Journal of Power and Energy Engineering, 2015, 3, 123-127
Published Online April 2015 in SciRes. http://www.scirp.org/journal/jpee
How to cite this paper: Savenkov, M. and Turner, M. (2015) New Robotic Technologies for Inspecting Two Pole Electric
Generators while the Rotor Remains in Place. Journal of Power and Energy Engineering, 3, 123-127.
New Robotic Technologies for Inspecting
Two Pole Electric Generators while the
Rotor Remains in Place
Mark Savenkov, Michael Turner
Alstom Power, Thermal Services Asia
Received January 2015
The electric generator is a highly stressed plant component requiring periodic inspection and
maintenance to reduce the risk of a costly forced outage. This paper briefly introduces two new
robotic technologies for performing fast and reliable inspections of two pole electric generators
with minimal mechanical disassembly requirements. The first robotic system is designed to in-
spect within the generator rotor and stator air gap, while the second robotic system is designed to
inspect the generator retaining rings. An overview of the design and construction of each system is
provided, along with an explanation of the capabilities and benefits they bring to the power sta-
Generator, Condition Based Maintenance, Robotic Inspection, Retaining Rings
The electric generator is a critical plant item in a power station. During its life time the generator is subject to
several kinds of stress, which are generally categorized as thermal, electrical, ambient or mechanical in nature.
These stresses affect the reliability of the machine and contribute to the risk of a costly forced outage. The
maintenance regime of the generator typically involves periodic disassembly, inspection of critical components,
and rehabilitative works as necessary.
This paper introduces two new technologies for inspecting the generator while the generator rotor remains in
place within the stator. Both systems are designed for two pole turbo generator types with a cylindrical rotor.
The new technologies discussed here leverage robotics to facilitate a rapid inspection of the generator, while
avoiding the time and costs associated with rotor removal. Such inspections can be performed between major
overhauls in order to increase machine condition information and thereby increase reliability, or alternatively,
such inspections could be used to justify prolonging the time between major overhauls and to optimize the out-
This paper is organized as follows. The first part of the paper is used to describe a new robotic system for
M. Savenkov, M. Turner
performing inspections within the air gap between generator rotor and stator. While the second part of the paper
is used to describe a new robotic system for performing ultrasonic inspections of the generator retaining rings.
An overview of the design and construction of each robotic system is provided, along with an explanation of the
capability of each system and its associated benefits to the power station owner. The paper also contains several
photos to depict the functionality of each technology within a real world context.
2. Robotic Inspection with Rotor in-Situ
2.1. Inspection within the Generator Air Gap
The company Alstom has been involved in the development of robotic tooling to access and inspect within the
air gap between generator stator and rotor for several decades. The original tooling base was geared towards
large hydrogen cooled generators  . The latest generation system to be introduced to the power market is
known as DIRIS® Small, where the acronym DIRIS is derived from the expression “Diagnostic Inspection with
Rotor In-situ”. This latest generation robotic system is highlighted in this paper as a novel device due to its abil-
ity to perform inspections within very narrow air gaps typically found on small air cooled generators with chal-
lenging geometrical constraints.
The DIRIS Small robot depicted in Figure 1 is designed such that it is positioned on one generator retaining
ring and has computer controlled axial and circumferential movement to facilitate the inspection process. Once
positioned circumferentially, the axial probe carrier can extend along the length of each stator slot section. This
process can be repeated a full 360 degrees around the stator bore. Typically one human operator can control the
entire process from outside the generator using proprietary software, while another human operator periodically
checks the position of the tooling for any unforeseen obstructions and acts as a secondary observer of mechani-
cal quality assurance.
The DIRIS Small robot is capable of performing critical tests of the generator stator iron core laminations,
stator radial wedging system and conducting a visual (video type) inspection of the inside surfaces of the rotor
and stator . These tests would normally be part of a typical overhaul regime after a lengthy process of remov-
ing the rotor and utilizing manual and semi-automated tooling. The low flux test permits the identification of
short circuits between the stator iron core laminations, which could otherwise develop into critical “hot spots”
and severely damage the generator . The tightness test of the radial wedging system permits the identification
of loose wedges, which could otherwise promote movement of the stator bars and damage to the stator winding
insulation system .
