Rabu, 30 Maret 2011

Working Principle of synchronous generator


Working Principle of synchronous generator

After we discuss here about the construction of a synchronous generator, then this article will discuss about the working principle of a synchronous generator. That would be a framework discussion this time is the operation of synchronous generators in load conditions, no load, determining the reactance and resistance by performing experiments without load (zero load), short circuit experiment and experiment anchor resistance.

As noted in previous articles, that the rotor speed and frequency of the voltage generated by a linear synchronous generator. Figure 1 will show the working principle of an AC generator with two poles, and let us assume only one winding is made ​​of two conductive in series, the Conductor of a and a '.

To be able to more easily understand, please refer to the animation generator working principle, here.
Figure 1. Generator Diagram Pole AC One phase two.

Winding as mentioned above is called "Circumference centered", the generator is actually composed of many loops in each phase are distributed in each of the stator groove and is called "Circumference distributed. " It is assumed rotor rotates clockwise, the rotor flux field moves according coil anchor. One lap in one second rotor produces one cycle per second or 1 Hertz (Hz).

If speed is 60 Revolution per minute (rpm), the frequency of 1 Hz. So for the frequency f = 60 Hz, the rotor should spin 3600 rpm. For the rotor speed n rpm, the rotor must rotate at a speed of n/60 revolution per second (rps). When the rotor has more than 1 pair of poles, for example P poles, each revolution of the rotor induces P / 2 cycle of voltage in the stator winding. The frequency of induced voltage as a function of rotor speed, and formulated by:
For three-phase synchronous generator, there should be three windings, each separated by 120 electrical degrees in space around the circumference of the air gap as shown in the coils of a - a ', b - b' and c - c 'in Figure 2. Each coil will produce a sine wave Flux different from one another 120 degrees electrical. In a balanced state of flux magnitude for a moment:

ΦA = Φm. Sin ωt
ΦB = Φm. Sin (ωt - 120 °)
ΦC = Φm. Sin (ωt - 240 °)
Figure 2. AC Generator Diagram Pole Three Phase Two

The amount of resultant flux is the sum of the three flux vectors are:
ΦT = ΦA + ΦB + ΦC, which is a function where (Φ) and time (t), then the large-magnitude total flux is:
ΦT = Φm.Sin ωt + Φm.Sin (ωt - 120 °) + Φm. Sin (ωt-240 °). Cos (φ - 240 °)

By using trigonometric transformations of the following:

Sin α. Cos β = ½. Sin (α + β) + ½ Sin (α + β),

then from the above equation is obtained:

ΦT = ½. Φm. Sin (ωt + φ) + ½. Φm. Sin (ωt - φ) + ½. Φm. Sin (ωt + φ - 240 °) + ½. Φm. Sin (ωt - φ) + ½. Φm. Sin (ωt + φ - 480 °)

From the equation above, when described the ethnic unity, third, and fifth
will eliminate cross. Thus from the equation will be obtained
total flux, ΦT = ¾ Φm. Sin (ωt - Φ) Weber.

So the resultant field is a rotating field with modulus 3 / 2 Φ with
turning angle of ω. So the voltage of each phase are:

E max = Bm. ℓ. ω r Volt

where:

Bm = maximum flux density of rotor field coil (Tesla)
ℓ = length of each coil in a magnetic field (Weber)
ω = angular velocity of rotor (rad / s)
r = Radius of the anchor (meters)

you can also read articles related to the discussion this time, at:

- Electromechanical in power systems-1, here.
- Electromechanical in power systems-2, here.

