BHEL has a services division at Bhopal and
one Motor repair shop at Mumbai.
BHEL takes following activities through services division:
Erection and Commissioning.
Troubleshooting.
Renovation and Overhauling.
TROUBLE SHOOTING
Vibration Problems
Noise Problem
Rotor Float Problem
High Stator Winding
Temperature
Temperature Rise in
Sleeve Bearings
Temperature Rise
in Anti-friction Bearings
Oil Leakage From Bearings
Low
Insulation Resistance(IR) )/PI Value of motor winding
Tripping of Machine on
Earth Fault
Excessive Wear of Brushes
Vibration
Problems
Regarding vibration Problem
following checks/preventive measures are suggested to eliminate the Problem.
- Decouple the machine and run the same to
asses the source of vibration (whether motor or driven equipment)
- If vibrations are reduced considerably then
check and correct the alignment. Again couple and observe the status while running.
- If vibrations still persist in uncoupled run
then remove half coupling and fit half key (up to level of shaft keyway) and run the
machine to detect coupling unbalance.
- Measure vibrations at both DE and NDE
bearings housings and on machine feet as well as on base frame in all 3 (H, V and Axial)
directions.
- The values of vibrations on machine feet and
base frame should not be more than approx. 10% of vibration measured on bearings.
- If vibrations are higher or equal at motor
base than on bearing, then check machine feet for correct seating /matching, check with
feeler gauges. If the Problem still persists, it indicates that motor support base / frame
is not sturdy enough and hence not damping out the vibrations and therefore, its
stiffening is required.
- If possible vibrations signature analysis to
be done on motor running uncoupled and also in coupled condition and send to BHEL for
detailed analysis to suggest further line of action.
Noise
Problem
In order to overcome the noise Problem,
following measures are recommended:
- Metallic rubbing noise may be due to
touching of external fan with air inlet guide provided at non-drive end fan housing.
Modify the running clearance between fan and air guide, as air guide is 2.5 tk sheet
metal.
- Check the dust and labrynth seals for
rubbing with shaft.
- Check loosening of fan at hub.
- If there is bearing noise, which reduces
slowly on shutting down, then provide grease in bearing and run the machine.
- Noise may also be due to vibration resonance
in the machine. Take vibration signature analysis to detect the source.
- Any loose fitting on motor frame/body to be
checked viz- Terminal box, Aux. Terminal box covers, dial thermometers, fan housing
covers, earthing strip etc.
Following information is required by
BHEL to analyze the Problem and to suggest further line of action:
- Type of noise i.e. humming, rubbing,
rattling, screeching and whether intermittent or continuous.
- Intensity of noise
- Whether noise due to mechanical or
electrical reason. The noise
due to electrical reason vanishes immediately on switching off.
- Bearing temperature rise of both DE and NDE
bearings.
- Vibration levels on both bearings and motor
feet in all three directions i.e. Horizontal / Vertical / Axial.
Rotor Float Problem
To arrest the rotor float the
following aspects to be checked and ensured:
- Check the level of rotor on both journal
surfaces, which should be equal on both ends. Run the machine in uncoupled condition and
ensure that motor is running in correct direction and also observe the tendency of rotor
float.
- During uncoupled run, the rotor may run on
either side due to air thrust of ventilating fan. Now couple the motor with its rotor in
mechanical center position (with almost equal axial clearance maintained at DE bearing).
To restrain the rotor float, it is mandatory to use the limited end-float coupling as
specified in OGA drawing.
High Stator Winding Temperature
Regarding Problem of high stator winding
temperature following measures are suggested :
- The winding temperature of class
‘F’ insulation motor is safely allowed upto 135 deg. C measured by RTD provided
between coil sides in the slot portion.
- In case winding temperature is observed at
higher side on full load, then measure RTD resistance at motor auxiliary RTD terminal box
and confirm the actual temperature. RTD are generally PT-100 i.e. 100 Ohms at 0 Deg. C and
138 ohms at 100 Deg. C., with linear rise (2.59 degrees/ Ohm rise or 0.385 Ohms/degree).
