ECE Undergraduate Laboratories
ECE 449 - Power Systems Laboratory

Experiment 6: Synchronizing an Alternator

Objectives

  1. To operate a DC machine as a prime mover to power an alternator.
  2. To study the conditions that must be met when paralleling two alternators or connecting an alternator to the system.
  3. 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. Examine the nameplate ratings of the DC machine and the AC machine coupled to it. Write down these ratings.
  2. Supply the DC machine with the appropriate voltage and practice controlling the speed of the motor.
  3. 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.
  4. 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.
  5. Observe the difference between the frequency of the generated voltage and the system frequency using a stroposcope.
  6. Adjust the generated voltage so that it is equal to that of the grid.
  7. 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.
  8. 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.
  9. Fig. 2. Equivalent circuit per phase (2)

    Figure 1. Line synchronization scheme, Copyright 2005, Missouri S&T, Used by permission.
  10. 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?
  11. Observe the rotor of the machine with a stroboscope.
  12. 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.