Chapter2-Synch Gen Fundamentals

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Synchronous Generator Fundamentals 1

Chapter 2:

Synchronous Generator Fundamentals

Figure 1-2 shows a magnet mounted on a rotor, and a

coil on a stator. When the shaft rotates the rotor flux

will induce a voltage in the coil. The frequency of thevoltage produced is:

( )mprinN

f rs ..60

= [2-1]

With Nrbeing the speed of rotation of the rotor.

If the rotor spins at a constant angular frequency

,driven by a mechanical PRIME MOVER, there willbe an induced voltage in the stator coil according to

dt

de

=

as tEe sinmax1= [2-2]Figure 1-2:

I

If one distributes the coils on the stator as in fig.2-2, one

can obtain a quasi sinusoidal induced voltage. Instead of astacked coil one can use slotted coils which are

distributed in a better way under a magnetic pole (N-S).These windings are connected in series in such a way that

the terminal voltage is near sinusoidal. Winding layouts isa special topic and quite complicated. In this introductory

course let us simply use an effective number of turns per

phase (Ns). This produces an induced voltage which canbe approximated as:

Figure 2-2

pse fNE = 44.4 [2-3]with p = flux per pole, and f the rated synchronous frequency and Ns the

number of turns on the stator

N

S

e1

N

S

coils in

series


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Finally fig. 2-3 shows that one can place 3 windingswith a PHYSICAL or GEOMETRIC placement 120

o

with respect to each other. When the magnet (NS)

rotates, the voltage induced in each coil will have thesame frequency, but out of phase (time delay) by 120o

One could write the equations of the 3 phases as:

( )

+=

=

=

3

2sin)(

3

2sin)(

sin)(

3

2

1

tEte

tEte

tEte

so

so

so

[2-4]

or draw a FRESNEL vector diagramwhich is a vector representation of each

voltage induced on a plane which rotates

in reverse direct with the angularfrequency.

Note the convention of (+) annotation

Figure 2-4:

The stator of a synchronous machine receives the ARMATUREwindings, which areconnected to the electrical load. When the physical phase angle of the coils are 90, we

have a 2 phase system. When the physical phase angle of the coils are 120, we have a 3

phase system.

The rotor of a synchronous generator provides the magnet (magnetic field), which

rotates with the speed of the rotor (driven by the mechanical prime mover). There are 3

types of rotors, also called excitation:

N

S

e1

e2

e3 figure 3

e1

e3 e2

Ns

Look at phases

going by


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1. Cylindrical Rotor (or non-salient):

Figure 2-5

Figure 2-5 shows a 2 pole machine. Note the 2 sets of windings with the appropriate

direction of currents in each, which produce a NORTH and SOUTH pole in the air gap.Because the windings are wound on a doughnut shaped cylinder, they have no physical

pole shapes, hence its name of cylindrical(or smooth) rotor.

2. Salient Pole Rotor:

Figure 6 shows the rotor of a 6 pole machine (or

3 pairs of poles), where the poles appear asdistinct metallic structures. In the gaps, the coils

are wound to provide with the DC excitation

providing the magnets.

Note that to produce a 50Hz armature voltage, a

2 pole, 3 phase machine has to rotate at 3000rpm,while a 4 pole machine rotates at 1500rpm, and a

6 pole machine at 1000rpm.

(In North America, where 60Hz is used, 3600rpm,1800rpm and 1200rpm).

Figure 2-6

3. Permanent Magnet Rotor

These motors are similar to the salient pole rotors, with the exception that the poles are

now made with permanent magnets, hence the excitation is constant.


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Excitat ion

The electrical excitation of a synchronous generator has to go into the rotor, and send aDC current in the pole windings. There 3 formal methods to do that:

1. Slip Ring:

As shown in figure 2-6, the rotor has 2 slip rings mounted at the end and connected in

series with the excitation windings. Two brushes ride the slip rings and provide the DCvoltage from an external source.

2. DC Generator mounted on the rotor

A common practice for older types of generators was to mount a DC generator directly

on the rotor of the synchronous generator. In a DC machine, the armature is the rotor, and

therefore the excitation is on the stator while the DC current supplied to the excitation of

the synchronous generator is on the rotor and there is no need for brushes. The control ofthe excitation of the DC machine is on the stator, and hence accessible by the controller.

Figure 2-7shows the layout.


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3. Brushless Exciter:

The DC generator, called the exciter has a high maintenance, and is a low efficiency

device, and its control is done by yet another control for supplying its field. A modern

design will use power electronics to provide both a high efficiency and ease of

controllability.

Typically a 3 phase synchronous permanent magnet generator (see dynamo) is used,

however it is inside-out. The rotor has the 3 phase armature windings, and the statorhas the excitation circuit. Figure 2-8 shows the inverted synchronous exciter providing

a 3 phase voltage (on the rotor windings) that can be rectified and connected to the field

windings of the large synchronous generator. Note that the excitation of the exciter isprovided by a controlled rectifier on the stator circuits. There are no brushes involved,

and this has minimum maintenance.

If

Synchronous generator

main field

ROTOR

STATOR

Exciter

armature

Exciter Field

3 phase low current controlled rectifer

Main Armature

3phase

Output

Figure 2-8


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Figure 2-9 shows the picture of a brushless salient pole rotor

Figure 2-9:


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Figure 2-10 shows a cutaway diagram of a large synchronous generator. Note the

various parts, salient poles, cooling fans, on shaft exciter etc.

Figure 2-10:

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Chapter2-Synch Gen Fundamentals