Analysis of high bracket vibration in HT motors.pdf
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Transcript of Analysis of high bracket vibration in HT motors.pdf
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7/28/2019 Analysis of high bracket vibration in HT motors.pdf
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SUBJECT : ANALYSIS OF HIGH BRACKET VIBRATIONS
IN 18 MW MOTORS
PROJECT : SRISAILAM LEFT BANK CANAL LIFTIRRIGATION
Srisailam Left bank Irrigation scheme is Equipped with 4 numbers
of 18 MW Synchronous motors to drive Vertical pumps of Francis
type with a discharge head of 81 m which is the highest in the
country. . The unit is supported by a Thrust and Guide bearing on
generator top , Lower Guide bearing at generator bottom and
Turbine Guide bearing at Turbine top. The pumps are designed to
start through Static frequency converter which brings the unit to
rated speed before synchronization. The pump discharge head and
volume are controlled by outlet guide vanes.
Direction of Rotation : Anticlockwise from top
Rated Speed : 427 RPM
Thrust Bearing No. of pads : 8
Upper Guide bearing No. of pads : 10
Lower Guide bearing No. of pads : 10
Turbine Guide bearing No. of pads : 10
Number of guide vanes : 20
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Balance weight locations : Rotor top -- 12 locations
Maximum balance weight at any location 14 KG
During initial commissioning trials Units 3 & 4 experienced bracket
vibrations of the order of 130 microns peak to peak on Motor
bracket which supports the guide bearings. BHEL Bhopal had
recommended 100 microns Peak to peak as acceptable levels on
brackets. The vibration levels on Units 1 & 2 were 60 microns
which was well within limits.
Bearing and shaft vibration measurements were carried on unit 3
using Bentley Nevada probes and analyser. The shaft and housing
vibration levels ( microns Peak Peak ) at 427 rpm , unit in
syncronised condition with guide vanes closed near Upper guide
Bearing are
1X Running Speed Component Phase Overall levels
Shaft 38 133 90-105
Bracket 103 305 120-125
Observation
1.The shaft vibration levels are less than Bracket vibrations2.The Shaft vibration running speed component is low
comparable to overall levels
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Since the bracket vibration levels were higher it was suspected
that the radial forces were not getting transmitted truly to the
foundation. The radial jacks 6 in number were checked and
tightened. The enclosed figure shows the sectional elevation of
motor with Upper guide bearings and radial jack arrangement.
The vibration levels measures after tightening are
1X Running Speed Component Phase Overall levels
Shaft 40 120 98Bracket 70 357 70-80
There is no change in shaft vibration levels , but there is
considerable reduction in bracket vibrations . The running speed
component of shaft vibration levels is still low compared to overall
levels . The other components in the shaft system could be because
of hydraulic forces since the measurement were carried out with
outlet vanes in closed condition with no water flow in the system.
With outlet vanes open and sufficient water in inlet surge tank the
pump operation should be smoother.
The above vibration levels are satisfactory for continuous operation
of the unit. Similar exercise of checking and tightening of radialjacks were carried out on unit 4. The bracket vibrations at rated
speed got reduced on Unit- 4 to 65 70 microns peak to peak
overall.
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Guide Bearing
Pad
Thrust Bearing
Pad
Shaft
Stator Frame
Stator coreRotor Hub
Rotor Rim
Rotor Coil
Radial jack
Lock nut
Thrust
Collar
FIG. CROSS SECTION OF MOTOR