3-Phase Induction Motor starting problems (Locked Motor & abnormal loud sound) 9

[TTojiiLee]

3-Phase Induction Motor starting problems (Locked Motor & abnormal loud sound)

When starting the motor, there is a problem in which a loud noise occurs as soon as power is applied
and the motor rotation is restricted(locked).

The driven machine is a integrally geared centrifugal compressor(3-stage, 2-pinion) and
I guess that the problem is caused by compressor jamming and excessive inrush current at DOL(Direct on line) startup.

If there is no jamming in the compressor(driven machine) or normal conditions, it is possible to start and operate normally without any problems.

When jamming of the compressor (possible seal, thrust collar, impeller, bearing, etc.) occurs,
the inrush current increases more than normal conditions and this problem is occurred a very rare phenomenon.

Can you tell how much the inrush current can cause this problem(Locked Motor & abnormal loud sound) from the attached motor data?

 

[Gr8blu]

1. Does the motor turn freely (by hand) when not connected to the compressor? Any grinding or squealing noise when doing so?
2. Is the "loud" noise coming from the motor? The compressor? The breaker? Any other unusual noises occurring - and where?

The amount of inrush current drawn is determined by: A) the voltage applied to the motor, where more voltage gives more inrush, and B) the torque required to overcome the drive train inertia - note that the train inertia includes the motor itself, the driven load (compressor), and any intermediate gearing or friction areas.

If the motor is locked during startup, then the developed torque is insufficient to overcome the required torque to get the inertia moving. This can be caused by:
1) insufficient voltage at the machine terminals; developed motor torque is proportional to the square of the per-unit applied voltage - which means having 90% volts develops 81% torque
2) internal damage in the motor (broken rotor bars, damaged bearings, no end play so the machine rides up against the thrust face, a broken shaft so that the rotor core makes contact with the stator core, damaged stator winding, etc.)
3) a lack of lubrication in any of the normally-lubricated areas (motor bearings, gearbox/pinion, compressor bearings, etc.)
4) internal damage to the compressor (pistons in contact with cylinders, bad seals, damaged crank, incorrect valve timing, etc.)
5) simply overloading the compressor - my guess is the system is designed to start unloaded, and after a shutdown (which may or may not occur as intended) some fluid remains in the compressor which then raises the torque requirement enough to stall out the motor at some point during its normal acceleration.

Converting energy to motion for more than half a century

 

[edison123]

Fix the damn compressor.

Muthu

http://www.edison.co.in

 

[Kiribanda]

Since it is an induction motor, based on its equivalent circuit, the maximum possible LRC=6.5 x FLA (47.8A) = 310A at 10kV(+/-10%) rated voltage & at 50HZ.
Refer to Line # 21 of page 1 of the data sheet and also I-N curve given in page 4 of the data sheet.
On other conditions it might vary.
Better to check your O/C relay settings (time & current) for LRC=310A.

 

[TTojiiLee]

Thanks,Gr8blu

1. Does the motor turn freely (by hand) when not connected to the compressor? Any grinding or squealing noise when doing so?
When stopped and checked by hand barring, it rotates freely without any unusual noise.
There is no damage to the compressor/motor or any special mechanical or electrical components. When the power is turned on again, normal startup/operation is possible.

2. Is the "loud" noise coming from the motor? The compressor? The breaker? Any other unusual noises occurring - and where?
"A VERY BIG SOUND" comes from the "MOTOR" as soon as electric power ON. It disappears when the electricity is cut off (Power OFF).

 

[FacEngrPE]

Only 2 possibilities I see - either the compressor is mechanically locked up, usually related to massive internal damage, or the motor is developing zero torque.
The most common cause of zero torque development in no particular order are
[ul]
[li]All of the items suggested by Gr8blu[/li]
[li]Single phasing.[/li]
[/ul]

Detecting single phasing on a 10kV circuit requires temporary installation of CT's on all three phase leads between the motor and the switch.
The condition might intermittent now

When the power is turned on again, normal startup/operation is possible.

probably not for long. Making a guess there may be an internal problem with your switch.

 

[TTojiiLee]

Thanks, Kiribanda

When the motor is power ON, Can I understand that if more than Max.LRC,310A(motor data sheet) is input, the motor will not rotate and will be locked?
I'd like to measure the current, voltage (normal condition vs abnormal condition(motor locked & big sound).