Generator Without Charges

If a synchronous machine functioned as a generator to be played at synchronous speed and rotor given field current (IF), then at anchor stator coil will be induced no-load voltage (Eo), which is:

Eo = 4.44. Kd. Kp. f. φm. T Volt

In the state of no-load current does not flow in the stator yoke, so there is no influence of the anchor reaction. Flux is only generated by the field current (IF). When the field current is increased, then the output voltage will also rise to the point of saturation (saturated), as shown in Figure 3. Generator no-load conditions can be described equivalent circuit as shown in Figure 3b.
Figure 3a and 3b. Curves and Equivalent Series Generator Without Charges

Generator load

When the generator load implies changing the size of the terminal voltage V will vary also, this is due to the loss of stress:
• Resistance of anchors Ra
• Reactance Xl leak anchor
• Anchor Xa Reaction

a. Resistance Anchor
Anchor resistance / phase Ra cause of the loss tense / phase (voltage fall / phase) and I. Ra which anchors the current phase with the grid.

b. Leaks reactance Anchors
When current flows through conductive anchor, some things did not induce flux in the path that has been determined, it is called Flux Leaks.

c. Reaction Anchor
The existence of current flowing in the coil when the generator loaded anchor would cause flux anchor (ΦA) which integrate with the flux generated in the rotor field coil (ΦF), so it will produce a resultant flux of:
The interaction between these two fluxes is referred as a reaction anchor, as shown in Figure 4. which illustrates the anchor reaction conditions for this kind of different loads.

Figure 4a, 4b, 4c and 4d. Reaction conditions Anchors.

Figure 4a, shows the reaction conditions when the generator loaded anchor resistance (resistive) so that the current phase with the grid emf He anchors Eb and ΦA be perpendicular to the ΦF.

Figure 4b, shows the reaction conditions when the generator loaded anchor capacitive, so the current anchor, he goes before emf Eb of θ and ΦA ΦF backward toward the corner (90-θ).

Figure 4c, shows the reaction conditions the anchor when loaded, resulting pure capacitive currents precede emf Eb He anchors at 90 ° and ΦA will strengthen that influence ΦF magnets.

Figure 4d, shows the reaction conditions when the current anchors are pure inductive load currents resulting anchor of emf Eb He was retarded by 90 ° and would weaken ΦF ΦA that affect magnets.

The amount of leakage reactance reactance XL and the anchor Xa called Synchronous reactance Xs.

Vector diagram for the load that is inductive, purely resistive, and capacitive shown in Figure 5a, 5b and 5c.

The interaction between these two fluxes is referred as a reaction anchor, as shown in Figure 4. which illustrates the anchor reaction conditions for this kind of different loads.
Figure 5a, 5b and 5c. Vector Diagram of Generator Expenses

Based on the picture above, it can determine its voltage falls that occur, namely:

Total Voltage Fall on Charges:

= I. Ra + j (Xa + I. I. XL)
= I {Ra + j (Xs + XL)}

= I {Ra + j (Xs)}

= I. Zs

Determining the Resistance and Reactance

To be able to determine the value of reactance and impedance of a generator, should be carried out experiments (test). There are three types of tests commonly performed, namely:

• Test without load (Zero Cost)
• Short circuit test.
• Resistance Anchor Test.

Test Without Charges

Test Without Charges made ​​on the speed of Sync with the anchor chain is open (no load) as shown in Figure 6. Experiments carried out by regulating the field current (IF) from zero to rated voltage output terminal is reached.

Figure 6. Generator Test series without expense.

Short circuit Test

To perform this test generator terminal short-circuited, and with Ampermeter placed between two conductive which is short circuited (Figure 7). Field current increased gradually until the maximum available current anchor. During the test process of the If current and short circuit current IHS recorded.
Figure 7. Generator Test series in short circuit.

From the results of the two tests above, it can be drawn in the form of characteristic curves as shown in Figure 8.
Figure 8. Characteristic Curves Without Charges and short circuit of a generator.

Sync Impedance searched based on test results, are:
If = constant

Resistance Anchor Test

With a series of open terrain, the DC resistance measured between two output terminals so that the two phases connected in series, Figure 9. Resistance per phase is half of that measured.
Figure 9. DC Resistance Measurement

In fact, the resistance value multiplied by a factor to determine the effective value of AC resistance, R eff. This factor depends on the shape and size of the plot, Conductor size anchors, and coil construction. Its value ranges between 1.2 s / d 1.6.

If the value of Ra is well known, the value of Xs can be determined based on the equation:

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