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