Generally speaking, the inspection of the air gap using such robotic tooling can be accomplished in less than a
week on a day-shift pattern, including the time required to perform mechanical disassembly and reassembly of
exterior components. As depicted in Figure 2 and Figure 3 the DIRIS Small robot accesses the air gap between
stator and rotor from one side of the machine only. Therefore it follows that the required disassembly of exterior
components for an air cooled generator is typically limited to a small amount of pipework, one outer end shield,
and an internal air baffle. It should also be noted that experience suggests that in most cases the rotor fan blades
are able to remain in place during the inspection.
Figure 1. Overview of the DIRIS small air gap robot.
Belt Circumferential Dive
M. Savenkov, M. Turner
Figure 2 DIRIS Small air gap robot deployed on a 60 MW generator of
European manufacturing origin.
Figure 3DIRIS Small air gap robot deployed on a 175 MW generator of
North American manufacturing origin.
2.2. Inspection of the Generator Retaining Rings
The retaining rings are a critical component of the generator. In service, the retaining rings provide mechanical
support to the generator rotor end windings which are subject to high levels of centrifugal force. The conse-
quences of retaining ring failure have been widely discussed in the literature and for this reason are ignored here
(see for example ). Historically the retaining rings have been inspected using a dye-penetrant test of the inside
surfaces. This process necessitates removal of the generator rotor, followed by removal of the retaining rings
using high levels of heat and significant human effort.
The robotic technology described here has also been developed by the company Alstom and provides the
flexibility to perform inspections of the generator retaining rings while the rotor remains in place and while the
retaining rings are mounted on the rotor. The robotic system for scanning retaining rings introduced in this paper
is known as TurboRotoscan-S and depicted in Figure 4 and Figure 5. The system is designated by an “S” (or
“Small”) as it represents an evolution of an older variant robotic tool intended for large hydrogen cooled gene-
rators. The system described here has an extremely lightweight chassis, designed to be mounted directly to the
M. Savenkov, M. Turner
Figure 4. Overview of the TurboRotoscan-S positioned on a simulated
generator retaining ring.
Figure 5 . TurboRotoscan-S deployed on a 40 MW generator of European
retaining rings and has computer controlled axial and circumferential movement of probe heads to map the body
of the retaining ring metal for defects. The system is intended for a wide range of types and sizes of turbo gene-
rators. Due to its lightweight structure the robotic tooling can be hand carried to the work site with relative ease.
In its nominal configuration the retaining ring scanner contains on-board probes for phased array ultrasonic
inspection, in order to check the inside surfaces of the retaining ring metal for cracks. The scanner also contains
an eddy current probe to check the retaining ring outside surfaces. The data acquisition is performed using third
party software. In a similar fashion to that described previously, the robotic retaining ring scanner utilizes two
human controllers, one to drive the movement from outside the machine and monitor data quality while a sec-
ondary human monitors for any unforeseen obstructions inside the end-winding portion of the machine.
Generally speaking, such an inspection of two retaining rings can also be performed in less than a week on a
day-shift basis. The required disassembly of exterior components on an air cooled generator is limited to a small
amount of pipework, any air guides on the exciter side of the generator, two outer end shields, and two internal
air baffles. Depending on the machine design the rotor fan blades may also need to be removed to grant access
to the axial rail portion of the scanner.
M. Savenkov, M. Turner
3. Summary Remarks
Two new robotic technologies to perform fast and reliable inspections of the turbo-generator have been de-
scribed in brief. Both of these systems are designed for two pole electric generators with cylindrical rotors and
bypass the need to remove the generator rotor from the stator. This results in significant savings in mechanical
disassembly, reassembly, and downtime costs for the power station owner. The capability of these robotic sys-
tems includes low flux stator iron core testing, stator radial wedging assessment, rotor and stator visual inspec-
tion, and ultrasonic inspection of the generator retaining rings. Both of these new systems were designed with
“real world” constraints in mind, such as consideration for a light weight and slim chassis, transportability,
challenging geometrical inspection constraints and a low level of ancillary human labor.
As partially evidenced by the supporting illustrations, both of these new systems have had immediate traction
in the power generation market, and have been deployed in a wide variety of generator types. From experience
of the authors, it can be stated that both forms of technology have already been deployed on generators of Euro-
pean, North American and Japanese manufacturing origin. The commercial logic for performing inspections
with robotic tooling, in place of traditional human means, is not a difficult one to grasp. The benefits of a rapid
robotic inspection include the flexibility to assess generator condition in between major overhauls and the ability
to make informed decisions to potentially prolong the time between major overhauls.
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