FIELD EXCITATION
The field windings of a synchronous machine are supplied from a dc source. It originates from batteries, solar converters, dc generators, or a rectified ac source. This is called the excitation of the machine. Permanent magnets are also used to excite synchronous machines. The excitation current controls the terminal voltage, power factor, short-circuit current, torque, and transient response of the machine.
A dc or ac exciter is used for large synchronous machines. The exciter is normally mounted on the same shaft as the synchronous machine. The ac exciter is normally a conventional synchronous machine similar to the main machine. The three-phase ac output of the exciter is rectified and fed to the field winding of the main machine. Slip rings and brushes are also used to supply the dc field directly to synchronous machines. The excitation is normally controlled by varying the dc field by using a variable resistor.
Most excitation currents (except the ones supplied from batteries) contain ripple. This is highly undesirable because it increases harmonics* in the power output from the main machine. Thus, a filter is required to reduce the ripple in the excitation system of the machine to an acceptable value.
Rotating Rectifier Excitation
Small synchronous machines use “brushless” excitation known as rotating rectifier instead of slip rings and brushes. The reason for this is that synchronous machines using slip rings and brushes require high maintenance that includes replacement of the carbon brushes on power.
The excitation system of a rotating rectifier machine uses an exciter mounted on the shaft as the main synchronous machine. The field of the exciter is on the stator, and the armature is on the rotor. The ac power output from the exciter armature is rectified and fed to the field of the main machine. The excitation of the main machine is controlled by controlling the field in the stator of the exciter.
The main disadvantage of the rotating rectifier machine is the long response time required to change the voltage at the output from the main machine. Since the variation of
*Harmonics are ripples that distort the ac voltage and current in the power output. They are normally signals that are superimposed on the main (60- or 50-Hz) signal. They normally have a frequency equal to a multiple of the main frequency. For example, if the main frequency is 60 Hz, the harmonic signal will have a frequency of 5 X 60 or 8 X 60 Hz. Harmonics are highly undesirable because they increase the heat losses and electromagnetic interference from the machine.
the field current to the exciter must affect the exciter output before it can vary the voltage at the terminals of the main machine, delays occur in the variations of the voltage from the main machine. A typical response time of a rotating exciter is around 0.5 to 1.0 s. The response time required to change the voltage of a machine using slip rings and brushes is around 0.2 s.
The rotating rectifier machines cannot be used in large modern power plants due to the requirement of short response time of the excitation system. This is necessary to be able to vary the voltage quickly to stabilize the power out of the machine.
Series Excitation
The excitation of a synchronous machine may also be derived from the output power of the main machine. This type of excitation is accomplished in conjunction with a sepa- rate excitation system that originates from a separate source, such as a battery bank.
The series excitation is appropriate for isolated synchronous alternators which are used to start motors requiring high inrush current. This type of excitation is rarely used with large synchronous machines.