Experiment 6 —
Synchronizing an Alternator
Objectives
1.
2.
3.
To operate a DC machine as a prime mover to power an alternator.
To study the conditions that must be met when paralleling two alternators or
connecting an alternator to the system.
To study how to control the real power and the reactive power delivered by an
alternator.
Theory
This theory can be found in your Energy Conversion textbook among other
resources.
To connect two alternators in parallel, the two alternators must be synchronized.
The synchronization process must be performed also when connecting an alternator to the
grid.
The purpose of synchronization is to ensure that at the moment of closing the circuit
breaker (closing the 3-pole single throw switch to connect the alternator to the grid in this
experiment), the voltages across the three phases of the breaker are as close to zero as
possible and remain so after the switch is closed. To ensure that, the following
conditions must be met:
1- The generated voltage must be approximately equal to the grid voltage.
2- The frequency of the generated voltage must be equal to that of the grid.
3- The phase sequence of the generated voltage must be the same as that of the grid.
4- The phase of the generated voltages relative to some reference must be very close
to the phase of grid lines.
These conditions can be understood by considering the voltage of one line of the grid,
vg1, to be connected to one line of the alternator, va1. Suppose
Vg1 = vg1(t) = Vmg sin (ωg1t + φg1)
(1)
Va1 = va1(t) = Vma sin (ωa1t + φa1)
(2)
The voltage across the switch is Vg1-Va1. It is obvious that for this voltage to be close
to zero and remains close to zero, the above four conditions must be met. In short, the
above two voltage waveforms must be on top of each other if seen on the oscilloscope.
The voltage Vg1-Va1 can be seen by applying this voltage to a light bulb. The brightness
of the bulb is an indication of the voltage across it. When the light bulb is totally dark,
the voltage across it is zero.
Procedure
1.
2.
3.
4.
5.
6.
7.
8.
Examine the nameplate ratings of the DC machine and the AC machine
coupled to it. Write down these ratings.
Supply the DC machine with the appropriate voltage and practice controlling
the speed of the motor.
Connect the circuit shown below and insert an ammeter in the appropriate
place to measure the AC line current. Take the precautions necessary to
protect the ammeter against inrush currents.
Bring the synchronous machine up to the synchronous speed by means of the
direct-current motor; then excite its field. The synchronous machine will now
be operating as a generator and will develop voltage across its terminals.
Observe the difference between the frequency of the generated voltage and the
system frequency using a stroposcope.
Adjust the generated voltage so that it is equal to that of the grid.
Observe the flashes of the light bulbs. If the light bulbs go on and off in
synchronism then the phase sequence of the generated voltages and that of the
system voltages are the same. If the light bulbs do not flash in synchronism
then the phase sequence is not the same and two grid wires coming to the
lamps switch mush be interchanged.
When the lamps go on and off very slowly (about less than once every few
seconds), and at the instant in time that is approximately in the middle of the
two bright instances (the lamps are darkest) the switch can be closed.
Figure 1. Line synchronization scheme, Copyright 2005, Missouri S&T, Used by
permission.
9.
10.
11.
Try to change the speed of the DC motor very slowly by changing the field
current of the DC machine as you did in step 2. What happens to the speed?
Observe the rotor of the machine with a stroboscope.
Observe the brightness of the light bulbs.
Report
In your report, use equations (1) and (2) to express mathematically the voltage
across the switch. Use phasor diagrams to explain your observations.