power system fundamentals ee 317 lecture 7 20 october 2010
TRANSCRIPT
Power System Power System FundamentalsFundamentalsPower System Power System FundamentalsFundamentals
EE 317EE 317Lecture 7Lecture 7
20 October 2010
Chapter 4Chapter 4
AC Machinery Types and Highlights The Rotating Magnetic Field Relationship between Frequency and Magnetic
Field Rotation Induced Voltage in AC Machines Induced Torque in an AC Machine
AC Machinery TypesAC Machinery Types
AC machines (generators and motors) Two major classes:
Synchronous machinesInduction machines
Two major parts of machinesStatorRotor
Stators and RotorsStators and Rotors
STATOR
ROTOR
N
S
Simple Way to RememberSimple Way to Remember
Stator is STATIONARY part of machine Rotor is ROTATING part of the machine
Where’s the Field?Where’s the Field?In most AC machines, the magnetic field
circuits and field currents are in the rotors…
THIS MEANS the induced (or supplied) AC field and currents are in the stator:
If POWER comes out torque goes in and this is an AC generatorIf POWER is supplied torque comes out and this is an AC motor
Synchronous MachinesSynchronous MachinesMotors and generators whose magnetic
field current for the rotor is supplied by a separate DC power source
Synchronous generators are used to produce nearly all the electric power produced in the world
Induction MachinesInduction Machines Motors and generators whose magnetic field
current is supplied by magnetic induction (transformer action) into the field windings of the rotor (a DC power source is not required)
Although induction machines can be motors or generators they have many disadvantages as generators. Thus, they are referred to typically as induction motors. Most popular type of AC motor
The Rotating Magnetic FieldThe Rotating Magnetic Field
STATOR
ROTOR
N
S
N S
CCW Rotation
How we create these fields…How we create these fields…The fundamental principle of AC
machine operation is that: If a three-phase set of currents, each of equal magnitude and differing in phase by 120o, flows in a three-phase winding, it will produce a rotating magnetic field of constant magnitude
3-phase stator winding3-phase stator winding
STATOR
B-Field
N
S
1 1
23
32
http://www.shermanlab.com/xmwang/javappl/acMotor1.html
How we create these fields…How we create these fields… In a synchronous generator: DC currents
create N-S poles in the rotor, which drive a rotating magnetic field flux through the stationary coils of the stator
In an induction motor: AC currents in the stator create a rotating magnetic flux which the internal rotor (with N-S poles) constantly chases
Magnitude of B-fieldMagnitude of B-field
B11
B22
B33
Relationship between Frequency and Magnetic Field Rotation
Relationship between Frequency and Magnetic Field Rotation
IN A 3-PHASE
STATOR
N
S
1 1
23
32
What is speed of field rotation?
What is relationship?What is relationship? Of electrical frequency and speed of the
magnetic field within the stator?
Multiple Pole AC machinesMultiple Pole AC machines
A 3-phase, two pole machine has 6 stator windings
If you want to have a four pole machine how many stator windings will it have?
Winding orientation is as follows: a-c’-b-a’-c-b’ (a-c’-b-a’-c-b’)
Four pole – 3-phase machinesFour pole – 3-phase machines When 3-phase currents move through the 4 pole
stator – 2 N and 2 S poles are created The governing equations then change for angle,
frequency and rotation speed:
In general, for multi-pole machinesIn general, for multi-pole machines If magnetic poles on the AC stator number P,
then there are P/2 repetitions of the winding sequence ( a-c’-b-a’-c-b’ ) around its inner surface
And…
me
me
me
P
fP
f
P
2
2
2
Reversing Direction of RotationReversing Direction of RotationIf the direction of the current in any two of
the three coils in a 3-phase stator is swapped, the direction of the magnetic field rotation will be reversed
Take the winding sequence when you start your HW and prove it to yourself…
3-ph voltages3-ph voltages
Reversal of any phaseReversal of any phase
Induced Voltage in AC MachinesInduced Voltage in AC Machines
The same way 3-phase currents in the stator produce a rotating magnetic field, a rotating magnetic field can produce a 3-phase set of voltages in the stator.
EquationsEquations
RMS Voltage in a 3-phase StatorRMS Voltage in a 3-phase Stator
Wye vs. Delta ConnectionWye vs. Delta Connection
Rms value depends upon connection type When Y connected it is …? When delta connected it is just ephase
exampleexample Question 1: Label where each of the following electrical quantities would be
found in both the "Y" and "Delta" three-phase configurations:
•Phase voltage
•Line voltage
•Phase current
•Line current
ExamplesExamplesQuestion 2:Explain the difference between
a balanced polyphase system and an unbalanced polyphase system. What conditions typically cause a polyphase system to become unbalanced
AC Machine Power FlowsAC Machine Power Flows
Electrical or copper lossesCore lossesMechanical lossesStray load losses