Page 163 - Steam Turbines Design, Applications, and Rerating

Governors and Control Systems 143

To restate and define, we should note the following:

■ Set point is the value of the controlled variable that the governor or
the regulator is set to maintain under fixed conditions.
■ Speed compensation is a device that responds to movement in the
governing system and loads the governor a predetermined amount.
This loading compensates for the inherent regulation of the governor
and results in a settled regulation of lower value than the inherent.
It is often referred to as reset device, droop correction, proportional
speed, or floating action.
■ Isochronous governor is a governor that controls for constant speed
or, theoretically, has no regulation.
■ Direct-acting governor systems allow only the speed sensitive portion
of the governor to supply the force to position the governor valve.
■ A relay governor system provides for a servo or a power supply ele-
ment to amplify the force positioning the governor valve.

8.3 NEMA Classifications
In the United States, governors in general are rated by NEMA
(National Electrical Manufacturer’s Association) classifications as
shown in Table 8.1 and are used by industry in specifying governors.
However, there are some modifications to these NEMA standards. For
example, a governor may meet the requirements of NEMA Class A but
not be capable of a 65 percent speed range. Further, a NEMA Class D

TABLE 8.1 NEMA Classifications for Speed-Governing Systems
Maximum steady-
state speed Maximum speed
regulation, % variation
NEMA Adjustable no load to plus or minus, % Maximum
class speed range, % full load at rated cond. speed rise, %
A 10–65 10 0.75 13
B 10–80 6 0.50 10
C 10–80 4 0.25 7
D 10–90 0.50 0.25 7
Overspeed Trip System Settings
NEMA class Trip speed, % above rated speed
A 15
B 10
C 10
D 10