Dear all 

Equipment Centerhung Fan 

Problem :starting Vibration at motor and fan upon start lasts for 1or two seconds reaches 8 mm./sec sometimes and it doesnt give same behaviour at every start runs between (4 and 8 mm./sec) then after than it damps again to 2 mm./sec 

the vibration occurs only at start for few seconds and then vanishes.

Motor :induction motor 3 MW with LRS (liquid resistance starter).

Tests performed :

Cascade plot before and during the start to capture the frequency of the amplification, using the multichannel analyzer to capture the motor and fan instantenoues (at the same time).

Peak-Hold Averaging to detect the maximum peak at the start.

 

Results: 

at all tests the captured frequency which amplifies the vibration at start is 50 hz at both fan and motor.

Spectrum 

 

this test was detected the freuquency but the amplitude was not very high 

any clarification 

 

Thank you

 

 

 

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Original Post

I also note you mention use of a liquid resistance starter which is commonly used for wound rotor motors.  This type starter/motor is susceptible to electrical unbalance causing a forward/backward torque so you might want to get an electrician to thoroughly check things from an electrical perspective. 

John from PA posted:

Some questions:

Is this behavior something new or has it always been present?

Any work on the machine done recently?  If so, what?

What type of couplings are utilized?

Has a analytical torsional study ever been done?

First of all Hello and thanks for quick reply

This is not bew behavior, gear coupling but it happens at flexible coupling also but dosent tranamit to fan.

No work done at all,  i have  no idea what torsional study

Thanks šŸ˜Š

John from PA posted:
John from PA posted:

I also note you mention use of a liquid resistance starter which is commonly used for wound rotor motors.  This type starter/motor is susceptible to electrical unbalance causing a forward/backward torque so you might want to get an electrician to thoroughly check things from an electrical perspective. 

From the electricval prespective the electric team. Insists thaT LRS Is new and works. Better than before and there is nothing to do from their side

Some questions:

Is this behavior something new or has it always been present?

Any work on the machine done recently?  If so, what?

What type of couplings are utilized?

Has a analytical torsional study ever been done?

First of all Hello and thanks for quick reply

This is not bew behavior, gear coupling but it happens at flexible coupling also but dosent tranamit to fan.

No work done at all,  i have  no idea what torsional study

Thanks šŸ˜Š

Hossam 2 posted:

First of all Hello and thanks for quick reply

This is not bew behavior, gear coupling but it happens at flexible coupling also but dosent tranamit to fan.

No work done at all,  i have  no idea what torsional study

Thanks šŸ˜Š

ā€œGear coupling but it happens at flexible coupling also...ā€ makes me think that you should reply and attach a sketch of the machine showing the details.  By details, distinguish the couplings and the type of coupling, bearing types (rolling element o fluid film), etc. 

Check damper position on startup. Required motor power is proportional to the airflow cubed. Typical large fan start is with closed dampers. This is for across the line starts without a VFD. The overall vibration could be associated with the change in damper position so check this.

ckyjohn posted:

Check damper position on startup. Required motor power is proportional to the airflow cubed. Typical large fan start is with closed dampers. This is for across the line starts without a VFD. The overall vibration could be associated with the change in damper position so check this.

Dampre is always zero and it was calibrated to be actually zero

Hossam 2 posted:

Dear all 

Equipment Centerhung Fan 

Problem :starting Vibration at motor and fan upon start lasts for 1or two seconds reaches 8 mm./sec sometimes and it doesnt give same behaviour at every start runs between (4 and 8 mm./sec) then after than it damps again to 2 mm./sec 

the vibration occurs only at start for few seconds and then vanishes.

Motor :induction motor 3 MW with LRS (liquid resistance starter).

Tests performed :

Cascade plot before and during the start to capture the frequency of the amplification, using the multichannel analyzer to capture the motor and fan instantenoues (at the same time).

Peak-Hold Averaging to detect the maximum peak at the start.

 

Results: 

at all tests the captured frequency which amplifies the vibration at start is 50 hz at both fan and motor.

