Filter
Top Products
TP-6200 10/12202 Appendix
Appendix C Voltage Regulator Definitions and Adjustments
The following definitions and adjustment/setting
specifications are intended for users planning to adjust
the voltage regulator beyond the default settings in order
to customize the alternator for a specific application.
This information is not intended to be a comprehensive
explanation of all the terms mentioned. There are
numerous documents available that define these terms
more completely than described herein. Any user
planning to change the generator set controller
adjustment settings or to apply the generator set to
these types of applications should understand these
terms.
This appendix contains references to other sections of
this manual. Please refer to these sections for further
information and explanation.
Paralleling generator sets can be a complicated and
dangerous exercise. Application programming must be
performed by appropriately skilled and suitably-trained
personnel.
Definitions
Underfrequency Unloading
Underfrequency unloading is a function used in the
alternator excitation control system to improve the
overall generator set system (engine and alternator)
response. In particular, underfrequency unloading
relates to large-block load applications. When applied to
engine-driven alternators, large-block loads cause a
subsequent transient torque load on the engine. This
torque load can reduce the engine’s speed below the
normal operating point. Typically, the engine speed
controller or governor will compensate for this by
commanding an increase in fuel. If, however, the fuel
system is inadequate to recover from a relatively large
load, the speed may never recover. In these instances,
other measures must be taken. This is where the
underfrequency unloading occurs.
When the excitation control system detects a drop in the
speed or electrical frequency below some
predetermined point, the control system enters an
unloading condition. This can be described as moving to
a lower voltage regulation point. By reducing the output
voltage of the alternator, the load on the generator set is
reduced. This can be shown mathematically by Ohm’s
law, which states that power is equal to the voltage
squared divided by the impedance. As the voltage is
reduced, the power delivered by the alternator
decreases by a squared relationship. Since it is the
power in the alternator that translates into engine torque,
the engine load is also reduced.
By changing various parameters of this compensation
technique, the controlling system can be tailored to
match the performance capabilities of most engine and
alternator combinations. The point at which the
unloading begins to act or how much unloading occurs
can be adjusted to impact maximum voltage droop,
maximum speed droop, or time to recover. Some
applications may not need unloading and, in these
cases, set the unloading parameter to disable the
function. These parameters are further described
below. An example is provided to help clarify the
relationship between these parameters.
Underfrequency Unload Slope
Underfrequency unload slope is the term used to
describe the amount that the voltage is reduced,
per-cycle-per-second or per-hertz (Hz), when in an
underfrequency condition. The slope or schedule is
sometimes called the volts-per-hertz slope. When the
electrical frequency drops below the cut-in point (see
below), the excitation control system temporarily
reduces the regulated voltage to reduce the subsequent
torque on the engine. The amount that the control
system reduces voltage is defined as the product or
multiplication of the slope and the amount of frequency
or speed below the cut-in point. For every Hz below the
cut-in point, the control system reduces the line-to-line
voltage by an amount equal to the slope.
Because each engine responds differently to the various
loads encountered, the slope may be adjusted to
improve the system response. If, when large loads are
applied to the generator set, the engine speed drops
below the acceptable limit (as determined by the
particular loads applied), the slope may need to be
increased. Increasing the slope will cause the voltage to
droop more during load applications, consequently
reducing the load torque on the engine and allowing the
speed to increase. If, however, the voltage drops below
an acceptable lower limit (as determined by the
particular loads connected to the generator set), a lower
slope may work better. The underfrequency unloading
function may be disabled by setting the slope to zero.