with two line wires and their potential coils connected

between these line wires and the common, or third, wire

that does not contain the current coils. The total true

power is equal to the algebraic sum of the two wattmeter

readings. If one meter reads backward, its potential coil

connections are first reversed to make the meter read

upscale, and the total true power is then equal to the

Generated voltage of a generator is expressed by the

difference in the two wattmeter readings. If the load

formula:

power factor is less than 0.5 and the loads are balanced,

the total true power is equal to the difference in the two

wattmeter readings. If the load power factor is 0.5, one

Where:

meter indicates the total true power and the other

indicates zero. If the load power factor is above 0.5, the

Eg is generated voltage

total true power is equal to the sum of the two wattmeter

K is a constant determined by the construction of

readings.

the generator

is the strength of the rotating magnetic field

N is the synchronous speed

The frequency of the ac generator voltage depends

Its impractical to vary the frequency of power

upon the speed of rotation of the rotor and the number

supplied throughout the ship in order to regulate the

of poles. The faster the speed the higher the frequency.

voltage generated, and the constant can't be changed

Conversely, the lower the speed, the lower the

once the machine has been designed and built; therefore,

frequency. The more poles there are on the rotor, the

the generated voltage of an ac generator is controlled by

higher the frequency is for a given speed. When a rotor

varying the dc excitation voltage applied to the rotor

has rotated through an angle so that two adjacent rotor

field winding thus varying

poles (a north and a south pole) have passed one

winding, the voltage induced in that winding will have

modulated through one complete cycle. For a given

frequency, the more pairs of poles, the lower the

speed of rotation. A two-pole generator rotates at twice

When the load on a generator is changed, the

the speed of a four-pole generator for the same

terminal voltage varies with the load. The amount of

frequency of generated voltage. The frequency of the

variation depends on the design of the generator and the

generator in Hz (cycles per second) is related to the

power factor of the load. With a load having a lagging

number of poles and the speed as expressed by the

power factor, the drop in terminal voltage with increased

equation

load is greater than for unity power factor. With a load

having a leading power factor, the terminal voltage tends

to rise. The causes of a change in terminal voltage with

load change are:

where P is the number of poles and N the speed in rpm.

armature resistance,

For example, a two-pole, 3,600-rpm generator has a

frequency of

armature reactance, and

armature reaction.

a four-pole 1,800-rpm generator has the same

frequency; a six-pole, 500-rpm generator has a

frequency of

When current flows through a generator armature

winding, there is an IR drop due to the resistance of the

winding. This drop increases with load, and the

terminal voltage is reduced. The armature resistance

drop is small because the resistance is low.

and a 12-pole, 4,000-rpm generator has a frequency of