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


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



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



3) Causes of vibration in electrical machines 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



3) Causes of vibration in electrical machines 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



3) Causes of vibration in electrical machines 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



3) Causes of vibration in electrical machines (continued) b) In some detail (continued) • 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



3) Causes of vibration in electrical machines • Mechanical force induced vibrations(continued) • 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



3) Causes of vibration in electrical machines • Mechanical force induced vibrations(continued) • 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



4) Some mitigation techniques of vibration in electrical machines • 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



4) Some mitigation techniques of vibration in electrical machines • 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



4) Some mitigation techniques of vibration in electrical machines • 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



4) Some mitigation techniques of vibration in electrical machines • 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