Electrical machines vibration - causes effects and mitigation - Mr Sunil Naik

Vibration in Electrical rotating machines – Causes, Effects and
Mitigation
By
Sunil Naik
Head – Design Engineering
Electrical Rotating Machines Division
TMEIC Industrial Systems India Pvt. Ltd.
Tumakuru, Karnataka, India

Vibration Analysis & Correlation-A Technology Symposium-May-2016
Vibration in Electrical rotating machines – Causes, Effects and
Mitigation
Presentation Contents
1) Introduction – Speaker & TMEIC India
2) Electrical machines construction & operation
3) Causes of vibration in electrical machines
4) Effects of vibration in electrical machines
5) Mitigation through –
a) Design
b) Process
c) Manufacture

Vibration in Electrical rotating machines – Causes, Effects and Mitigation
1) Introduction – Speaker & TMEIC India
• My name is Sunil Naik and by qualification - an Electrical Engineer
• My work experience is in the field of design, analysis, manufacture,
testing & development of rotating electrical generators and motors
• I presently work for M/s TMEIC Industrial Systems India Pvt. Ltd. at its
rotating machines factory in Tumakuru, Karnataka about 90 km from
Bengaluru
• M/s TMEIC is in the business of manufacture of rotating electrical
machines and drives for various Industrial applications both safe area
and explosive atmospheres – a Japanese MNC formed by the
amalgamation of the industrial systems divisions of TOSHIBA &
MITSUBISHI ELECTRIC
• Today, I will deliver a brief talk on the phenomenon of vibrations in
electrical rotating machines, their causes, effects and mitigation
principles as practiced in industry today
• My thanks to the organisers of this symposium – M/s ProSIM in
general & Dr Shamasunder in particular – for giving me an opportunity
to share my views and experience

2) Electrical machines construction & operation
• In general an electrical rotating machine coverts energy from one form
to the other – generator : rotational mechanical to electrical energy
- motor : electrical to mechanical rotational energy
• Two main parts – a non-moving stator and a rotating part – rotor
• The stator comprises of electrical grade silicon steel core housing a
winding and terminal connections
• The rotor comprises either of a salient/cylindrical electrical grade
silicon or high tensile steel core in which copper windings or bars/end
rings are housed. Aluminium die cast rotors are common in motors.
• Synchronous machines have slip-rings or exciter while wound rotor
motors also have slip-rings
• To ensure rotational motion, the stator and rotor are separated by an
air gap by means of bearings housed in end shields
• To keep machine temperature under limits, there are self or forced
cooling units of either air or water mounted. Oil/grease lubrication of
bearings is in vogue

2) Electrical machines construction & operation(cont.)
• When the stator windings of a motor are connected to a power
source, the flow of currents causes a rotating magnetic field which in-
turn induces a voltage in the rotor.
• If the rotor circuit is closed (permanently as in squirrel cage motors or
by connecting the slip rings in wound rotor motors) the induced
voltage sends a current that in-turn produces a rotating magnetic field.
The interaction of the rotor & stator magnetic field causes the rotation
of the motor
• Thereby a motor converts electrical to mechanical rotational energy
• Similarly, when the rotor, that carries an excitation current, is rotated
by an external prime mover, a voltage is generated in the stator
windings of a generator
• If this generator is connected to a power grid or to an external local
load, the induced voltage sends a current to the load/grid
• Thereby a generator converts mechanical rotational energy to
electrical energy

a) In brief
• Electromagnetic forces
• Mechanical forces
• The movement of air or water to cool the machine
• Structural characteristics of the machine housings – Cast iron vs.
Fabricated frames, L/D ratios in vertical machines
• Mounting methods – foundations , base frames, rocking
• Load characteristics & non sinusoidal supplies – Electrical imbalances,
VSD-PWM
• Electrical faults – 1-ph, 2-ph or 3-ph faults, unbalanced magnetic pull
• Load-side - coupling misalignments, driven eqipment imbalances
• Mechanical – rubbing, loose items, bearings, shaft critical speeds, soft
foot, rotor eccentricities, bent shaft extensions, thermal imbalances in
rotor, elliptical bearing journals, broken rotor bars, inadequate
clamping of laminated structures, improper keys, broken fan blades
• Imbalances due to large terminal boxes cantilevered to the machine
frame

