AC Machines: Three-Phase Alternators -- part 3

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Sharing the Load

After the alternators have been paralleled, the power input to Alternator 2 must be increased to permit it to share part of the load. E.g., if the alternator is being driven by a steam turbine, the power of the turbine would have to be increased. When this is done, the power to the load remains constant. The power output of the base alternator decreases, and the power output of the second alternator increases.

Field-Discharge Protection

When the DC excitation current is disconnected, the collapsing magnetic field can induce a high voltage in the rotor winding. This voltage can be high enough to arc contacts and damage the rotor winding or other circuit components. One method of preventing the induced voltage from be coming excessive is with the use of a field-discharge resistor. A special double-pole single-throw switch with a separate blade is used to connect the resistor to the field before the switch contacts open. When the switch is closed and DC is connected to the field, the circuit connecting the resistor to the field is open. When the switch is opened, the special blade connects the resistor to the field before the main contacts open.

Another method of preventing the high voltage discharge is to connect a diode in parallel with the field. The diode is connected in such a manner that, when excitation current is flowing, the diode is reverse-biased and no current flows through the diode.

++++17 Switch in closed position. Field Field-discharge switch Field-discharge resistor DC supply

++++18 Switch in open position. Field Field-discharge switch Field-discharge resistor DC supply

++++19 Normal current flow. Diode Field DC excitation

++++20 Induced current flow. Diode Field DC excitation

When the switch opens and the magnetic field collapses, the induced volt age is opposite in polarity to the applied voltage. The diode is now forward-biased, permitting current to flow through the diode. The energy contained in the magnetic field is dissipated in the form of heat by the diode and field winding.


¦ There are two basic types of three-phase alternators: the revolving-armature type and the revolving-field type.

¦ The rotating-armature type is the least used because of its limited voltage and power rating.

¦ The rotor of the revolving-field-type alternator contains electromagnets.

¦ DC must be supplied to the field before the alternator can produce an output voltage.

¦ The DC supplied to the field is called excitation current.

¦ The output frequency of an alternator is determined by the number of stator poles and the speed of rotation.

¦ Three factors that determine the output voltage of an alternator are a. the length of the conductor of the armature or stator winding.

b. the strength of the magnetic field of the rotor.

c. the speed of the rotor.

¦ The output voltage is controlled by the amount of DC excitation current.

¦ Before two alternators can be connected in parallel, the output voltage of the two machines should be the same, the phase rotation of the machines must be the same, and the output voltages of the two machines must be in phase.

¦ Three lamps connected between the two alternators can be used to test for phase rotation.

¦ A synchroscope can be used to determine phase rotation and difference of frequency between two alternators.

¦ Two devices used to prevent a high voltage being induced in the rotor when the DC excitation current is stopped are a field-discharge resistor and a diode.

¦ Many large alternators use a brushless exciter to supply DC to the rotor winding.


1. What conditions must be met before two alternators can be paralleled together?

2. How can the phase rotation of one alternator be changed in relationship to the other alternator?

3. What is the function of the synchronizing lamps?

4. What is a synchroscope?

5. Assume that Alternator A is supplying power to a load and that Alternator B is to be paralleled to A. After the paralleling has been completed, what must be done to permit Alternator B to share the load with Alternator A?

6. What two factors determine the output frequency of an alternator?

7. At what speed must a six-pole alternator turn to produce 60 Hz?

8. What three factors determine the output voltage of an alternator?

9. What are sliprings used for on a revolving-field-type alternator?

10. Is the rotor excitation current AC or DC?

11. When a brushless exciter is used, what converts the AC produced in the armature winding into DC before it’s supplied to the field winding?

12. What two devices are used to eliminate the induced voltage produced in the rotor when the field excitation current is stopped?


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