FIELD OF INVENTION
The present invention relates to a portable device for dynamic testing of large
turbo generator decks in three directions and a method for the same.
BACKGROUND OF THE INVENTION AND PRIOR ART
Devices for dynamic testing developed till now have one or more of the following
disadvantages.
• They are not portable and feasible for testing large TG decks, in the sense
the test specimen needs to be mounted on the device.
• Three directional testing of structure is not possible with a common base
plate.
• Force and speed rating are less than 50000 Newtons force and 3600 RPM
respectively.
• The device is to be located at two places as against one place in the
present invention.
• The weight and the cost of the equipment are prohibitively high compared
to the present invention.
The present device is a portable one which can be fixed at one location for
testing in three directions using a common base, with force rating and speed
reaching 50000 Newtons and 3600 RPM respectively. Due to in house
development the device is highly economical and the cost of the total system is
ten times lower than imported devices.
OBJECTS OF THE INVENTION
Therefore, it is an object of the invention to propose a portable device for
dynamic testing of large Turbo generator decks (TG) which is capable of
producing a vibration response in vertical, horizontal and axial directions by fixing
the device at one location only.
Another object of the invention is to propose a portable device for dynamic
testing of large Turbo generator decks (TG) which is highly economical.
A further object of the invention is to propose a portable device for dynamic
testing of large Turbo generator (TG) decks which is capable of working in speed
range of 100 to 3600 rpm and generating a force upto 50,000 Newtons.
BRIEF DESCRIPITON OF THE ACCOMPANYING DRAWINGS
Figure 1 shows single rotating mass and force diagram.
Figure 2 shows force diagram for counter rotating eccentric masses.
Figure 3 shows a mechanical excitation system.
Figure 4 shows a photograph of a mechanical excitation
system with drive and controller.
Figure 5 shows a mechanical excitation system under testing
on concrete foundation.
Figure 6 shows a mounting arrangement for excitation in various directions.
Figure 7 shows coast down vibration Bode Plot of large thrust bearing test rig.
DETAILEP DESCRIPTION OF A PREFERRED EMBODIMENT
OF THE INVENTION.
The dynamic characterization of large size supporting structures, base frame and
pedestals of Steam / Gas / Nuclear Turbo Generators is essential before erection
of the set due to heterogeneous nature of the system. Full scale forced vibration
testing is a useful tool for obtaining the dynamic characteristics like natural
frequency and damping. However, portable and full scale forced vibration
testing systems are not readily available and need to be designed and developed
as is done in this project. For the above requirement, several excitation systems
like electro-mechanical and hydraulic systems suitable for exciting the deck of
the turbine generator sets are studied. Specification for the system including the
type, peak force, frequency range etc., has been finalized based on site
requirements. Conceptual design to meet the above specifications had been
carried out based on the principle of counter rotating eccentric masses
generating sinusoidal forces. The general assembly and detailed drawings of the
system were verified and approved for manufacturing. However, this being a
developmental work, a lot of iterations took place in the design and manufacture
of the system to meet the functional requirement of the system.
After series of inspections of the mechanical excitation system, modifications
were made to overcome practical shortcomings. The excitation system was fixed
to ground on a concrete bed with fixing bolts to avoid support structure flexibility
during testing. The system was tested at works and its performance is found to
be satisfactory meeting the functional requirements. Large thrust bearing test
rig was tested in the laboratory using the above system and transient plots
obtained which gave the resonance characteristics of support structure. The
system developed was found to be useful to obtain the dynamic characteristics
of not only large supporting structures but also foundation and support
structures of medium / smaller systems like PA fan, ID fan etc.
Dynamics of Vibration Generators
Shakers and vibration and shock testing equipment are force generators or
transducers that provide a vibration, shock or modal excitation source for testing
and analysis. Shakers are used to determine product or component performance
under vibration or shock loads, detect flaws through modal analysis, verify
product designs, measure structural fatigue of a system or material or simulate
the shock or vibration conditions found in aerospace, transportation, power
utilities or other areas.
Shakers can operate under a number of different principles. Mechanical shakers
use a motor with an eccentric on the shaft to generate vibration. Electro
dynamic models use an electromagnet to create force and vibration. Hydraulic
systems are useful when large force amplitudes are required, such as in testing
large aerospace or marine structures or when the magnetic fields of electro
dynamic generators cannot be tolerated. Pneumatic systems, known as "air
hammer tables", use pressure air to drive a table. Piezoelectric shakers work by
applying an electrical charge and voltage to a sensitive piezoelectric crystal or
ceramic element to generate deformation and motion.