b) In some detail
• Electromagnetic force induced vibrations
• The interaction of the stator magnetic flux and rotor currents give
rise to two components of force – Tangential & radial
• The tangential force is responsible for torque & rotation
• Radial force is cause of vibration. If the natural freq of the
machine coincides or is close to the radial force freq then
vibrations are set up
• Broken rotor bars/badly brazed rings/electrically shorted rotor
coils/turns
– Absence of current causes a differential flux distribution
and creates 1 x fL vibration or high amplitudes at high freq
Broken bar spectrum
Stator coil
fault
spectrum

b) In some detail
• Electromagnetic force induced vibrations
• Stator tooth acts like cantilever beam supported at the root by
the core. Its fr is a function of the length & width –
Lower width & longer length will reduce vibration
but gets saturated magnetically
• Unbalanced magnetic pull (UMP) and non uniform air gaps
create 1 x Nr, 2 x Nr, & 2 x fl vibration

3) Causes of vibration in electrical machines (continued)
b) In some detail (continued)
• Electromagnetic force induced vibrations(continued)
• 2-pole machines are particularly sensitive to the fact that half of
the machine is alternatively “pulled” and ”pushed” every cycle of
operation creating a 2 x fl vibration. This
has serious mechanical mounting
implications like soft foot flatness, frame
& base stiffness
• Shorted rotor laminations give rise to 1 x Nr
vibration
The best distinguisher of an EM related vibration is its disappearance on
switching off power
• Mechanical force induced vibrations
• Today’s industrial scenario demand economic machines resulting
in frames that are less stiffer. Higher magnetic flux densities in the
stator cores add to the higher magnetic forces.
• Together, they make the machine more sensitive to vibration &
noise

• Mechanical force induced vibrations(continued)
• In 2-pole machines, the 2 x fl vibration can send the laminated
stator into vibration fatigue cycles that could result in stator
winding loosening & insulation failure & stator core fretting
corrosion
• Additionally the 2 x fl vibration being independent of the machine
load is sensitive to foot flatness, frame & base stiffness & air gap
concentricity
• Looseness of assembly parts due to improper fit causes a whole
host of integral frequencies (2x to 10x) and ½, 3.5, 4.5, 5.5 x fL of
vibration with amplitudes greater than 20% of fundamental
• Rubbing of static and
rotating parts produces
nearly same spectrum as
looseness
Before After
tightening
Looseness of parts

• Imbalance caused by shaft mass centre line not coinciding with
the geometric centre gives rise to static (one force only), coupling
(2 forces 180⁰ out-of-phase) or dynamic(both static & coupled)
• Caused by improper mfg. process, accumulation of debris,
breakage of fan blades, loss of balancing elements or additions to
a shaft
• Vibrations caused by imbalance generally show high radial
magnitudes in X & Y planes & 1 x Nr freq
Unbalance spectrum

• Bent shafts either inside the machine
or at the extensions also give rise to
vibrations similar to imbalance condition
in axial direction. Checking of shaft run-
out or coupling alignment can distinguish
between the two. 1x, 2x & 3x fL are predominant axial frequencies
• Eccentric rotor with respect to bearing journals causes 1 x Nr
vibration with beats (amplitude changes with time at same freq.)
• Thermal imbalances caused due to uneven rotor heating also
bend the rotor giving rise to a whirl during rotation (thermal bow)
will cause 1x & 2x fL radial vibrations. Could also occur when
transitioning from cold to hot state.
• Improper shaft keys raise shaft unbalances and hence ½ key
balancing is common.

• Weak mounting base or foundations cause vibrations of the order
1x & 2x fL . It is very significant to note that seasonal variations
cause the mud below the concrete to bloat or contract or
concrete itself deteriorates with time giving rise to vibration
modes unpredicted. Seismic activities generally contribute to
these type of defects
• Bearing are the most exhaustive and significant causes of
vibration.
• Antifriction bearings-ball &
roller-have specific frequencies
associated with them
(ball passing freq.) and defects
are detected by the presence
of high freq. in the spectrum

• Common antifriction brg. problems include
Bearing misalignments – Radial vib at the DE & NDE side 1x to
& Loose bearing fit & 6 x fL
Worn out or damaged rolling elements
– 1 - 20kHz high acceleration
Seals rubbing or wear – High amplitude
radial vib 2 x fL
• In case of hydrodynamic/static bearings (sleeve brgs)
• Oil whirl vibrations caused by high bearing clearances in
the low speed range generate
vibrations in the 0.42 to 0.45 x Nr
range

