Download Lecture 7

Power System Fundamentals
EE 317
Lecture 7
20 October 2010
Chapter 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 Types
 AC machines (generators and motors)
 Two major classes:
Synchronous machines
Induction machines
 Two major parts of machines
Stator
Rotor
Stators and Rotors
STATOR
S
ROTOR
N
Simple Way to Remember
 Stator is STATIONARY part of machine
 Rotor is ROTATING part of the machine
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 generator
If POWER is supplied  torque comes out and this
is an AC motor
Synchronous Machines
 Motors 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 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 Field
STATOR
S
N
ROTOR
N
CCW Rotation
S
How we create these fields…
 The fundamental principle of AC
machine operation is that: If a threephase 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 winding
3
2
STATOR
S
B-Field
1
1
N
3
2
 http://www.shermanlab.com/xmwang/javappl/ac
Motor1.html
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-field
B33
B11
B22
Relationship between Frequency
and Magnetic Field Rotation
IN A 3-PHASE
2
3
S
STATOR
1
1
3
N
2
What is speed of field rotation?
What is relationship?
 Of electrical frequency and speed of the
magnetic field within the stator?
Multiple 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 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 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
P
 And…
e 
m
2
P
fe 
fm
2
P
e 
m
2
Reversing Direction of Rotation
 If 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 voltages
Reversal of any phase
Induced 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.
Equations
RMS Voltage in a 3-phase Stator
Wye vs. Delta Connection
 Rms value depends upon connection type
 When Y connected it is …?
 When delta connected it is just ephase
example


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
Examples
 Question 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 Flows
Electrical or copper losses
Core losses
Mechanical losses
Stray load losses