Figure 1-34.-Watt, VA, and VAR relationship.

30-kW load at unity power factor will result in a current

Saturation may be defined as the condition in the

magnetic material where an increase of magnetomotive

of only 38.5 amperes per line. The difference in the

force causes no increase in flux. The coupling of the

output current with identical kilowatt loads is the result

primary and secondary voltage is accomplished only

of the flow of reactive current in the load circuit. This is

known as the reactive volt ampere component of the

when there is a flux change; consequently, when the core

material is forced into the condition where no flux

load and is abbreviated VAR.

change can take place, the coupling of the primary and

This VAR component of the load is caused by the

secondary voltages becomes nonlinear, and the effect of

current of the generator being out of phase with the

de- coupling the secondary winding is produced. Fig-

voltage. The mathematical relationship of power factor,

ure 1-31 indicates the path of dc flux when the start of

watts, VA, and VAR is shown in figure 1-34.

is positive. Naturally, when the applied voltage

It is possible for the current to either lead or lag

polarity reverses itself, the fluxes, 01 and 02, also

behind the voltage, and, if it is lagging (for inductive

reverse themselves; but the dc flux through the control

reactive loads), the power factor would be a lagging

winding then forces 01 into saturation before 02 is

power factor.

forced into saturation. Since the load on the saturable

potential transformer secondary is magnetically coupled

The phase angle of the current in relation to the

to the primary of the saturable potential transformer, the

voltage of the generator output in combination with the

variable control current through the winding

will

magnitude of the output current is used by the

produce a variable secondary output voltage. The

power-sensing network to produce an output signal.

control current versus the output voltage characteristic

That signal will vary in magnitude in relation to the

of the saturable potential transformer is shown in fig-

useful output (kW) of the generator and will produce no

ure 1-33. The saturable potential transformer is designed

output when the generator load is entirely VAR. Any

to operate in the linear position of the characteristic

combination of VAR and kW will produce an output

curve, as shown in figure 1-33.

signal that is directly proportional to the kW load only.

The amount of power required by the motor to drive

POWER-SENSING NETWORK

the generator is also directly proportional to the kW

output of the generator. This makes it possible to use the

The power-sensing network functions to balance the

output signal of the power-sensing network with

load between generators operating in parallel. In single

changing load.

generator operation, the power-sensing network is not

used.

This network is designed to sense real power or the

kilowatt (kW) output of the generator only, as opposed

to kilovolt amperes (KVA) output.

This generating system has an output rating of 30

kW at 0.8 power factor, 37.5 KVA. The current in each

line with this load will be 48.5 amperes at 450 volts. A

Figure 1-33.-Characteristic curve of a saturable potential

Figure 1-34.-Watt, VA, and VAR relationship.

transformer.

1-28