• In case of hydrodynamic/static bearings (sleeve brgs)
• Bearing wear due to shaft-babbit rubbing gives rise to
very high radial vibration amplitudes at frequencies
from 0.5 to 6 & above x Nr
• Journal out-of-roundness
brings out a 2 x Nr
predominant frequency
• Presence of bearing debris composed
of dust, dirt can disrupt the oil film, damaging the
surface of the bearing

• External load coupling induced vibrations could be due to mis-
alignments in
• Angular direction giving rise to a bending
moment on the shaft & 1 x Nr high axial freq on DE
• Parallel direction due to coupling centre offsets causing a 2 x Nr
freq. > 1x Nr radial component on DE
• Oversized couplings can change critical speeds
• Load imbalances contribute a good amount of vibrations when rigidly
coupled to an electrical machine. Vibration frequencies in the range of
0.5 x Nr to producing a number of sidebands
in the freq spectrum characterise them. Gear
misalignments are classic examples with
broken teeth or backlash issues

• Vertical machines are always prone to high axial vibrations that are
contributed by
• Weak base flanges
• Inadequate flange bonding to coupling frame
• Loose coupling bolts
• High L/D ratios
• Top mounted thrust bearings
• Unbalanced masses
• Improper coupling alignments

4) Effects of vibration in electrical machines
• Reduces the reliability of the machine
• Increases audible noise
• Causes incipient faults that over a period of time get amplified to a
catastrophic failure
• Loss of process or production on the factory resulting in material
wasted and monetary loss running into crores of rupees
• Unplanned maintenance preceded by shut downs
• Early bearing failures due to excessive loading caused by imbalances
• Induces shaft fatigue
• Damage adjacent or connected equipment
• Loosen windings and erode insulation by flaking & fracture
• Make the conductors brittle
• Damages the fragile overhang portions of the stators

4) Some mitigation techniques of vibration in electrical machines
• By Design
• At the machine development stage follow a rigorous analysis route
consisting of :
• EM Finite element analysis of a wound stator rotor model
• Compute the air gap flux densities & resulting EM forces by principles
of virtual work

• By Design (continued)
• Using structural analysis FEA, compute the resultant vibration modes
• Stator deformation for each mode (upto 20) can be computed along
with the natural frequencies

• By Design (continued)
• Compute the harmonic components & response of the stator frame
• In vertical motors, check the L/D ratios < 1 and analyse as a 2-mass 2-
spring system
• Increase stiffness to counter the high bending modes and re-analyse
• Always design a stiff shaft with the 1st critical speed is 15-20% off the
max speed of operation

• By practical counter measures
• Vibration measurements data & FFT analysis plots – velocity preferred
• Systematic approach to data analysis
• Use the diagnostic techniques outlined earlier – based on published
data & experience
• Process & mfg refinements to achieve specified drawing tolerances
• Choice of proper machining techniques – CNC vs Conventional
• Use of welding & machining fixtures
• Stress relieving of welded components
• Two level or three level planned machining sequences
• Multilevel balancing with ½ key
• Eliminate rotor bends during mfg through rigorous inspection,
correction and proper handling
• Adopt tight rotor bars by machine
controlled swaging in the ends and
along the rotor slots

• By practical counter measures (continued)
• Hot trim balancing in-situ
• Key & coupling balancing with rotor
• Accurate run-out machining of the coupling
• In case of flexible rotors, balance them at rated speed
• Adopt policy of supplying rigid base frames instead of sole plates
• Vertical motor coupling side/DE flanges to be completely bolted and
spigot fitted
• Tight tolerances for the thrust bearing fits

5) In conclusion,
We have (re)learnt that vibration (and hence noise) are dependent on sources
of vibration forces, the structure and mechanical characteristics of the machine
It is possible to build a database of possible causes of vibrations through a
systematic collection and analysis of vibration data
This knowledge base can be used to design machines to reduce the effects of
vibration thereby improving their reliability in service

Once again…………………………………….
We thank the audience for a patient hearing
and
M/s ProSIM - sponsors of this symposia for having given us an
opportunity to outline and share our thoughts on
Vibration in electrical rotating machines-causes, effects and mitigation
Thank you

Electrical machines vibration - causes effects and mitigation - Mr Sunil Naik

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