Spectrum 

 

this test was detected the freuquency but the amplitude was not very high 

any clarification 

 

Thank you

 

 

 

hello hossam

have you ever check structure looseness and resonance?

have you ever done bump test?

do you have check valve at out line?  

aziz58 posted:
Hossam 2 posted:

Dear all 

Equipment Centerhung Fan 

Problem :starting Vibration at motor and fan upon start lasts for 1or two seconds reaches 8 mm./sec sometimes and it doesnt give same behaviour at every start runs between (4 and 8 mm./sec) then after than it damps again to 2 mm./sec 

the vibration occurs only at start for few seconds and then vanishes.

Motor :induction motor 3 MW with LRS (liquid resistance starter).

Tests performed :

Cascade plot before and during the start to capture the frequency of the amplification, using the multichannel analyzer to capture the motor and fan instantenoues (at the same time).

Peak-Hold Averaging to detect the maximum peak at the start.

Results: 

at all tests the captured frequency which amplifies the vibration at start is 50 hz at both fan and motor.

Spectrum 

this test was detected the freuquency but the amplitude was not very high 

any clarification 

Thank you

hello hossam

have you ever check structure looseness and resonance?

Structure looseness would give a permenant vibration besides it will give 1X this vibration lasts for only second or two seconds. and i think resonance is the same ,, why 50 Hz every start?!

have you ever done bump test?

i didnt conduct a bump test actually 

do you have check valve at out line?  

what does that mean ?

Regards'

 

Have you rolled the motor in decoupled condition if yes are you observing similar behavior in solo condition 

Bump test on coupling,  motor DE and NDE bearing pedestal suggested 

Is both DE and NDE bearing ehiws high initial vibs or any one... .

In which direction the vibs are high horizontal or vertical.... 

If it's a VFD drive can you hold the speed just below 50-100 rpm of operating speed and see the vibration trend

Nagaraja 

 

Hossam 2 posted:
aziz58 posted:
Hossam 2 posted:

Dear all 

Equipment Centerhung Fan 

Problem :starting Vibration at motor and fan upon start lasts for 1or two seconds reaches 8 mm./sec sometimes and it doesnt give same behaviour at every start runs between (4 and 8 mm./sec) then after than it damps again to 2 mm./sec 

the vibration occurs only at start for few seconds and then vanishes.

Motor :induction motor 3 MW with LRS (liquid resistance starter).

Tests performed :

Cascade plot before and during the start to capture the frequency of the amplification, using the multichannel analyzer to capture the motor and fan instantenoues (at the same time).

Peak-Hold Averaging to detect the maximum peak at the start.

Results: 

at all tests the captured frequency which amplifies the vibration at start is 50 hz at both fan and motor.

Spectrum 

this test was detected the freuquency but the amplitude was not very high 

any clarification 

Thank you

hello hossam

have you ever check structure looseness and resonance?

Structure looseness would give a permenant vibration besides it will give 1X this vibration lasts for only second or two seconds. and i think resonance is the same ,, why 50 Hz every start?!

have you ever done bump test?

i didnt conduct a bump test actually 

do you have check valve at out line?  

what does that mean ?

Regards'

 

at the discharge line what do you have? maybe check valve or  flow control valve if yes ,have you check it to insure to correctly open at time  when starting 

Ron Brook posted:

Have had a fan jump in vibration level drastically from smooth to very rough several times.  Was the result of faulty dampers and the fan being starved for air.  Check pressure differential across the fan.  Then check damper settings.

Ron

 

Hello 

could you help with the procedure ? and what is the solution.

John from PA posted:

Some questions:

Is this behavior something new or has it always been present?

Any work on the machine done recently?  If so, what?

What type of couplings are utilized?

Has a analytical torsional study ever been done?

seems like i missed something 

The coupling is Spring couping

thanks

Because the vibration is at startup, It could very well be due to fan loading.  Again, check the static pressures on inlet and discharge sides.  You may have to start the fan with the discharge damper throttled back.  Then open once fan is up and running.  Check the pressure readings against the fan performance curve.  Wide open may not be acceptable at any time, even after running.

 

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