3MW, 11kV Motor No Load Trail

[krisys]

We have to commission a Crude Oil Pump which will be driven by a 3000 kW, 11kV Motor. The motor is having an Ex "d" enclosure suitable for Hazardous area. Physically the motor is very large due to the Ex "d" enclosure.

The subject motor is totally enclosed, fan cooled (Air cooled type IEC IC-511). But the bearings are forced oil lubricated. The lube oil pump is electric motor driven and is common for pump and motor bearing lubrication.

Now, the motor is ready for No-Load Trail, but the Lube oil pump is not yet ready. How to take the motor No-Load trail?

The vendor says, the three (3) hour No-Load trail (de-coupled from the pump) is not necessary. Only the kick start to check the direction of rotation is sufficient.

Any experience to share or any views?

I personally feel that the motor no load trail for three (3) hours shall be performed before giving clearance for the coupling.

 

[edison123]

Why do you want a 3-hour no-load trial run?

Why not wait for the lube oil pump to be ready?

Muthu

http://www.edison.co.in

 

[jraef]

Seriously! Rotation check bump, ok maybe. Run for 3 hours with no lube? Are you crazy or just a gluten for punishment?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ScottyUK]

Given the size of the machine and fact that it is using forced oil lubrication suggests it is using white metal bearings. A non-lubricated run, even a bump test, could wreck the bearings. That machine is a very high value piece of plant, why risk damaging it for the sake of a test which isn't even required?

 

[waross]

We can almost guarantee that the motor will fail the test.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wayne440]

...the motor is ready for No-Load Trail, but the Lube oil pump is not yet ready...

With all due respect, your reasoning appears to be fundamentally flawed. For all practical purposes, the motor is not "ready" until the associated lube oil system is functional.

 

[krisys]

All the above feedbacks are highly appreciated.

Now I am convinced that the No Load trail (for certain duration, if not three hours !!!)) is required. However, we should insist the Contractor to make the related lube oil system ready before the motor trial run.

 

[edison123]

"Now I am convinced that the No Load trail (for certain duration, if not three hours !!!)) is required."

How? All of the posters here are saying the opposite.

Muthu

http://www.edison.co.in

 

[krisys]

edison123 (Electrical),
Appreciated

No one has posted indicating that the No Load trail is not required. Everyone has the concern on the readiness of Lube Oil system.

Regarding the duration of No Load Trial people have differing view. However, the duration of No Load Trial is left to the users. In my region, the established practice is to run the motor for at least three (3) hours. The significance of three (3) hours is to ensure that the motor is reached to its steady state temperature. Most of the motors have heating time constant of less than three hours. Also the original bearing problem (persisting, if any) will also be revealed. Thus the motor can be confidently handed over to mechanical for coupling.

Please note that the initial no load trail will be a good reference document for the future.

So to clarify, Now I am convinced that the bump start and the No Load trail is required. However, only after the completion of lube oil system for the motor.

 

[LionelHutz]

But the motor won't reach any meaningful steady-state temperature without a load.

 

[krisys]

The idea is to isolate the motor problems from the mechanical related problems, misalignment, vibrations etc. in case it happens after coupling during the load trial.

 

[edison123]

Misalignment comes after you couple the load.

As for motor uncoupled vibrations, bearing temperatures etc. didn't the supplier give you a factory test certificate? This is part of standards requirement.

Muthu

http://www.edison.co.in

 

[electricpete]

3 hours may not be unreasonable for no-load test. It is a fairly common requirement to run the machine until stable temperatures are reached. Stable can be tough to quantify, but 2 or maybe 3 hours is a reasonable (conservative) duration imo. Alternatively, you can pick a degree per hour limit to define stabilization. And note the temperatures of interest during no-load run are primarily the bearing temperatures (which can show their problems when the machine is not under load).

The test when finally coupled up is of course more representative, but in some circumstances it's far better to find any possible problem on the motor before that test. Of course, you've got a vendor more familiar who's saying the 3-hour run is not required, they may be aware of the followup testing and overall project risks and better placed to make a judgement. On the other hand, if this is something that is builit into specification and the project is already accepted at fixed cost, the vendor may be influenced by reducing his own cost.


=====================================
(2B)+(2B)' ?

 

[krisys]

Muthu,

The motor was delivered about two (2) years ago and was lying in the yard. But space heaters were kept ON. The motor was also covered with tarpaulin sheet.

Nevertheless, who can predict the internal condition of the motor. Only the no load trial can reveal something, if there is anything drastically wrong.

 

[edison123]

krisys

Ah, two years in the yard means no OEM warranty any more? Was it preserved and protected well in the yard? If not, I suggest you consult with OEM how to go about ensuring the motor is ok.

Muthu

http://www.edison.co.in

 

[krisys]

Yes Muthu, it is a valid point.

The motor was preserved reasonably. The vendor's representative has visited the yard couple of times and inspected and given his report as satisfactory.

Since it is part of the pump package, the warranty is part of the pump vendor and in turn the warranty is extended by the EPC Contractor. So the warranty should not be an issue here.

 

[krisys]

The motor vendor is supervising the commissioning of the motor. He has again cautioned the customer saying that; as a manufacturer they are not recommending the no-load trial. In spite of this, if the customer wants to run at no load, the supplier is not responsible for any damage to the motor.

Further I have checked in the motor manual and found that this motor is an "overcritical" machine. Thus it is not allowed to operate the motor without a counter torque. Because the machine can be damaged, when it is switched OFF and runs down without a counter torque.

Does any one in this forum has any experience on such motors?

 

[waross]

Make sure that all of your instructions are clearly stated in writing.
If the customer destroys this motor he will probably be looking for scape goats.
In your case I may be checking the job market just in case everything goes sideways.
A suggestion: extend the run-on time of the lube pump(s). That motor may coast a long time without a load. You want the lube pump to be running until the motor has stopped. Loss of power during the test or run-down may allow the bearings to run dry.
I don't envy your position. Good luck.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

While I don't see the point of running the motor unloaded, I also question the OEM's ridiculous claim that the motor cannot be run unloaded without a braking torque. As long as the motor is properly secured in its bed, has proper continuous lubrication and has proper ventilation (in case of external cooling), all the motors can be run for hours on no-load.

Edit:
One exception could be vertical motors, which may require a preload on the thrust bearings. But in most cases, the weight of the motor rotor is good enough of a preload.

Muthu

http://www.edison.co.in

 

[krisys]

waross, edison123,
Thanks for your inputs.

I did not understand the term "overcritical machine" clearly. Though this is something to do with the rotor dynamics, which is purely a mechanical subject by nature. However, while dealing with such a large sized motor, we need to know some basics of rotor dynamics and torsional analysis.
I guess the OEM's concern is something related to the mechanical issue.

Based on the study of rotor dynamics, the manufacturers identify the lower critical speed (LCS) and the higher critical speed (HCS). These are basically the mechanical resonance points and the rotor is vulnerable to damage at these speeds.
While switching on the motor it accelerates slowly, but at the lower critical speed (say at 75-85% of the rated speed) the acceleration is so high that it crosses the LCS without risking any damage. But if the motor is having high inertia, I guess, when the motor is switched off it takes a longer time to come to standstill.

While decelerating slowly, it will be spending relatively longer time at the LCS. At that time, the rotor is vulnerable to damage.

This is based on by basic understanding of rotor dynamics. Probably someone from the rotating equipment (Mechanical) side can give a better explanation.