
resistance=Ra=0.04Ω and brush drop=2V
Assignment-6
UNIT-4B:D.C.MOTORS
1.
(A).Explain the principle of operation of DC motors.
(B). A 220V series motor runs
at 800RPM, when taking a current of 15A the motor has Ra=0.3ohm and Rf=0.2ohm.find
the resistance to be connected in series with the armature. If it has to take
the same current at the same voltage at 600 RPM.assume flux is proportional to
current.
2. (A) Derive the torque equation of a DC
motor.
(B) A 250 V. D.C shunt motor takes
a line current of 20 A. Resistance of shunt field winding is 200ohms and
resistance of the armature is 0.3ohms. Find the armature current and the back
e.m.f.
3. (A) what
is the significance of back emf generated in a DC machine hence explain the
principle of operation of a DC motor.
(or)
Explain
how back emf makes d.c motor self regulating.
(B) A 100V series motor takes 45 A when running at 750 rpm. Its armature
resistance is 0.22 ohms, while the series field resistance is 0.13 ohms. Iron
and frictional losses amounts to 750 W.Find the shaft power.
4. (A) Draw the
power flow diagram of a DC shunt motor and also explain the following terms
i. Electrical
Efficiency,
ii. Mechanical
Efficiency and
iii. Commercial
Efficiency.
(B) A 220 V D.C shunt motor runs at 500 r.p.m when the
armature current is 50 A. Calculate the speed if the torque is doubled. The
armature resistance of the motor is 0.2 Ω
5. (A) Write the
applications of Dc motors.
(B) A
460V D.C series motor runs at 1000RPM taking a current of 40A.caluculate the
speed and percentage change in torque if the load is reduced so that the motor
is taking 30A.total resistance of the armature and field circuits is
0.8Ω.Assume ‘Ø’ is proportional to filed current.
6. (A) Explain
the losses in D.C. Motor (b) What happens if D.C series motor is started
on no load?(C) why series motors are applicable for traction applications
Assignment-7
UNIT4C:A.C.MACHINES
(3 PHASE INDUCTION MOTOR)
1. (A) Explain the principle of operation of a 3 phase
induction motor.
(B) . Three phase induction motor is wound
for 4 – poles and is supplied from a 50 Hz supply. Calculate
i) The synchronous speed
ii) The speed of the motor
when the slip is 3 %
iii)
The rotor frequency.
*****2. (A) Explain with the help of diagrams
how a rotating magnetic field is produced in a 3 phase induction motor. (Or)
Explain why 3-phase induction motor is self stating induction motor.
(Or)
A rotating magnetic field is produced in the air gap of a 3-phase
induction motor when a balanced supply is given to the stator. Justify the
statement with necessary mathematical equations and phasor diagrams.
(B).power input to a
three phase induction motor is 75kw.the stator losses total 1.5kw.find the total
mechanical power developed and the rotor copper losses per phase if the motor
is running at a slip of 3%.
3. How the induction motors are classified? Explain the types
of induction motors in detail.
4. (A) Explain why a 3-phase induction motor cannot develop torque when running at synchronous speed.
(B) Explain why induction motor is called rotating transformer
(C). A 3 phase 4 pole 50 Hz induction motor at stand still has 180v induced across its star connected terminals. The rotor resistance and stand still reactance per phase are 0.6ohm and 0.4ohm respectively. Calculate the speed when the rotor is drawing a current of 6A at a particular load. Also calculate the speed at which the torque is maximum and the corresponding value of the rotor.
4. (A) Explain why a 3-phase induction motor cannot develop torque when running at synchronous speed.
(B) Explain why induction motor is called rotating transformer
(C). A 3 phase 4 pole 50 Hz induction motor at stand still has 180v induced across its star connected terminals. The rotor resistance and stand still reactance per phase are 0.6ohm and 0.4ohm respectively. Calculate the speed when the rotor is drawing a current of 6A at a particular load. Also calculate the speed at which the torque is maximum and the corresponding value of the rotor.
5. (A) Derive the condition for maximum torque under running
condition of 3-phase induction Motor
(B) a three phase induction motor has starting torque of 100% and a maximum torque of 200% of the full load torque. Find the slip at maximum torque.
(B) a three phase induction motor has starting torque of 100% and a maximum torque of 200% of the full load torque. Find the slip at maximum torque.
6. (a) Explain the effect of slip on the rotor circuit.
(b) A 6 pole induction motor is fed by 3-phase supply and running with a full load slip of 3% .find the full load slip of 3 phase induction motor and also the frequency of rotor emf.
(b) A 6 pole induction motor is fed by 3-phase supply and running with a full load slip of 3% .find the full load slip of 3 phase induction motor and also the frequency of rotor emf.
Assignment-8
UNIT-5: BASIC INSTRUMENTS
1. (A)
explain the classification instruments
(B)Explain the operating principles
of Electrical instruments
2. Explain
the essentials of indicating instruments.
3. (a)
Compare spring control with gravity control
(b) Explain about damping curves.
4. (a)Explain
the construction and operation of Permanent Magnet Moving Coil instruments with
a neat diagram.
(b) Explain PMMC ammeters and
voltmeters.
(c) a moving coil voltmeter with a
resistance of 20Ω gives a full scale deflection
of 120°
When a potential difference of
100mV is applied across the moving coil. The moving coil has dimensions of 30mm×25mm
and is wound with 100turns.the control spring constant is 0.375×10-6 Nm/deg.find the flux density in the air
gap .find also the diameter of copper wire of coil
winding if 30 percent of instrument resistance is due to coil winding. The
specific resistance for copper =1.7×10-8 ohm-meter.
5. (a) Prove that deflecting torque is Proportional
to the Square of the current in
Attraction type MI instruments.(General
torque equation of MI instruments)
(b)
Explain the construction and operation of Moving iron attraction type
instruments with a Neat diagram.
(c) The inductance of a moving iron instrument
is given by L=(10+5ⱷ-ⱷ2) µh where “ⱷ” is the deflection in radian
from zero position. The spring constant is 12×10-6 nm/rad. Estimate
the deflection for a current of 5A.
6. (a) Explain the construction and operation
of Moving iron Repulsion type instruments with
a Neat diagram.
(b) Explain
advantages and disadvantages and errors in MI instruments.