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EE301 – Lesson 36

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AC GENERATORS PART I

Learning Objectives 

(a) Understand the operation of a single phase two pole AC generator.(b) Describe the operation of a simple AC generator.

(c) Identify and define the components of a three phase two pole AC generator to include rotor, stator,armature, field windings, slip rings and brushes.(d) Understand the effects of applying a DC voltage power supply to a two pole rotor's field windings via

 brushes and slip rings.

(e) Understand the induced effects that result from rotating the rotor's electromagnetic field past thearmatures (Faraday's Law).

(f) 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.

(g) Understand the relationship between the number of poles and rpm of the rotor to the induced ACcurrent's frequency. 

A motor is a machine that converts electrical energy into mechanical energy. A generator does exactly

the opposite: it converts mechanical energy into electrical energy. 

Advantages of AC Power 

•  Motors: AC is natural for rotary motors

•  Voltage Transformation: AC transformers allowed efficient changing of voltage to enable power transmission

•  Power Transmission: AC power can be transmitted hundreds of miles whereas DC transmission

limited to ~1 mile

The AC Generator First recall the DC motor: an armature current (I) produces force (F) in the

armature causing rotation.

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EE301 – Lesson 36

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What if, instead, we remove the voltage source (VT) and we provide the torque?

Turning the armature results in an induced emf (e AA′  ) across the load (Faraday’s Law). The voltage e AA′   will be single phase AC given by e AA′   = Vm sin ω t  [V, volts].

What determines ω ?

What if we added two additional armature coils?sin

sin( 120 )

sin( 120 )

 AA m

 BB m

CC m

e V t 

e V t 

e V t 

ω 

ω 

ω 

′

′

′

=

= −

= +

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EE301 – Lesson 36

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Converting to phasors:

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):

 Negative phase sequence (ACB):

Unlike our generator model with a fixed magnetic field and rotating armature, shown above, in practice

it is more practical to fix the armature windings and rotate the magnetic field on large generators.

This is because a rotating armature requires slip rings and brushes to conduct the current from the

armature to the load. The result for higher voltage alternators is arc-overs and short circuits and hence

inefficiency and damage at the slip ring/brush interface at high generated voltages.

02

120

2

1202

m AA

m BB

mCC 

V 

V 

V 

′

′

′

= ∠

= ∠ −

= ∠ +

E

E

E

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EE301 – Lesson 36

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The revolving-field type alternator has a stationary armature and a rotating magnetic field. Despite this

major difference, the physics and the operation of the alternator is essentially the same as discussed for 

the elementary generator (a conductor in a magnetic field with relative motion between them).

Again, the advantage of having a stationary armature is that the generated voltage can be connected directly to the load without having to pass across the slip rings and brushes. Therefore, high-voltage

alternators are usually of the rotating field type. The voltage applied to generate the rotating field is a

small DC voltage (called a field excitation voltage) and, therefore, the problem of arc-over at the sliprings is not encountered.

The 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.

The rotor contains the 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 notcommutating and are not as subject to wear.

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EE301 – Lesson 36

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The slip rings allow DC current to flow to the field windings on the rotor to produce the magnetic field.

The amplitude of the voltage output from the AC generator is a function of the current supplied to thefield windings. The stronger the current, the larger the magnetic field, and 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 a multiple number of poles (2, 4, 6, etc.) then

Synchronous Speed versus Poles for a 60Hz Machine:

P

(poles) 2  4  6  8  10 

 N  

(RPM) 3600  1800  1200  900  720 

Example. For a 4 pole, 60 HZ generator, what is the speed in rpm of the rotor?

Solution.

Example. What would be the frequency of a 6 pole machine spinning at the same rpm as the machine in

the prior example?

Solution.