Correct the scanner if discrepancy is observed.
- Confirm the load current in all 3 phases by
calibrating ammeter or by use of additional ammeters and compare with full load rated
current to ensure that motor is not overloaded. Also ensure that machine is running at
rated voltage.
- If the cooling fan in the motor is
unidirectional, then confirm direction of rotation as shown on motor frame. Proper
functioning of coolers or external blowers to be ensured.
- Confirm that the cooling air circuit/ tubes
/filters/ inlet mesh etc. is not choked. Clean these air inlet passages to ensure correct
quantity of airflow.
- If external parameters as mentioned above
are found in order then dismantle the motor to check blockage of radial cooling ducts of
stator and rotor. If required ventilating ducts are to be cleaned thoroughly by blowing
out blockage with dry compressed air. If sticky deposits are there, clean them with
thinner, trichloroethylene, white sprit and if nothing is available then with clean dry
cloth soaked in petrol and squeezed.
Temperature Rise In Sleeve Bearings
Regarding bearing temperature rise
Problems following measures are suggested to overcome the Problem:
- Check grade of oil, it should be exactly
according to recommended grade.
- Oil flow quantity to bearing inlet must be
in line with recommendations given on OGA drawing and bearing lubrication instruction
plate. Temperature rise may take place due to inadequate oil supply. The bearing oil inlet
pressure should be within 0.2 to 0.5 bar.
- Check axial running clearances of shaft
journal w.r. to bearing side faces and ensure that shaft collar does not touch to bush
face during running condition. Axial clearances should be in line with machine OGA
drawing. To restrict the axial float of rotor and consequential rubbing of shaft collar
against bearing it is mandatory to use limited end float coupling, in line with technical
requirement mentioned in this regard in our OGA drawing.
- Check quality of oil for suspended
impurities and contamination, and if condition of oil found detoriated, change it
completely.
- Check free rotation of oil rings, and oil
level through sight glasses
- Decouple the machine and run to assess
bearing temperature rise trend. If the bearing temperature becomes normal, it indicates
higher misalignment (with driven equipment), which has to be improved to acceptable
limits. Simultaneously check the level of the machine and if required necessary
improvements to be carried out.
Temperature Rise In Anti-friction
Bearings
Regarding higher bearing temperatures
rise Problems in motors with anti-friction bearings , following measures are
suggested:
- The temperature rise may be due to excessive
grease in the bearings, hence machine should be run for little longer time, and till the
bearing temperature stabilizes. In subsequent runs bearing temperature may come down.
- Re-lubrication of bearings to be carried out
as per the bearing instruction plate by which only 50-60 gm of grease may be required to
be pumped-in, during normal running of motor.
- Motor to be run in uncoupled condition to
ensure that no external thrust is coming from driven machine. If the temperature rise is
normal in uncoupled condition then improve alignment and ensure that no thrust comes on
motor bearing from driven equipment. The alignment to be carried out within 0.02 mm in
case of rigid coupling and 0,05 in case of flexible coupling.
- If the temperature rise is associated with
noise from bearings, then it may be a case of dry run. Apply some grease and run the
machine and if still Problem persists then open the bearing and check its conditions. The
bearing may be required to be replaced.
Oil Leakage From Bearings
To arrest oil leakage from bearings,
check and ensure the following :
Check all the pipe and
nipple joints for leakage. If leakage found retighten them by applying Teflon tape on
threads.
Check and ensure that rated
oil flow and oil pressure at bearing inlet is as per the specified values in OGA drawing
and bearing instruction plate. If required suitable orifice, oil flow switch and pressure
switch may be incorporated in line to maintain required flow and pressure. Excess pressure
can lead to leakages even if oil flow is maintained by oil regulating valves/ orifice.
