Download The operation of a rotating magnetic machine is

JRE SCHOOL OF Engineering
CLASS TEST-I
Subject Name Electrical machine
Roll No. of Student
Date
23rd March 2015
For AEI branch only
SECTION – A
Subject Code
Max Marks
Max Duration
Time
March 2015
EIC 602
30
1 hr
9.20 am- 10.20 am
(5*3 marks =15marks)
NOTE: ATTEMPT ALL QUESTION
1.
Discuss the concept of rotating magnetic field in 3 phase distributed winding.
Ans: a rotating magnetic field is that which is constant in magnitude but whose
axis of direction rotates in space as field system of a dc machine. When a 3
phase winding is energized by a three phase supply, a rotating magnetic field is
developed that tends the rotor of the machine to move in the direction of
rotation.
The operation of a rotating magnetic machine is based on the production of
rotating magnetic field. Energy transfer phenomenon occurs due to such
rotating magnetic field.
2.
Draw a neat sketch of electrical dc machine showing all main parts.
3.
Draw the characteristics of dc shunt generator.
4. Why are starters necessary for dc motor?
Ans: Starting of DC Motor: The starting of DC motor is somewhat different from the
starting of all other types of electrical motors. It has a very high starting current that
has the potential of damaging the internal circuit of the armature winding of dc motor if
not restricted to some limited value. To limit this starting current we need the starter.
Starter is a device containing a variable resistance connected in series to the armature
winding so as to limit the starting current of dc motor to a desired optimum value
taking into consideration the safety aspect of the motor.
the basic operational voltage equation of the dc motor given by,
V = Eb + Ia Ra, where V is the supply voltage, Ia is the armature current, Ra is the
armature resistance, and the back emf is given by Eb. The back emf, in a dc motor, is
very similar to the generated emf of a dc generator as it is produced by the rotational
motion of the current carrying armature conductor in presence of the field. This back
emf of dc motor is given by
and has a major role to play in case of the starting of dc motor.
From this equation we can see that Eb is directly proportional to the speed N of the
motor. Now since at starting N = 0, Eb is also zero, and under this circumstance
the voltage equation is modified to
V = 0 + Ia Ra,
Therefore, Ia = V/ Ra
For all practical practices to obtain optimum operation of the motor the armature
resistance is kept very small usually of the order of 0.5 Ω and the bare minimum
supply voltage being 220 volts. Even under these circumstance the starting current, I a is
as high as 220/0.5 amp = 440 amp.
Such high starting current of dc motor creates two major problems.
1) Firstly, current of the order of 400 A has the potential of damaging the internal
circuit of the armature winding of dc motor at the very onset.
2) Secondly, since the torque equation of dc motor is given by
T = Φ Ia
Very high electromagnetic starting torque of DC motor is produced by virtue of the high
starting current, which has the potential of producing huge centrifugal force capable of
flying off the rotor winding from the slots.
5. State advantages of synchronous machine over dc machine.
ANS: Synchronous generators/ alternators are used to produce high ac voltage, so
(1)
commutator is absent.
(2)
The field winding is on the rotor while the armature is on rotor. Stationary
armature helps to provide large space to accommodate large numbers of
conductors. It is easy to collect large current from stationary members.
SECTION – B
(1*5marks = 5 Marks)
NOTE: ATTEMPT ANY ONE
6.
Derive the expression for E.M.F equation of a dc machine.
E.M.F. Equation of a DC machine:
Let,

Z
= Flux per pole in Weber
= Total Number of armature conductors
= Number of slots x Number of conductors per slot
P
= Number of generator pole
A
= Number of Parallel path in Armature
N
= Armature rotation in revolution per minute (i.e., in rpm)
E
= e.m.f. induced in any parallel path in armature
According to the Law of electromagnetic Induction, the generated or induced e.m.f
is proportional to rate of change of flux with respect to time and if only one
conductor is taken in to consideration then the induced e.m.f is given by
E
d
dt
(1)
Initially let us consider only one conductor, which is rotating inside a magnetic
field having ‘P’ number of poles. If the conductor makes one complete rotation
then to total flux cut by conductor
Total Flux  P
(2)
If ‘N’ is the speed in rpm at which the conductor is rotating in that magnetic field
then time taken by the conductor for one complete rotation
Time for one complete rotation =
1
60
min utes 
sec onds
N
N
(3)
There fore
total flux cut in one revolution
Average e.m. f induce in one conductor 
E
P
60 / N
time taken for one complete revolution