The motor data sheet's LRT/FLT=85%, and when starting normally without a problem, the starting time(0 to motor rated rpm) is 5 seconds.
If the motor is designed so that the LRT/FLT is larger than the current level(LRT/FLT=85%), the motor T-N performance curve changes upward and the starting time under the same load machine (Load GD2, same Required T-N curve of the load machine) is expected to be shorter than 5 seconds, and conversely, if the LRT/FLT is designed to be smaller than the current level (LRT/FLT=85%), the motor T-N performance curve will change downward and the acceleration torque will become smaller, resulting in a starting time is judged to be longer than 5 seconds.

This problem is that the rotational acceleration torque is TOO LARGE for "DOL starting" compared to Reactor starting or Soft-Starter starting, so the starting time is too short (acceleration rapidly), so smooth rotational acceleration is not possible, and there may be (slight) jamming in the driven machine(load machine).

In some cases, it is judged that the Max.LRC (310A) is exceeded, causing a restriction in motor rotation and a loud noise.
During DOL startup, the acceleration torque, which is the difference between the motor T-N Curve and the load T-N Curve, is sufficient and even too large, so the startup time is too short during normal startup without any problems, and in case of jamming, there is a problem of locked motor rotation & abnormal big sound. I think it is.

I think the solution is

1) How to reduce sudden acceleration by designing LRT/FLT= to be less than 85% to reduce motor T-N performance and ultimately increase startup time by reducing motor acceleration torque.

2) How to reduce sudden acceleration by reducing motor torque and increasing start-up time by applying reactor start-up or soft-starter start-up method.

If you have any comments or additional considerations regarding my thoughts/judgment, please give me advice.

by TTojiiLee
​

 

[jraef]

I’m wondering about some unexpected axial thrust from the motor causing a locked rotor in the compressor. That sort of thing doesn’t show up when manually rotating it.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[edison123]

Maybe the system is designed to start the compressor unloaded and you're starting it loaded?

Muthu

http://www.edison.co.in

 

[waross]

You have a problem and I don't think that it is one of your guesses.
A centrifugal compressor doesn't much care about DOL starting torque.
You may have a broken tooth in a gear train in the compressor.

Fix the damn compressor.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FacEngrPE]

We have a bunch of 400 HP centrifugal and a few screw compressors. All of them are started across the line, with a stiff MV power supply. Fast acceleration has never resulted in a mechanical problem.
Starting with the centrifugal compressor loaded - rather than unloaded will result in excessively long acceleration times, and might result in the motor being unable to develop enough torque to reach normal speed. This condition does not match your described symptoms.
Broken tooth in the gearbox is a possibility, when the compressor does start that should show up in the on board vibration sensors.

 

[panter]

TT ,

A good question for maintenance is whether someone has been working on something at high voltage and whether there was perhaps a phase change and consequently change direction of rotation of the motor.

 

[TTojiiLee]

Thanks All.

1. The compressor is started unload conditions always with Full closed IGV(Inlet Guide Vane) & Full opened Discharge BOV(Bolw Off Valve.
2. We opened the compresor gearbox.
We inspected and checked the 1-bull gear & 2-pinion gear teeth, gear thrust collar,
Low speed 2-bearings for bull gear(Taper-Land Thrust & sleeve journal bearing) & High speed 4-bearings for pinion (5-tilting pad journal bearing),
air seal(labyrinth type-Mat'l aluminium(AL6061) & oil seal(labyrinth type-aluminium(AL6061), inlet shroud for impeller, impeller, diffuser and etc.
But we could not find the any fault in the gear & shaft, bearing, shroud, impeller, diffuser. we found just the labyrinth teeth's small rubbing.
If there is the mechanical damage in the compressor or motor, the compressor could not be operated normal operation after the abnormal start problem.
Now the compressor is OK, but we are worried about this startup problem(Motor locked & Very Big Sound) that may occure again at any time.
I am worried that the compressor or motor may be damaged mechanically due to this startup problem.
3. I'll inspect/check "about some unexpected axial thrust from the motor causing a locked rotor in the compressor" if possible.

Your replies are very helpful. Thank you again.

by TTojiiLee

 

[edison123]

Check for damaged/loose couplings, keys and coupling bolts on both motor and compressor sides.

Muthu

http://www.edison.co.in

 

[TTojiiLee]

edison123, I checked the coupling, but could not find any fault in the copling. Thank you!