Common features of shakers are an integral slip table and active suspension. An
integral slip allows horizontal or both horizontal and vertical testing of samples.
The slip table is a large flat plate that rests on environmental or floating platform
variations.
The most important specifications for shakers are peak sinusoidal force,
frequency range, peak-to-peak displacement, peak acceleration and peak
velocity. Some of these specifications may be ratings without a load, as the
manufacturers cannot always predict how the shakers will be used.
For testing of large structures like the foundation decks and overall machinery
installations, portable mechanical shakers are required. These shakers need to
be rigidly fixed to the structures to obtain the dynamic characteristics of the
same.
Single- Frequency Excitation
The alternating excitation forces developed by mechanical shakers are centrifugal
inertia forces produced by rotating eccentric masses or unbalances. Practical
demands necessitate the wide use of vibration generators whose rotors are
statically unbalanced.
The absolute magnitude, spatial orientation and character of time variation of the
exciting forces and moments of centrifugal (unbalanced-mass) vibration
generators depend on the motion of the working member vibrated by means of
the generator and on the properties of the driving motor which rotates the
eccentric masses. The motion of the working member is determined, in
particular, by the position of the generator, the type of connections between the
working member and the generator, and by interactions with the medium with
which the working member is in contact either directly or through intermediate
elements. Hence it is necessary to evaluate the dynamics of the whole system
comprising the drive, vibration generator proper, working member and external
medium as well as the connections and intermediate elements.
Several schemes of the generation of excitation are considered, assuming that
the vibration generator body and the axes of rotation of the rotors are fixed and
the unbalanced masses are not acted upon by any forces or moments, i.e., that
they rotate at a constant angular velocity. Out of a multitude of possible
schemes only the simplest ones covering the greater part of practical applications
have been selected.
Considering first plane diagrams with statically unbalanced rotors, the centrifugal
forces of all the unbalanced masses are coplanar and lie in the plane of the
drawing. The vectors of the moments of forces, if there arise any, are
perpendicular to the plane of the drawing. If there is only one rotating
unbalance (Figure 1), we have a circular exciting force (a rotating exciting force
of constant modulus). The hodograph of the exciting-force vector is a circle.
The numbers designate the successive positions of the unbalance and the
corresponding excitation-force vectors. If the static mass moment of the
unbalance with respect to the axis of rotation is "Me", then the exciting force
modulus is determined from the expression

Where 'M' is the eccentric mass, 'e' is the eccentricity and V is the angular
velocity of rotation of the unbalanced mass; the vector itself can be represented
in the form

if the unbalanced mass rotates in the positive sense, and in the form

if it rotates in the negative sense.
A circular exciting force reduced to the midpoint of the generator, which usually
coincides with its center of gravity, can be also produced in more complicated
systems, for instance, in the presence of two equal and in phase counter rotating
eccentric masses (Figure 2). If the static mass moment of each of the eccentric
masses with respect to the axis of rotation is "Me", then the exciting force
modulus is given as follows:

The generator midpoint is at the middle of the segment connecting the axes of
rotation of the unbalanced masses. The forces generated in vertical direction
add up while those in horizontal direction cancel each other as shown in
Figure 2.
Designs of Mechanical Exciter
• Various exciter systems like electro-mechanical and hydraulic systems
suitable for exciting the deck of the turbine generator sets are studied.
Specification for the system including the type, peak force, frequency
range etc., has been finalized based on site requirements. Conceptual
design to meet the above specifications had been carried out based on
the principle of counter rotating eccentric masses generating a sinusoidal
force as described above. The description of mechanical system is such
that the system shall consist of mechanical and electrical drive units. The
system shall generate the force by using the principal of eccentric mass
rotating within the desired speed range. The force is to be generated by
using two equal counter rotating weights, which produce a directional
sinusoidal force. The weights are to be rotated by an AC variable speed
drive mounted separately and connected by either a flexible shaft or
multiple shafts with universal joints or belt drive.
The general assembly and detailed drawings of the system was verified and
approved for manufacturing. The technical specification of the mechanical
exciter system is as follows:
• Peak force: 50, 000 N at maximum speed
• Operating speed: 600 - 3600 RPM
• Drive: 400 V A.C variable speed drive with controller
• Mechanical transmission by a flexible shaft or multiple shafts with
universal joints or belt drive.