Check and ensure minimum
slope of 1:20 on the return oil line of the machine. For the safety of bearings it is
imperative that from trial run stage of the machine the bearing temperatures are monitored
through scanners set for alarm and trip. The values should be as per our drawing/
technical data supplied for the motor.
Low Insulation Resistance(IR) )/PI Value of
motor winding
Regarding low IR and PI values of motor
winding it is recommended to carry out following:
- Check terminal boxes and neutral box for
water/moisture deposition. Clean and dry with hot blower.
- Remove leads from terminals/connections.
- Check IR-phase to earth and phase to phase.
- In case of IR or PI values still low then
winding needs drying out. For PI value below 1.5 drying is mandatory, between 1.5 and 2.0,
it is recommended and above 2 it is not required.
- Drying of winding can be carried out as per
either of following methods:
- By producing heat losses in the machine
itself i.e. by operating the machine on short circuit in case of alternators and locked
rotor condition in case of motors with low voltage supply to stator.
- By feeding current from external energy
source to produce heat losses in the winding e.g. with the aid of welding sets having
controllable high current rectifiers.
- By providing a flow of hot air from external
source after covering with tarpaulins.
- During dry out IR value to be measured
intermittently (after every 4 hrs) and when last four readings show improvement the drying
process may be stopped. For HT motors, minimum IR value should be approx. 50 M-Ohms at
ambient temperature.
Tripping of Machine on Earth Fault
For Problem of motor tripping on
earth fault, following actions are suggested :
- Open the terminal boxes, main and neutral
and inspect for the local flash over. Check T.B. insulating items and rectify/replace them
if required.
- Meger the individual phases to detect fault
in particular phase.
- If star point is inside the motor, measure
the resistance AB, BC and CA and analyze for particular phase for earthing on low IR.
- Dismantle stator for access to winding
physically. Inspect the winding for burning or any black spot. If not found then:
- Separate out the phase group and then coil
group and detect particular zone/coil.
- If the earth fault is in slot portion on top
coil side or burning/melting on overhang portion then contact BHEL for local repair.
- If earth fault is in bottom coil then motor
need shifting to BHEL works for repair.
- May earth fault tripping occur due to low
setting of earth fault relay, setting can be increased with the concurrence of relay
supplier/ motor designer. But this should be done when the health of the winding is
assessed and good IR value is found.
Excessive Wear of Brushes
To overcome the Problem of fast
wearing carbon brushes, following measures are suggested:
- The brush grades are as specified in
datasheet.
- All brushes should be from one supplier.
There should not be mixing of brush grades as well as make.
- Brush pressure should be approx 200-gm/sq cm
uniform on all brushes.
- The brush box to slipping gap should be
within 1.5 to 3 mm for all the brushes.
- Number of brushes should be according to the
datasheet. The current density should be between 12 to15 Amps/sq cm
- The brushes should be used up to 2/3rd
of its total length.
- There should not be any loose connection of
rotor cables, pigtails etc. coming on slipping.
- The slipping surface temperature should not
exceed 100 degree centigrade.
- The slipping surface should be quite smooth.
Surface must be free from scratches and indentations. Oxide skin must not be destroyed by
unnecessary grinding or otherwise even not after replacement of brushes.
- Healthiness of rotor leads, lugs and
crimping is to be ensured
- Normal brush life is 4 to 6 months depending
on number of hours running (Approx 4000 running hrs / annum)
If all above is ok then send following
information to BHEL with the Problem:
- Existing carbon brush grade and number of
brushes on each ring
- Rate of brush wear.
- Condition of slipring surface whether smooth
or having indentations, scratches etc.
- Status of brush contact with slipring in
percentage terms. It should be more than 80% and uniform on all brushes.
Following information is
required by BHEL to analyze the Problem and to suggest further line of action:
- Whether the edges of brushes are chipped or
not?
- What is the pressure of carbon brushes?
- What is the gap between brush box and
slipping?
- Is the brush moves in brush box easily or
not?
- Any overheating marks seen on brushes, rings
or on rotor connections to sliprings.
