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
