PRINCIPLE OF OPERATION
MagnaPLUS
®
generators are brushless, self excited,
externally voltage regulated, synchronous AC generator. The
generator is made up of six major components: main stator
(armature), main rotor (field), exciter stator (field), exciter
rotor (armature), rectifier assembly, and voltage regulator. In
understanding the above terminology, note the following:
stators are stationary, rotors rotate, a field is a DC electrical
input, and an armature is an AC electrical output. These
system components are electrically interconnected as shown
in Figure 1 and physically located as shown in Figure 2.
The generator’s exciter consists of a stationary field and a
rotating armature. The stationary field (exciter stator) is
designed to be the primary source of the generator’s residual
magnetism. This residual magnetism allows the exciter rotor
(armature) to produce AC voltage even when the exciter
stator (field) is not powered. This AC voltage is rectified to DC
by the rotating rectifier assembly and fed directly to the main
rotor (field). As the generator shaft continues to rotate, the
main rotor (field) induces a voltage into the generator's main
stator (armature). At rated speed, the main stator’s voltage
produced by the residual magnetism of the exciter allows the
automatic voltage regulator to function. The regulator
provides voltage to the exciter field resulting in a build-up of
generator terminal voltage. This system of using residual
magnetism eliminates the need for a special field flashing
circuit in the regulator. After the generator has established the
initial residual voltage, the regulator provides a controlled DC
field voltage to the exciter stator resulting in a controlled
generator terminal voltage.
Voltage Regulation
In the standard configuration (shunt excited), the automatic
voltage regulator receives both its input power and voltage
sensing from the generator's output terminals (See Figure 1).
With the optional PMG configuration, the regulator receives
input power from the PMG. The regulator automatically
monitors the generator's output voltage against an internal
reference set point and provides the necessary DC output
voltage to the exciter field required to maintain constant
generator terminal voltage. The generator's terminal voltage
is changed by adjusting the regulator's reference set point.
Consult the regulator manual for specific adjustment and
operating instructions.
MOTOR STARTING
When a motor is started, a large surge of current is drawn by
the motor. This starting current is equivalent to the motors
locked rotor or stall current and is 5 to 10 times normal full
load current. When the generator supplies this in-rush of
starting current, the generator voltage dips temporarily. If the
motor is too large for the generator, the generator’s voltage
dips greater than 30 percent. This may result in the motor
starter de-energizing or the motor stalling. MagnaPlus
®
generators generally supply .3 to .4 horsepower per
generator KW in motor starting capability. For specific data
contact Marathon Electric.
PARALLEL OPERATION
All MagnaPlus
®
generators are built with 2/3 pitch main
stator windings and full amortisseur (damper) windings.
These features make the MagnaPlus® generators suitable for
parallel operation when equipped with the proper voltage
regulators and voltage regulator accessories. Consult with the
factory for further information relative to parallel operations.
NONLINEAR LOADING
Solid state electronic control devices (variable frequency
drives, precision motor controls, battery chargers, etc.) utilize
electronic switching circuits (thyristors, SCRs, Diodes, etc.).
These switching circuits introduce high frequency harmonics
which distort the normal wave form of the generator. This
creates additional heat in the generator windings and may
cause the generator to over-heat. Problems which can occur
are not limited to the generator. Poor wave shape may
adversely effect various loads connected to the generator.
Consult Marathon Electric for further information relative to
nonlinear loads.
INSTALLATION
PREPARATION FOR USE
Although the generator has been carefully inspected and
tested in operation prior to shipment from the factory, it is
recommended that the generator be thoroughly inspected.
Check all bolts for tightness and examine the insulation on
lead wires for chafing prior to proceeding with installation.
Remove all shipping tapes, bags, skids and rotor support
blocking. For two bearing units, rotate the shaft by hand to
ensure that it rotates smoothly without binding.
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