15145

4
th
Quarter, 2005


GMS NEWS
GMS Expands Capabilities to Increase
Brushless Exciter Availability and Reliability






















GMS has become an industry leader in
demonstrating that brushless exciters, some
in operation for up to 40 years, can be
overhauled and retrofitted to allow 20 years
of extended, reliable operation. This is an
attractive option to users who are faced with
the prospect of replacing existing equipment
with expensive static excitation systems that
use brushes and slip rings to transfer very
large DC currents to the generator field
windings. The slip ring system impacts the
vibrational behavior of the entire turbine
generator shaft and may cause operational
problems.


During the past three years GMS has
overhauled, repaired and rewound over 20
Westinghouse brushless exciters with
outputs ranging from 250 kW to 4.2 MW.


The main components of a brushless
exciter are the stationary DC field windings,
the rotating 3-phase armature winding, and
the diode wheel with electrical components.
The diode wheel supports rotating diodes,
capacitors and fuses. The armature winding
and phase leads are supported against
centrifugal forces by glass banding. Cooling
is achieved using air or hydrogen in a closed
circuit. GMS can overhaul, modify and/or
replace all exciter components.

In a recent project, GMS completely
refurbished a Westinghouse 4.2 MW
brushless exciter that had experienced an
operating failure caused by improperly
secured hardware. Arcing below the diode
wheel destroyed the diode wheel and many
electrical components. With extensive
engineering and manufacturing experience,
GMS used an in-stock steel forging to
replace the diode wheel and repair the entire
exciter in 11 weeks, starting with only 24
hours advance notice.












New Diode Wheel after Machining

In addition to replacement of the diode
wheel, the overhaul included replacement of
diode wheel components, redesign and
modification of the phase leads, a complete
rewind of the rotor and stator, replacement
of the base wiring, high speed balance/over-
speed testing. In parallel with work on the
exciter, a Positive Pressure Ventilation
Modification (PPVM) was installed on the
exciter house to avoid contamination of the
exciter during future operation.
The rewind included an engineering
modification of the phase leads and balance
resistors. All work was performed in
accordance with GMS quality procedures.









Installation of Coils during Rewind


CONTACT GMS
If you want to eliminate tooth-
top cracking on your rotors,
replace retaining rings, obtain
field service on your stator or
overhaul your brushless
exciter contact:
Bob Fuller
GMS Marketing Manager
(412) 829-7500
rfuller@gmsinternational.com


GMS GENERATOR &
EXCITER SERVICES:


Condition Assessment

Stator and Rotor Repairs
and Rewinds

Generator and Exciter
Overhaul and Repair

Positive Pressure Exciter
Housing

Life Extension Programs

Monitoring and Diagnostics

Visit our web site at

www.gmsinternational.com GMS Can Provide Emergency Service for Generators,
Exciters and PMGs
GMS Engineering also redesigned
the phase lead support blocks to include
axial band support, which allowed the
completion of the polyester banding
process without use of additional tooling.












Phase Lead Modification


New tooling prepared for this project
included a lathe mounted radiant heater
box, designed to allow all curing and
banding operations to be completed at a
single work station. This configuration
gave improved control of heat and
restricted heating to only affected zones
during the banding process.

Project completion schedules were
maintained using continuous project
management oversight and adjustment of
technician work hours as required to
prevent delays. Our 8 to 11 week exciter
rewind program provides the generating
industry with a valuable tool for
maintaining brushless exciters.

BASE WIRING UP-GRADES
Base wiring on many older exciters
contains asbestos insulation. During
exciter overhauls GMS tests and replaces
connections, terminal blocks and wiring
with improved material.


New Terminals and Wiring
HIGH SPEED BALANCE AND
OVER-SPEED TESTING


GMS has developed a close working
relationship with two well-established
contractors in the Pittsburgh area who
operate spin pits capable of handling the
exciters we overhaul. Because of the
different discrete components the exciter
shaft must carry, the shaft acts as a
flexible element with multiple mode
shapes. Typically exciter rotors are
balanced (up to seven planes) at 3600
rpm and over-speed test at 3960 rpm after
overhauls. Experienced GMS balancing
engineers can reliably balance even the
largest of rotors within two days.










Exciter Loaded in High Speed Spin Pit


PMG OUTPUT TESTING


The primary purpose of high speed
testing to record displacement data that is
used to balance the exciter rotor. During
the past outage season GMS strengthened
its impressive list of exciter testing
capabilities by adding PMG output
testing, which is also conducted with the
unit running at rated speed while under
load.










PMG Output Testing @ 3600 RPM

This running test can be done after
the high speed balance with the PMG
attached to the exciter shaft as shown or
with the PMG removed from the exciter
shaft using a tooling shaft. The purpose
of this testing is to verify adequacy of all
repairs completed on the PMG stator or
magnets to insure trouble free start up.


COOLER REPAIRS
Forced outages can be caused by
water leaks caused by pipe corrosion and
joint deterioration in exciter coolers.
GMS performs extensive inspection and
tests of exciter coolers. When required,
coolers are refurbished to avoid cooler-
induced outages.


Installation of Refurbished Cooler


REPLACEMENT ELECTRICAL
COMPONENTS
GMS recently entered into a formal
agreement with an established after-
market supplier to provide new diodes,
capacitors and fuses for exciters. This
eliminates need for customers to procure
only OEM replacement electrical parts,
reducing costs and repair cycle time
delays.

GMS HAS NOW PROVEN
MANY TIMES THAT BRUSHLESS
EXCITERS CAN BE REFURBISHED
IN AN ECONOMICAL WAY AND
WITHIN THE TIME WINDOW OF A
STANDARD PLANNED OUTAGE.
EXCITERS ARE RETURNED
TO CUSTOMERS READY TO RUN
FOR AN ADDITIONAL 20 YEARS -
AND MORE.