Operating Principles Of Machines
DC Motor and DC Generator
Transformers & Rotating Magnetic Fields in AC Machines
Induction Motor and Synchronous Generator
Construction Of A Phase Transformer
Components
Transfomer immersed in a cooling oil to dissipate heat via natural convection
Conservator Tank
Oil in the tank will expand as they absorb heat
Induction Motor
Synchronous Generator
Lenz's Law
Direction of magnetic field created oppose the change in current through the rotor windings
Formulas
Synchronous Speed, Ns = rate of rotation of stator magnetic firld
Ns = 120 x fe/p
Slip, s is the difference between Ns and operating speed
s = Ns - (Nr/Ns)
Components
Rotor
Stator
Operation
3 phase power supply provides a rotating magnetic field in an induction motor
Stator's Magnetic field changes or rotates relative to the rotor
This induces an opposing current in the rotor
The rotating magnetic flux induces currents in rotor windings
Waveform of the generated voltage is synchronized with the rotation of the generator
Principles
DC Current is applied to the rotor winding
Produces a rotor magnetic field
Rotor is turned by prime mover
produces rotating magnetic field(RMF)
At stator/ armature winding, rmf induces a three phase set of voltages
Formulas
f= RPM x p/120
rate of rotation of the magnetic fields = the stator electrical frequency, fe
fe = Nm x p/120
DC Motors
Components
Commutator Rings
Supplies DC power
Always has reverse polarity with the stator brush
Torque action is always in the same direction
Stators
Supplies a constant magnetic field
Armature
Must have 2 to avoid irregular motion
Produces Back EMF
Back EMF: internal EMF that opposes applied input voltage
reduces armature current by a large amount
Back EMF is directly proportional to speed of the motor
When starting, back EMF is too low, thus armature current, EMFin is too high, causing motor to burnout.
To reduce back EMF, put coil in magnet to oppose direction of magnetic field
Definition
Converts DC electricity into kinetic energy
Electromagnet
Field Coils
Rotors Windings (Parallel)
Shunt Motor
Low starting torque
Able to run at constant speed regardless of load
Rotor Windings (Series)
Series Motor
Good starting torque
Speed drops with load
DC Generators
Definition
Converts mechanical/kinetic energy to electrical energy through magnetic induction
Components
Magnetic Circuit
Yoke
Protecting shield
Provides mechanical support
Cast Iron for small generators
Cast steel or rolled steel for large generators
Pole Core
spreads out flux
reduces reluctance of magnetic path
Supports field coils
Field Coils
produce magnetic flux
Armature Core
Path of Low Reluctance to Flux through armature
Armature Winding
Lap Winding
No. of Brushes = No. of Parallel Paths = No. of Poles
For High Current, Low Voltage Machines
Wave winding
No of Parallel Path = 2
For High Voltage, Low Current Machines
Commutator
Collects current from armature conductors
Converts AC to unidirectional current
Brushes
Collects current from commutator
Bearings
Principle Of Transformers
AC system so that EMF alternates, magnetic field will also fluctuate
Primary Winding
Secondary Winding
Current carrying conductor produces magnetic field
AC EMF system produces fluctuating magnetic field
Magnetic field causes electromagnetic induction in the secondary winding
Formula of Transformers
Es = (Ep/Np) x Ns
Es x Is = Ep x Ip
3 Phase Transformers
High voltage windings
High voltage insulated bushings
Transformer core made out of thin insulated steel laminations that are stacked together
Disc type windings
Low voltage windings
Connected in series through outer and inner crossovers
Connected in a star configuration
Connected in a delta configuration
Energy losses
Eddy current loss
Hysteresis loss
click to edit