ElecticalQuestion 1: What is Electrical machine?
machine is an electro mechanical device which converts input motive
power ( prime mover like turbine in case of generator or electrical
supply in case of dc motor) to output motive power (Electrical power in
case of the generator or mechanical output in case of motor). This uses
the principle of electro magentic induction principle.
Electrical Question 2: What is the advantage of DC motors over AC motors?
the advert of the ac power generation and transmission over long
distances with less losses using electrical transformers, dc machines
were out ruled by the ac machines in industrial and home applications.
However dc motors still exist in the industries because of some special
properties of the dc machines where ac drives have failed to attain.
DC motors have some special properties such as:
- High Starting Torque
- Excellent speed control capability
traction applications in electrical trains and cranes motor drives
require to have high starting torque. DC series motor has very good
starting torque capabilities compared to induction motors and
synchronous motors. So still in present traction applications DC series
motors are used
In some applications such as paper mills and roll
mills speed control of the motor drive should be accurate else the
process will be failure. In such applications of finer speed control DC
shunt machines are widely used.
Electrical Question 3: What is the use of Commutator in DC machine?
is the part in DC machine that makes the difference from the ac
machine. In DC machine the current flow in the armature is ac. This
current is made uni directional with the help of commutator.
machines slip rings are used to collect the armature current from the
armature and supply to the load. In DC machines this slip rings are
replaced by the split rings. Split rings consists of 2 conductor
materials separated by insulating material.
In first half cycle
the induced current will be taken by the split rings or commutator which
is in one position.During the other half cycle when the induced current
in the armature changes the split rings position also changes results
in the delivering the unidirectional current to the external load.
Electrical Question 4: Explain essential parts of DC machine?
- Field System
- Brearings and Shaft
Electrical Question 5: Why field poles are laminated in DC machine ?
poles of a dc machine is laminated in order to reduce the eddy
currents. Eddy currents are the circulating currents that are induced in
the iron by the alternating magnetic field. These eddy currents
produced will create a flux which opposes the main flux. So this eddy
current production is considered as loss in the dc machine.Hence in
order to reduce this eddy current losses thin laminations are considered
and insulating coating is provided in between the laminations.
Electrical Questions 6: Do commutator convert ac to dc or dc to ac ?
in a dc machine converts both ac to dc current and dc to ac current.
Consider the dc generator the current produced in the armature due to
the electro magnetic induction is ac. However commutator converts the bi
directional current to uni directional current and supplies the
current the external load circuit.
In DC motor the voltage we
apply at the terminals is dc. This dc current is converted to ac and
supply to the armature circuit so as to produce the rotational torque.
Hence Commutator converts both ac to dc variables and dc to ac variables.
Electrical Question 7: Why armature teeth of DC machine is skewed ?
air gap length changes between the slot and teeth of the armature when
the armature is rotating under constant magnetic field. This results in
the production of small variation in the torque hence this results can
be observed as small vibrations in the frame of the machine. So in order
to avoid this vibration effect armature teeth is skewed.