Analysis of high bracket vibration in HT motors.pdf

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

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Analysis of high bracket vibration in HT motors.pdf