MM0704, Lesson 3
In DC circuits, where the value
of the voltage and current are constant, the product
of volts and amperes
is equal to the power in watts (P = E x I). In AC Circuits, the product of volts and amperes is equal to the power in
watts only, when the volts and amperes are in phase. When the voltage and current are out of phase, the product of the
volts and amperes is greater than the watts consumed by the circuit. For this reason the product of volts and amperes is
called the volt-amperes (VA) of the circuit. This product is also frequently called the apparent power of a circuit.
VA = V X amp or VA = E X I
Power Factor. The ratio of the actual watts consumed by a circuit to the VA of that circuit is called the power factor
The value of the power factor may vary from 0 to 1.
Power. The actual power consumed by a circuit may be calculated by the following equation.
P (W) = E(V) x I (amp) x PF
AC generators are rated in terms of VA or Kilovolt-ampere (KVA) output, at a specified frequency and power factor.
For example, a single-phase 60 Hz AC generator rated at 100 KVA would deliver 100 KW of power with a load
producing a power factor of 1. This same generator would provide 80 KW of power with a load at 80-percent power
factor. The maximum load rating of a particular generator is determined by the internal heat it can withstand. As the
power factor goes down, more and more current is required to pass through the generator windings in order to provide
the same amount of voltage (power out). The 100 KVA generator, required to supply a 100 KVA load at 20-percent
power factor would most assuredly have excessive heating in the generator field windings because of the required
increase in DC field current needed to maintain the required power output. Power companies strive to keep the power
factor of their load as high as possible in order to obtain the best generating efficiency for their system. Power
companies often give special rates to large commercial users who maintain loads with high power factors.
Basic Functions of Generator Parts. Almost all rotating-field AC generators are actually two generators in one. A
typical machine consists of an AC generator and a smaller DC generator built into a single unit (figure 3-11). The
output of the AC generator supplies alternating current to the load for which the generator was designed. The DC
generator's only purpose is to supply the DC needed to maintain the generator field. This DC generator is referred to as
Any rotary generator requires a prime moving force (figure 3-11-B1) to rotate the AC field and exciter armature. This
rotary force is transmitted to the generator through the rotor drive shaft and is usually furnished by a combustion
engine, turbine, or electric motor. The exciter shunt field (2) creates an area of intense magnetic flux between its poles.
When the exciter armature (3) rotates in the exciter field flux, voltage is induced into the exciter