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Lesson 33
AC 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 Phase
Three Phase
Three-Phase AC Generator

Voltages as a function of time
eAA  Vm sin t
eBB  Vm sin( t  120 )
eCC  Vm sin( t  120 )
Three-Phase AC Generator

Phasor representation
E AA
E BB
E CC 
Vm

0 
2
V
 m   120
2
V
 m   120 
2
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.
eAA  Vm sin t
eCC   Vm sin( t  120 )
eBB  Vm sin(t  120 )
Negative phase sequence (ACB)

The ACB or negative sequence is produced when
the generator rotates clockwise.
E AA 
ECC  
E BB 
Vm
2
Vm
2
Vm
2
0
  120
  120
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

2

rotor  
 2 f (rad/sec)
 Poles 
NP 
120 f
(RPM)
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
120 f
N
P
[RPM]
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 f
NP 
(RPM)
Poles