Introduction to Generator I

3. Operation of Generators

Generators can be operated in three separate modes:

The operating characteristics of generators vary depending on which mode is applicable at the time. 

Generated and Terminal Voltage of a Generator

To assist with the explanation of generator characteristics it is necessary to distinguish between:

Generated Voltage

Generated voltage is the voltage induced in the stator windings by the action of the magnetic field produced by the rotor when the generator is supplying no load (i.e. open circuit). The value of voltage generated is dependant on the frequency (i.e. rotor speed) and the strength of the rotor field. The amount of rotor current determines the strength of the rotor field. Therefore with a constant speed the generated voltage is dependant on rotor current.

When a generator is supplying load the rotor field is modified due to the effect of what is termed as armature reaction. This armature reaction influences the value of the voltage actually induced in the stator windings. Armature reaction will be covered in detail later in this module.

Terminal Voltage

Terminal voltage as the name suggests is the voltage present at the output terminal of the generator. Terminal voltage is only present when generated voltage is present and is equal to generated voltage only when a generator is open circuited and supplying no load. When the load on a generator is increased the terminal voltage falls below that of the generated voltage due to internal voltage drops. The greatest difference between terminal voltage and generated voltage occurs when the generator is at full load. 

Load Characteristic of a Generator

The load characteristic of a generator is defined as the relationship between the terminal voltage and the stator load current with the excitation and speed being kept constant at normal no-load values.

The load characteristic of a generator is best represented by a curve whose shape will vary with the power factor of the load being supplied. Figure 6 shows load characteristic curves for various leading, lagging and unity power factor loads.

Figure 6:  Generator Load Characteristic Curves

From Figure 6 you can see that if the load is inductive (lagging) then the terminal voltage will reduce as the load is applied. In this instance it is necessary to increase the excitation as load is applied to maintain the desired terminal voltage. If however the load is capacitive (leading) then as the load is increased so does the terminal voltage. Under these circumstances the excitation will need to be reduced as load is increased so as to maintain terminal voltage. 

The behaviour of the terminal voltage of a generator to the load and power factor of that load is attributable to two factors:

Stator Windings Impedance

Due to the resistance and inductive reactance of the stator windings there will be a voltage drop within the generator windings whenever a current is flowing. The value of the voltage drop will depend on the product of the current and the impedance (i.e. E = IZ). Also accompanying this voltage drop is a phase displacement between the generated voltage and the terminal voltage. 

Figure 7 shows a circuit diagram representation of a generator on load. The generated voltage (E_G) for an ideal generator is reliant on the excitation and assumes the generator of having no impedance. In actual fact there is a component of resistance (R) associated with the stator windings along with an inductive reactance (X_L). These components of resistance and inductive reactance have been shown in Figure 7 as a series combination for simplicity. The generated voltage (E_G) minus the two series voltage drops will give us terminal voltage.

Figure 7:  Circuit Diagram of Generator Load

Effect of Armature Reaction

If the generator is on load and therefore a current flowing in the stator windings another magnetic field is produced around the stator windings. This additional magnetic field rotating in synchronism with the rotor magnetic field affects the rotor magnetic field and is referred to as armature reaction.

This armature reaction either weakens, strengthens or distorts the rotor magnetic field depending on the power factor and load on the generator.

As explained earlier for a given rotor speed the generated voltage E_G is dependant solely on the strength of the magnetic field which is governed by the rotor current. When the generator is on no load there is no current flowing in the stator winding and therefore no armature reaction. Under this no load condition the generated voltage is that which is actually induced in the stator windings. When the generator is loaded and armature reaction occurs the voltage actually induced in the stator windings depends on the modified or resultant rotor magnetic field. The generators terminal voltage when on load is therefore not only dependant on rotor current and stator reactance but also the effect of armature reaction.

When the generator supplies an inductive load the rotor field is weakened by the armature reaction and therefore the terminal voltage decreases. Excitation must be increased to return the terminal voltage to the correct value. If the generator is supplying a capacitive load then the rotor field is strengthened and terminal voltage tends to rise above set value. In this case the rotor current is reduced to compensate for the terminal voltage rise. If however the generator is supplying a highly capacitive load such as a lightly loaded long transmission line then the terminal voltage may rise such that rotor current is at minimum and the terminal voltage is still above set value.

Table 1 summarises the effect of armature reaction on the rotor magnetic field for various power factor loads. When armature reaction creates distortion of the rotor magnetic field this takes place in the air gap between the rotor and the stator as the field assumes a position that differs from that of no-load. This distortion of the field causes a reduction in terminal voltage.

Type of load on generator	Power factor of load	Effect on rotor field
		Magnitude	Distortion
Pure inductive Resistive-inductive Pure resistive Resistive-capacitive Pure capacitive	zero lag 0.8 lag unity 0.8 lead zero lead	weakens weakens slight strengthens strengthens	none slight yes slight none

Table 1:  Effects of Armature Reaction with Various Generator Loads