Dc Machines Flashcards
is a device which
converts mechanical energy into electrical
energy or vice versa.
ELECTRICAL MACHINES
– interaction of both magnetic and
electric field
– electromagnetic induction
BASIC Principles:
Electromagnetics
– are the fundamental
mechanism by which energy is converted from
one form to another in motors, generators, and
transformers.
Magnetic Fields
Four basic principles describe how magnetic fields are used in these devices:
- A current-carrying wire produces a
magnetic field in the area around it. - A time-changing magnetic field induces a voltage in a coil of wire if it passes
through that coil. (This is the basis of
Transformer Action). - A current-carrying wire in the presence
of a magnetic field has a force induced
in it. (This is the basis of Motor Action) - A moving wire in the presence of a
magnetic field has a voltage induced in
it. (This is the basis of Generator Action).
– the induced emf in a closed loop
equals to the negative of the time rate of
change of magnetic flux.
FARADAY’S Law
– The direction of any magnetic induction effect is such as to oppose the cause of the effect.
LENZ’S Law
– can work as generators, motors and
brakes
DC Machines
The machine is driven by a prime mover
(such as a steam turbine or a diesel engine)
with the mechanical power converted into
electrical power.
GENERATOR MODE
The machine is drives a mechanical load
with the electrical power supplied converted
into mechanical power.
MOTOR MODE
The machine decelerates on account of
the power supplied or dissipated by it and,
therefore, produces a mechanical braking
action.
Brake mode
Applications of DC Machines
DC Generators: 1-4
- early power systems (until late 1800’s)
- power sources for standalone systems
(cellular & telephone towers) - used to control speed and torque of
motors used in lifts, cranes, trains, - voltage regulators to make up for the
voltage drop in DC distribution feeders
such as railways.
Applications of DC Machines
DC Motors: 5-7
- commonly used in household (in toys,
electric shaver, electric toothbrushes,
printers, etc…) - small DC motors are used as control
devices - medical equipment which needs small
and precise of fluids such as a dialysis
machine.
Applications of DC Machines
DC Motors: 8-10
- industrial uses (steel, rolling mill motors
and railway traction motors)
. - power windows, car fans, starting motors in vehicles.
- as actuators and as speed and position
sensing devices (tachogenerators for
speed sensing and servomotors for
positioning and tracing.
- is an electrical machine which converts
mechanical energy (input) into direct
current electricity (output). This energy
conversion is based on the principle of
production of dynamically induced
emf.
DC Generator
- is a mechanical machine that drives the
rotor of a DC generator - this machine will serve as an input to
the armature of a generator.
Prime mover
• internal Combustion Engines (ICE)
➢either diesel or gasoline engine
• hydraulic turbines
• steam turbines
Prime movers for large DC generators
• synchronous motors
• squirrel cage induction motors
• internal combustion engines.
Prime movers for small DC generators
Three main parts of DC generator : MAC
- Magnetic field system
- Armature
- Commutator and brush gear
Other parts of DC generator:
MPFABBS
- magnetic frame and yoke
- pole core and pole shoes
- field or exciting coils
- armature coils and windings
- brushes
- bearings
- shaft
- is the stationary or fixed part of the machine
- it produces the main magnetic flux
MAGNETIC Field System
MAGNETIC FIELD system Consists of: FPF
- frame or yoke
- pole core and pole shoes
- field or exciting coils.
Frame or yoke
- for small generators, it is made
of cast iron
- for large generators, it is made
of cast steel or rolled steel
Frame or yoke
Purposes of frame or yoke
- serves as mechanical support
for the entire assembly - provides mechanical
protection to the inner parts of
the machine - provides a low reluctance path
for the magnetic flux - for small generators, it is made
of cast iron - for large generators, it is made
of cast steel or rolled steel
Purposes: - serves as mechanical support
for the entire assembly - provides mechanical
protection to the inner parts of
the machine - provides a low reluctance path
for the magnetic flux
- fixed to the magnetic frame
or yoke by bolts - has a curved surface which
are made of thin cast steel or
wrought iron laminations
which are riveted together
under hydraulic pressure - poles are laminated to
reduce Eddy current loSS.
Pole Core and Pole Shoes
- supports the field or exciting
coils - spread out the magnetic flux
over the armature periphery
more uniformly - increases the cross-sectional
area of the magnetic circuit,
as a results, the reluctance of
the magnetic path is reduced.
Pole Core and Pole Shoes
Purposes:
- placed over a pole core to
produce a magnetic field - enameled copper wire is
used for its construction - wound in the former and
placed around the pole core - when direct current passes
through the field winding, it
magnetizes the poles, which
turns produces the flux
Field or exciting coils
- the rotating part of
the DC machine or
DC generator - consists of a shaft
upon which a
laminated cylinder,
called armature
core is placed.
ARMATURE
- cylindrical or drumshaped and keyed to
the rotating shaft - the outer periphery has
groves or slots which
accommodate the
armature winding - is made of silicon steel
material to reduce
hysteresis loss
ARMATURE Core
- armature core is
laminated with a
stamping of about 0.3
to 0.5 mm thickness to
reduce Eddy current loss - each lamination is
insulated from the other
by a coating of varnish.
ARMATURE Core
- it houses the
conductors in the
slots - provides an easy
path for the
magnetic flux
ARMATURE Core
Purposes:
- the insulated conductors are
placed in the slots of the
armature core - conductors are wedged,
and bands of steel wire
wound around the core and
are suitably connected - this conductor arrangement
is called armature winding - The HEART of the DC
machine
ARMATURE Windings
- a place where the conversions of
power takes place - where mechanical power is
converted into electrical power
ARMATURE Windings
CLASSIFICATIONS of Armature Windings;
- LAP WINDING
- WAVE WINDING
– rotates with the armature
– cylindrical in shape and is
made from a number of
wedge-shaped hard drawn
copper bars or segments
insulated from each other
and from the shaft
– the segments form a ring
around the shaft of the
armature
– each commutator segment
is connected to the ends of
the armature coil.
COMMUTATOR
COMMUTATOR
Purposes:
– connects the rotating
armature conductors to
the stationary external
circuit through brushes
– converts the induced
alternating current in the
armature conductor into
the unidirectional current in
the external load circuit
– carbon brushes are placed
or mounted on the
commutator
– with the help of two or more
brushes current is collected
from the armature winding
– is supported in a metal box
called brush box or brush
holder
– made of high-grade carbon
(conducting material)
– provides lubricating effect on
the commutator surface
BRUSHES
– made of mild steel with
maximum breaking
strength
– Used to transfer
mechanical power from
or to the machine
– The rotating parts like the
armature core,
commutator, cooling
fans, etc… are keyed to
the shaft
SHAFT
– are fitted in the end
housings
– reduce the friction
between the rotating
and stationary parts of
the machine
– made of high carbon
steel as it is a very hard
materials
BEARINGS
