EE 2201 – Electrical Engineering
2 Mark Questions and Answers
UNIT -I DC MACHINES
1.
State the
basic parts of a DC machine.
Stationary
Parts: Frame, Main pole, field coils, interpoles, interpole winding Rotating
Parts: Armature core, Armature winding, Commutator, Shaft.
2.
Name the
various parts of a DC machine that control the magnetic circuit.
Poles,
Air-gap, Armature core, Yoke.
3.
What is
prime mover?
The
basic source of mechanical power which drives the armature of the generator is
called prime mover.
4.
How is
voltage generated in rotating machines?
In
rotating machines voltage is generated in windings or group of coils by
rotating them through a magnetic field or by mechanically rotating a magnetic
field past the winding or by designing the magnetic circuit so that the
reluctance varies with rotation of the rotor.
5.
Write down
the emf equation for d.c generator.
E =
(ΦNZ / 60)(P/A) V Where, P= number of poles Z= Total number of conductors A=
number of parallel paths Φ= flux per pole N= speed in rpm
6.
Why is
Commutator employed in d.c machines? Or what is the function of a commutator in
a DC generator?
·
Conduct
electricity between armature and fixed brushes
·
Converts
alternating emf into unidirectional emf and vice versa
7.
How will
you change the direction of rotation of a d.c motor?
Either
the direction of the main field or the direction of current through the
armature conductors is to be reserved.
8.
What is
back emf in d.c motors?
As
the motor armature rotates, the system of conductor come across alternate North
and South Pole magnetic fields causing an emf induced in the conductors. The
direction of the emf induced in the conductors is in the direction opposite to
the current .As this emf always opposes the flow of current in motor operation
it is called back emf.
9.
Under what
condition the mechanical power developed in a dc motor will be maximum?
Condition
for mechanical power developed to be maximum is Eb=Va/2 or Ia= Va / 2Ra
10.
What is
the function of a no-voltage release coil provided in a dc motor starter?
As
long as the supply voltage is on healthy condition the current through the NVR
coil produce enough magnetic force of attraction and retain the starter handle
in the ON position against spring force. When the supply voltage fails or
becomes lower than a prescribed value the electromagnet may not have enough
force and the handle will come back to OFF position due to spring force
automatically. Thus, a no-voltage or under voltage protections is given to the
motor.
11.
Define
critical field resistance in dc shunt generator.
Critical
field resistance is defined as the resistance of the field circuit which will
cause the shunt generator just to build up its emf at a specified field.
12.
Why is the
emf not zero when the field current is reduced to zero in a dc generator?
Even
after the field current/magnetizing force is reduced to zero the machine is
left out with some flux as residue. Emf due to this residual flux is available
when field current is zero.
13.
Define the
term „critical speed‟ in dc shunt generator.
Critical
sped is defined as the speed at which the generator is to be driven to cause
self-excited generator to Build up its emf for the given field circuit
resistance.
14.
On what
occasions dc generators may not have residual flux?
·
The generator may
be put for its first operation after its construction.
·
In previous
operation the generator would have been fully demagnetized.
15.
What are
the conditions to be fulfilled for a dc shunt generator to build up emf?
·
The generator
should have residual flux
·
The field winding
should be connected in such a manner that the flux set up by the field winding
should be in the same direction as that of residual flux
·
The field circuit
resistance should be less than critical field resistance
·
Load circuit
resistance should be above its critical load resistance
16.
What are
the types of DC starters?
1.
Two point starters 2. Three point starters 3. Four point starters
17.
What are
the major categories of losses in a DC machine?
Magnetic
losses, Electrical losses, Mechanical losses
18.
Name the
different types of DC motors.
Shunt
motor, Series motor, cumulative compound motor, differential compound Motor.
19.
Name any
four applications of DC series motors.
Electric
traction, Food mixies, Hoist work, Drilling machine
20.
Why
starters are used for DC motors? Or Why a starter is necessary for a DC motor? Starters are used in DC motors to limit the starting
current within about 2 to 3 times the rated current by adding resistance in series
with the armature circuit. Apart from starting resistances starters are
invariably fitted with protective devices such as No-voltage protection.
21.
Why are
carbon brushes preferred for dc machines?
The
high contact resistance carbon brushes help the current in the coil undergoing
commutation to attain its full value in the reverse direction at the end of
commutation. The carbon brushes also lubricate and give less wear and tear on
commutator surface.
22.
Name any
two applications of DC series generator.
Booster,
electric welding, Constant current source, Constant illumination
23.
What is
the basic principle of a dc generator?
Basic
principle of a dc generator is Faraday’s law of electromagnetic induction. i.e.
whenever a conductor is moved in a magnetic field, dynamically induced emf is
produced in that conductor.
24.
What is
the purpose of yoke in a dc machine? Or The outer frame of a DC machine serves
double purpose. What are they?
·
It acts as a
protecting cover for the whole machine and provides mechanical support for the
machine.
·
It carries the
magnetic flux produced by the poles. The flux per pole divides at the yoke so
that; the yoke carries only half the flux produced by each pole.
25.
What are
the causes of failure of dc shunt generator to exite?
·
The residual
magnetism may not be present in the poles.
·
The field winding
may not be properly connected with armature.
·
Under no load
condition, the shunt field resistance should be greater than the critical field
resistance.
·
Under loaded
condition, the shunt field resistance should be less than the critical field
resistance.
26.
Why a dc
shunt motor is also called a constant flux motor or constant speed motor?
In
shunt motor, flux produced by field winding is directly proportional to the
field current i.e. (Φ α Ish). Here, the input voltage is constant and so the flux
is also constant. Therefore, DC shunt motor is also called a constant flux
motor or constant speed motor.
27.
Why series
motor cannot be started without any load?
In dc
series motor, flux is directly proportional to armature current. i.e. (Φ α Ia). Under no load
condition, the armature current is very low and flux also be less. By using the
formula N α (1/ Φ), here Φ is less; the motor speed will be very high. Due to
this motor will be damaged. Hence dc series motor should always be started with
some load on the shaft.
28.
What is
the function of starters in DC motor?
·
To limit the
starting current.
·
To protect
against low voltage and over load condition.
29.
List the
important parts of a DC starter.
Starting
resistance, Handle, over load relay, No voltage relay
30.
What are
the drawbacks of brake test on DC machines?
·
The brake test
can be used for small motors only, because in case of large motors, it is
difficult to dissipate the large amount of heat generated at the brake.
·
This method
cannot be used for determining internal losses.
·
The output of the
motor cannot be measured directly.