Page 168 - Steam Turbines Design, Applications, and Rerating

148 Chapter Eight

may be changed, but governor speed should remain within the follow-
ing limits: Normal speed between 250 and 1050 r/min, but a low speed
of 150 r/min and a high speed of 1500 r/min can be obtained if required.
Governor speed changes of from 8 to 120 r/min per psi (8 to 120 r/min
per 0.07 bar) control air pressure change can be effected for PG-PL gov-
ernors.
The PG-PH governor is a modified PG governor used for turbine-
generator units where the regulation is required to permit the genera-
tor to parallel a utility system or another generator.
The output of the power piston is fed back into the speeder spring.
The governor valve movement is, therefore, proportional to spring
force. This provides the governor with regulation. The governor also
has a built-in regulation adjustment. The regulation can be adjusted
from 0 to 6 percent. The regulation adjustment control is mounted out-
side the governor permitting the regulation to be readily set for each
application. This governor also has a load limit feature.

8.6 Electronic Governors

The Woodward electric type governor is used on large numbers of
mechanical drive steam turbines and is described here because it is
rather typical of modern devices. It is designated by EG and is in-
tended for speed control of steam turbines driving generators, pumps,
fans, compressors, and paper mill machinery. The EG governor consists
basically of three separate assemblies: a control box, a speed-adjusting
potentiometer, and a hydraulic actuator.
There are two basic types of EG control boxes. The EGA model is
used primarily for generator drive and receives its power supply and
speed signal from the generator system. The EGM model is used pri-
marily for mechanical drive applications and requires a separate
power source. The speed signal originates from the electrical impulses
generated by a magnetic pickup.
The operation of the EGM governor system is as follows (see Fig. 8.7).

1. A magnetic proximity speed pickup is mounted on the turbine to
sense speed from a gear mounted on the turbine shaft. The pickup is
mounted as close to the bearing as possible to reduce speed errors
caused by shaft vibration and runout.
2. The speed reference signal from the pickup is fed into the EGM con-
trol box. These speed pulses are then converted into a dc voltage pro-
portional to speed. This voltage signal is then fed from the control
box to the electrohydraulic transducer portion of the governor sys-
tem. The EGM consists of a chassis, cover, and two encapsulated