PN
60
(4)
If the machine has total ‘Z’ number of conductors and has ‘A’ number of parallel
path, number of conductors in each parallel path is
Number of conductor / Parallel Path 
Z
A
(5)
The conductors in each parallel path are connected in series hence the total e.m.f
is added. Total e.m.f induced in all conductors will be
E
PN
60

Z
A
(6)
Or
7.
E
ZNP
60 A
Explain with a neat sketch working principle of dc motor.
(7)
SECTION – C
(1*10 marks = 10 marks)
NOTE: ATTEMPT ANY ONE
8.
What is armature reaction? Explain with neat sketches.
Ans: The flux in a d.c. machine due to the main poles called main flux. However,
current flowing through armature conductors also creates a magnetic flux (called
armature flux) that distorts and weakens the flux coming from the poles. This
distortion and field weakening takes place in both generators and motors. The action of
armature flux on the main flux is known as armature reaction.
With no current in armature conductors, the M.N.A. coincides with G.N.A.However,
when current flows in armature conductors, the combined action of main flux and
armature flux shifts the M.N.A. from G.N.A. In case of a generator, the M.N.A. is shifted
in the direction of rotation of the machine. In order to achieve sparkless commutation,
the brushes have to be moved along the new M.N.A. Under such a condition, the
armature reaction produces the following two effects:
1. It demagnetizes or weakens the main flux.
2. It cross-magnetizes or distorts the main flux.
9.
Describe the different methods of speed control of dc motor.
Ans: We know, back emf of a DC motor Eb is the induced emf due to rotation of
the armature in magnetic field. Thus value of the E b can be given by the EMF
equation of a DC generator.
Eb = PØNZ/60A
(where, P= no. of poles, Ø=flux/pole, N=speed in rpm, Z=no. of armature
conductors, A=parallel paths)
Eb can also be given as,
Eb = V- IaRa
thus from above equations
N=
E 60A/
b
PØZ
but, for a DC motor A, P and Z are constant
Nα K
E /
b Ø
(where, K=constant)
thus, it shows speed is directly proportional to back emf and inversely
proportional to the flux per pole.
Speed Control Methods Of DC Motor
Speed Control Of Shunt Motor
1. Flux Control Method
It is seen that speed of the motor is inversely proportional to flux. Thus by
decreasing flux speed can be increased and vice versa.
To control the flux, a rheostat is added in series with the field winding, as shown
in the circuit diagram. Adding more resistance in series with field winding will
increase the speed, as it will decrease the flux. Field current is relatively small
and hence I2R loss is small, hence this method is quiet efficient. Though speed
can be increased by reducing flux with this method, it puts a limit to maximum
speed as weakening of flux beyond the limit will adversely affect the
commutation.
Armature Control Method:
Speed of the motor is directly proportional to the back emf Eb and Eb = V- IaRa.
That is when supply voltage V and armature resistance R a are kept constant,
speed is directly proportional to armature current Ia. Thus if we add resistance in
series with armature, Ia decreases and hence speed decreases.
Greater the resistance in series with armature, greater the decrease in speed.
3. Voltage Control Method
A) Multiple voltage control: In this method the, shunt filed is connected to a
fixed exciting voltage, and armature is supplied with different voltages. Voltage
across armature is changed with the help of a suitable switchgear. The speed is
approximately proportional to the voltage across the armature.
B) Ward-Leonard System:
This system is used where very
sensitive speed control of motor is required (e.g electric excavators, elevators
etc.) The arrangement of this system is as required in the figure beside.
M2 is the motor whose speed control is required.
M1 may be any AC motor or DC motor with constant speed.
G is the generator directly coupled to M1.
In this method the output from the generator G is fed to the armature of the
motor M2 whose speed is to be controlled. The output voltage of the generator G
can be varied from zero to its maximum value, and hence the armature voltage of
the motor M2 is varied very smoothly. Hence very smooth speed control of
motor can be obtained by this method.
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