I counted the number of motor start/stop(Motor Power On/Off) in 8 month compressor operation.
During 8 months of operation, there were 24 starting(Motor power ON), 3 of which were starting problem(Motor locked & abnomal very big sound) without ALL mechanical/electrical component/part
and it is still operating with good running conditions (compressor vibration & bearing temp, motor winding temp & brg. temp., lube oil pressure&temp, voltage & current, etc)

Thanks, All

by TT

 

[waross]

Possibly one pole of the contactor is malfunctioning intermittently and the motor is single phasing from time to time.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[zlatkodo]

Hi, TT,
I see two possible scenarios here:
- Damage to one or more bars on the rotor, causing the motor to sometimes fail to start (depending on the position in which the rotor stopped).
- It's possible that the power supply to the motor is weak at certain moments (for example, if another large motor is simultaneously started with this motor).
BTW, who is the manufacturer of the motor? Does the motor have the original winding of the stator and rotor bars, or has it been repaired? Copper bars on the rotor provide a smaller starting torque than aluminum ones.
ACW

 

[electricpete]

There was lots of good suggestions in this thread. For example the last two talked about low or unbalanced voltage can certainly reduce starting torque (and of course single open phase can result in no starting torque). And problems in rotor cage can be intermittent. (typically I'd check rotor condition by current signature analysis under load).

This problem is that the rotational acceleration torque is TOO LARGE for "DOL starting" compared to Reactor starting or Soft-Starter starting, so the starting time is too short (acceleration rapidly), so smooth rotational acceleration is not possible, and there may be (slight) jamming in the driven machine(load machine).

In some cases, it is judged that the Max.LRC (310A) is exceeded, causing a restriction in motor rotation and a loud noise.
During DOL startup, the acceleration torque, which is the difference between the motor T-N Curve and the load T-N Curve, is sufficient and even too large, so the startup time is too short during normal startup without any problems, and in case of jamming, there is a problem of locked motor rotation & abnormal big sound. I think it is.

I think the solution is

1) How to reduce sudden acceleration by designing LRT/FLT= to be less than 85% to reduce motor T-N performance and ultimately increase startup time by reducing motor acceleration torque.

2) How to reduce sudden acceleration by reducing motor torque and increasing start-up time by applying reactor start-up or soft-starter start-up method.

I've got to admit I've never heard of anything like that. But it's worth trying to think through because you are a lot closer to the situation than any of us. In most trains, torque doesn't cause any radial force. But in a gear, torque can translate to radial loading of the pinions which in theory can increase the static friction in the bearings during start (and higher torque could mean higher static friction). And of course, since your compressor is stepping up the speed in the direction from motor to compressor, that means it is stepping up the torque in the direction from compressor to motor, so changes in static friction of the compressor / pinion can have a bigger effect on the motor side.

Then you have 5 radial pads which adds a complication to the static friction picture. I can imagine for example if the starting torque transmitted from bull to pinion creates a radial force on the pinion pushing in a direction between two pads, that might also increase startup friction. Do you by any chance have sketch or description showing relative positions of bull and 2 pinions and the pads on the pinions.

Also I think Jeff mentioned axial loading. It would be interesting to know more about the motor bearing configuration. I assume they are rolling bearings (since you mentioned grease) but is there any external endplay for the motor? What type of coupling?

Also I wanted to clarify - was the shaft visually confirmed to be stationary during any of these failed starts? Or is it just a supposition.

 

[TTojiiLee]

Hi, zlatkodo,

Your suggestions is good & very helpful (depending on the positions in which the rotor stopped).
I can't say the motor manufacturers in this thread and the motor have the original winding of the stator and rotor bars.
I'll check whether it is Copper bar or an Aluminum bar.

Hi, electricpete,
We are keeping all possibilities open at MOTOR & COMPRESSOR and checking them and eliminated one by one now.
The the single helical gear loading(radial/axial load) & static friction effect is good suggestion & I'll sketch the bearing pads positions.
I'm checking the Your radial loading and Jeff's axial loading, motor & compressor bearing configuration & motor end play.
The coupling is flexible disc coupling.
I could not see the starting problem situation visually, but based on our engineer said and I checked all compressor logging data for confirming motor locked
(oil supply pressure change , vibration change,coast down time & vib. change, current, power on/off time, etc at Normal starting conditions vs Abnormal starting conditions, Comparison).

Thanks again, All !

by TT

 

[edison123]

Check for cracked disc(s). Most likely cause for this abnormal sound.

Muthu

http://www.edison.co.in