Lesson 33AC Generators
Learning Objectives Understand the operation of a single phase two pole AC generator.
Describe the operation of a simple AC generator.
Identify and define the components of a three phase two pole AC generator to include rotor, stator, armature. field windings, slip rings and brushes.
Understand the effects of applying a DC voltage power supply to a two pole rotor's field windings via brushes and slip rings.
Understand the induced effects that result from rotating the rotor's electromagnetic field past the armatures (Faraday's Law).
Given the armature coil sequence and their physical location, plot the induced AC voltages for a three phase two pole AC generator as a function of time and as phasors.
Understand the relationship between the number of poles and rpm of the rotor to the induced AC current's frequency.
Producing Electricity A generator is a machine that converts
mechanical energy into electrical energy. Motors and generators perform exactly the opposite
function However, motors and generator are essentially the
same device
Advantages of AC Power Motors
AC induction motors could be made more powerful
Voltage Transformation AC transformers allowed efficient changing of
voltage to enable power transmission
Power Transmission AC power can be transmitted hundreds of miles DC transmission limited to ~1 mile
Motor to Generator: Rotating DC Armature current (I) produces
force (F) in the armature causing rotation.
What if we remove the voltage source (VT) and we provided the torque?
Equivalent circuit representation
Motor to Generator: Rotating DC What if we remove the voltage source (VT) and we
provided the torque?
Basic Single-Phase AC Generator Turning the armature results in
induced emf (eAA ) across the load (Faraday’s Law). REMEMBER LAST LAB.
The voltage eAA will be single phase AC given
eAA = Vm sin t [V, volts] What determines ? Rotor
Three-Phase AC Generator What if we added two additional armature coils?
Single PhaseThree Phase
Three-Phase AC Generator Voltages as a function of time
)120sin(
)120sin(
sin
tVe
tVe
tVe
mCC
mBB
mAA
Three-Phase AC Generator Phasor representation
1202
1202
02
mCC
mBB
mAA
V
V
V
E
E
E
Phase sequence The phase sequence is the time order in which
the voltages pass through their respective maximum values.
Phase sequence is important because it determines the direction of rotation of a connected motor.
Positive phase sequence (ABC) The ABC sequence or positive sequence.
Example Problem 1
Plot the three phase voltages in the phasor and time domain if the generator was spun the opposite direction (start with phase A as the reference).
What are the equations?
Negative phase sequence (ACB) The ACB or negative sequence is produced when
the generator rotates clockwise.
sin
sin( 120 )
sin( 120 )
AA m
CC m
BB m
e V t
e V t
e V t
Negative phase sequence (ACB) The ACB or negative sequence is produced when
the generator rotates clockwise.
02
1202
1202
mAA
mCC
mBB
V
V
V
E
E
E
Large AC generator•Unlike our generator model with a fixed magnetic field and rotating armature, it is more practical to fix the armature windings and rotate the magnetic field on large generators.
•Brushes and slip rings pass EXCITATION voltage to the field windings on the rotor to create the magnetic field
•Minimizes current flow through brushes to rotor windings
Generator Stator Stator is slotted with integer multiple of 6 slots. Three pairs of slots contain identical coils of wire, each
with NS turns. These windings are called the armature.
Generator Rotor Rotor contains rotating electromagnet called
the field winding. The electromagnet is powered by a DC current
via slip rings and brushes. Unlike in the DC motor application, brushes are
not commutating and are not as subject to wear (less frictions).
Slip Rings Allow DC current to flow to the field windings
on the rotor to produce the magnetic field
Generator Output The amplitude of voltage output is a function of
the current supplied to the field windings. The stronger the current, the larger the
magnetic field, the larger the output voltage
Generator Frequency The frequency f (in Hz) of the AC voltage is a function
of speed of the rotor N (in RPM)
N = 60 f [RPM]
If the rotor contains multiple number of even poles (2, 4, 6, etc.) then
22 (rad/sec)
120 (RPM)
rotor
P
fPoles
fN
Poles
Synchronous Speed Synchronous Speed (speed of rotation of B)
versus Poles for a 60Hz Machine
P (poles)
2 4 6 8 10
N (RPM)
3600 1800 1200 900 720
[RPM] P
fN
120
Example Problem 2
For a 4 pole, 60 HZ generator, what is the speed in rpm of the rotor? 1800 rpm
What would be the frequency of a 6 pole machine spinning at the same rpm? 90 Hz
120 (RPM)P
fN
Poles