• Unbalance: 3.5 x 105 gram-mm approximately at a radius of 125 to
150 mm.
Manufacture of mechanical exciter
The exciter system as shown in the schematics given in Figure 3 was
manufactured. The system was tested subsequently with 3 H.P. motor and
observations are as follows:
• Initial trial run with flexible links between the drive motor and the exciter
system showed that the system functioning was stable up to 1000 rpm.
Higher speed could not be achieved due to twisting of the flexible link and
high unbalance reaction force acting on the drive motor.
• As an alternate arrangement the flexible link was replaced with long pipe
around 3 m long with universal joints on both sides of the propelling shaft.
However, with this configuration the system could be run up to 1400 rpm.
The present invention has adapted belt pulley drive as a flexible link between the
motor and the exciter system. The motor was mounted directly on the exciter
body and connected to gears through belt (as shown in Figure 4).
The rated speed of 3600 rpm could be achieved with the belt-pulley arrangement
and 10 H.P, 2890-rpm motor in order to meet the desired peak force of 50, 000
Newton at 3600 rpm. This arrangement of drive motor being mounted on the
exciter system has made the system compact, which will facilitate easy
transportation of the mechanical excitation system to sites for carrying out
resonance test. The final technical specification of the mechanical exciter system
is given below:
• Peak force: 50, 000 N at maximum speed
• Operating speed: 600 - 3600 RPM
• Electric motor: 10 HP, 3000 RPM with variable frequency speed controller
• Drive voltage: 3 phase, 400 volts AC 50 Hz
• Eccentric weights: Variable weights at 110 - mm radius from 500 gms to
3000 gms.
Testing of mechanical exciter at manufacturing works
The system is subsequently inspected, sweep tested fixing to concrete
foundation for excitation in vertical direction and analyzed with dual channel
analyzer and accessories as shown in Figure 5. System is run at different
eccentric masses, the vibration velocity components corresponding to rotating
speeds in both vertical and horizontal directions are recorded in the following
tables.
Record 1: Unbalance 500 gm
Speed (RPM) Vertical direction (mm/sec) Horizontal direction (mm/sec)
1026 0.073 0.009
2030 0.879 0.138
3040 3.150 2.400
3560 5.050 3.300
The system is found to be satisfactorily meeting the functional requirements.
Low frequency components in the spectrum though do not effect the functional
requirements, is attributed to belt flutter. Horizontal levels are high at higher
speeds due to low stiffness in that direction on the test bed. Figure 6 gives the
interface base plate requirement for testing in horizontal, vertical and axial
directions.
The device has three configurations of positioning and bolting arrangement as
shown in Figure 6. For each configuration, the testing can be done in each
direction, viz., Vertical (1), Horizontal (2) and Axial (3) directions as shown in
Figure 6.
When the device is mounted with base as (1) then the excitation is obtained in
vertical direction. When the device is mounted with base as (2), then the
excitation is in horizontal direction. Finally when the device is mounted with
base as (3), then the structure is excited in axial direction.
A portable mechanical excitation system is developed using counter rotating twin
eccentric mass system, generating a maximum force of 50, 000 Newton in the
speed range of 100 to 3600 RPM. The system is tested to excite a large thrust
bearing test rig and distinct resonant frequencies are obtained in the coast down
Bode plots. The above system which is used to test the dynamic characteristics
of large support structures and foundation decks in triaxial directions can also be
used for testing of supporting structures of medium / smaller systems like
industrial turbines, GT skids, pumps and compressors, PA/ID/FD fans etc.
The final device consists of only belt pulley drive, with motor mounted on the
device itself. Other methods like flexible shaft, pipe drive are discarded as they
have disadvantages like limitations in speed, portability problems etc. The
present type of arrangement with belt drive and motor mounted on the device
itself is found to be the best feasible option for achieving desired speed,
compactness and portability as shown in Figure 3.
This device uses the principle of mechanical shakers where excitation is created
by centrifugal inertia forces produced by counter rotating eccentric masses or
unbalances. The counter rotating eccentric masses are selected so that the
resultant forces are accumulated in one direction only and in the other direction
the resultant forces cancel out. When the device is mounted in vertical direction,
the forces in horizontal direction are cancelled out. Similarly when the device is
mounted in Horizontal direction, the forces in vertical direction stand cancelled
out.
