Guardian 4389 Repair Manual page 17

GENERAL INFORMATION
With alternating current (AC), the electrons flow first
in one direction, then reverse and move in the oppo-
site direction. They will repeat this cycle at regular
intervals. A wave diagram, called a "sine wave"
shows that current goes from zero to maximum posi-
tive value, then reverses and goes from zero to maxi-
mum negative value. Two reversals of current flow is
called a cycle. The number of cycles per second is
called frequency and is usually stated in "Hertz".
VOLT:
The VOLT is the unit used to measure electrical
PRESSURE, or the difference in electrical potential
that causes electrons to flow. Very few electrons will
flow when voltage is weak. More electrons will flow as
voltage becomes stronger. VOLTAGE may be consid-
ered to be a state of unbalance and current flow as an
attempt to regain balance. One volt is the amount of
EMF that will cause a current of 1 ampere to flow
through 1 ohm of resistance.
OHM:
The OHM is the unit of RESISTANCE. In every circuit
there is a natural resistance or opposition to the flow
of electrons. When an EMF is applied to a complete
circuit, the electrons are forced to flow in a single
direction rather than their free or orbiting pattern. The
resistance of a conductor depends on (a) its physical
makeup, (b) its cross-sectional area, (c) its length,
and (d) its temperature. As the conductor's tempera-
ture increases, its resistance increases in direct pro-
portion. One (1) ohm of resistance will permit one (1)
ampere of current to flow when one (1) volt of electro-
motive force (EMF) is applied.
Figure 4. Electrical Units
OHM'S LAW
A definite and exact relationship exists between
VOLTS, OHMS and AMPERES. The value of one
can be calculated when the value of the other two
are known. Ohm's Law states that in any circuit the
current will increase when voltage increases but
PART 1
TESTING, CLEANING AND DRYING
resistance remains the same, and current will
decrease when resistance Increases and voltage
remains the same.
Figure 5.
If AMPERES is unknown while VOLTS and OHMS
are known, use the following formula:
AMPERES = VOLTS
If VOLTS is unknown while AMPERES and OHMS
are known, use the following formula:
VOLTS = AMPERES x OHMS
If OHMS is unknown but VOLTS and AMPERES are
known, use the following:
OHMS =
INSULATION RESISTANCE
The insulation resistance of stator and rotor windings
is a measurement of the integrity of the insulating
materials that separate the electrical windings from
the generator steel core. This resistance can
degrade over time or due to such contaminants as
dust, dirt, oil, grease and especially moisture. In
most cases, failures of stator and rotor windings is
due to a breakdown in the insulation. And, in many
cases, a low insulation resistance is caused by mois-
ture that collects while the generator is shut down.
When problems are caused by moisture buildup on
the windings, they can usually be corrected by dry-
ing the windings. Cleaning and drying the windings
can usually eliminate dirt and moisture built up in the
generator windings.
THE MEGOHMMETER
GENERAL:
A megohmmeter, often called a "megger", consists of
a meter calibrated in megohms and a power supply.
Use a power supply of 500 volts when testing stators
or rotors. DO NOT APPLY VOLTAGE LONGER
THAN ONE (1) SECOND.
TESTING STATOR INSULATION:
All parts that might be damaged by the high megger
voltages must be disconnected before testing. Isolate
all stator leads (Figure 2) and connect all of the stator
SECTION 1.4
OHMS
VOLTS
AMPERES
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