The natural frequency of any system can be obtained by sweep test wherein the
frequency of excitation is changed from maximum speed to standstill and a bode
plot is generated which has speed as X-axis and vibration amplitude / phase on
the Y-axis. This is achieved either by running the machine from the rated speed
to standstill or by fitting an external device like the present one. Natural
frequency or the critical speed is identified from the bode plot as a steep change
in the vibration and phase as shown in Figure 7.
From the above Figure 750 RPM is the resonant frequency of the thrust bearing
test rig, which is obtained using the present portable device fitted on to the test
rig.
The mechanical excitation (M) system as shown in Figure 3 consists of a drive
motor (1) that drives the discs (6) from 100 RPM to 3600 RPM. A drive
controller (2) that is programmed to drive the motor with predefined speed
range, acceleration and deceleration rates. This is the bought out device which
is used to control and set the speed range of the motor as required. A V-belt (7)
connects the motor (1) and the disc (6). The main exciter body (4) housing the
drive and driven gear discs and having three machined surfaces for mounting in
three direction. In order to increase the excitation force, unbalance weights (5)
of various configurations are provided in the main exciter body (M) housing. The
interface plates (A, B, C) which are used to mount the mechanical exciter (M) to
obtain excitation in vertical, horizontal and axial directions.
Following is the method of dynamic testing of a structure:
Step 1: Interface plate is fixed on the structure to be tested using four grouting
bolts which are spaced 425 mm apart as shown in the interface plate drawing of
Figure 6.
Step 2: The portable device (Mechanical exciter M) is fixed on the interface plate
A, B, C using the appropriate holes as required for horizontal, vertical and axial
direction.
Step 3: Controller and the motor are given electrical connections (three phase
415 V).
Step 4: Vibration recording instrumentation is made ready with signal inputs
from the probes fixed on the TG deck beams using embedded metal plates.
Step 5: The portable device is started using the controller from 0 to 3000 RPM
and vibrations are recorded to obtain the bode plots during coast up. Then the
device is tripped at 3000 RPM and the vibration recordings are continued while
the device is coasting down to 0 RPM.
Step 6: The above exercise is repeated by tilting the portable device and fixing it
again on the interface plate to give excitation in other two directions.
Step 7: Using the recorded vibration data, bode plots are generated, which give
the coast down response and the natural frequencies of TG deck.
WE CLAIM
1. A portable device for dynamic testing of large turbo generator decks in
three directions comprising:
- a drive motor (1) for driving the discs (6);
- a drive controller (2) for controlling the drive motor (1);
- a vee-belt (7) for connecting the motor (1) and the disc (6);
Characterized in that,
- the main exciter body (4) houses the drive and driven gear discs having
three machined surfaces for mounting on interface plate (A, B or C) to
obtain excitation in three directions wherein unbalance weights (5) of
various configurations are provided in the main excitor body (M) housing
for increasing the excitation force.
2. A portable device for dynamic testing as claimed in claim 1, wherein
counter rotating twin eccentric masses generates a maximum force of
50, 000 Newton in the speed range of 100 to 3600 RPM.
3. A method for dynamic testing of large Turbo generator decks in three
directions comprising;
- fixing the interface plate (A, B, C) on the structure to be tested;
- grouting the plate with a plurality of grouting bolts spaced at pre-
determined distance;
- fixing the portable device (mechanical exciter M) on the interface plate
(A, B, C) with appropriate holes required for horizontal, vertical and axial
direction;
- connecting the controller (2) and the motor electrically;
- making the vibration recording instrument ready with signal inputs from
the probes fixed on the TG deck beams with embedded metal plates
wherein, the portable device is started putting the controller form 0 to
3000 rpm and recording the vibrations to obtain the bode plots during
coast up when the device is tripped at 3000 RPM and the vibration
recordings continued while the device coast down to 0 RPM and the
process is repeated for other two directions when vibration data is
recorded to generate bode plots to obtain coast down response and the
natural frequencies of TG deck.

A portable device for dynamic testing of large Turbo generator decks in three
directions consists of a drive motor (1) for driving the discs (6), a drive controller
for controlling the drive motor (1), a vee-belt connecting the motor and the disc.
The main exciter body (4) houses the drive and driven gear discs having three
machine surfaces for mounting on interface plate (A, B, C) to obtain excitation in
three directions wherein unbalance weights (5) of various configuration are
provided in the main excitor body (M) housing for increasing the excitation force.