Large Induction Motor Experience 3

[KB3HHC]

I'm looking for any experience feedback or comments on the use of induction motors in the range of 10,000 hp - 25,000 hp, at any speed. I'm not concerned about starting methods, etc, since we can design the system to adequately take care of starting. I'm more concerned about machine issues such as reliability, failure modes that might be more pronounced in this size range, operating issues, etc. Any personal experience feedback in this regard, or pointers to studies or surveys would be greatly appreciated. I have reference lists from several suppliers, but am interested in the user's perspective.

Thanks

 

[oldfieldguy]

KB3HHC-

I've some experience in caring for motors in your size range. I find no particular accelerated incidence of failure related to size.

You may want to consider on-line diagnostic equipment such as offered by Iris or others.

Obviously, protection of your investment is a consideration, and I'd recommend good protective relaying with advanced capabilities, i.e., GE UR or SR series, or Schweitzer. I'm familiar with those.

You will also want to include a very comprehensive regimen of inspection and testing during periodic maintenance. Since these things aren't portable, in situ preventive maintenance is an issue.

Actually, the only real problems I've had are related to starting issues: utility voltage sags due to offsite switching issues, incomplete cycle on synchronous motors due to operational issues, etc.

old field guy

 

[arcflash99]

We use IRIS PD monitoring on a 13.8 KV 12,000 HP induction motor and have been sucessful in validating the health of the motor. We were able to monitor the increase in PD and made the decision to have it pulled during our last outage. As it turns out, the coils were moving in the slots, some coils were raised in the slots, some blocking had fallen out of the ends of the coils and the insulation was showing signs of burning. If not for this condition based system, we would not have known when was the right time to spend our money to send out the motor.

I do know that motors of this size do not like to be started very often. The physical movement of the coils and the stresses on the rotor take their toll.

 

[KB3HHC]

Thanks to old field guy and arcflash99 for this helpful info. arcflash - about how long has that motor been in service?

 

[jraef]

Too bad electricpete has apparently not seen this thread, he has a lot of experience in large rotating machine reliability.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[Laplacian]

This is a lot to consider for a machine this size.

You really need to prepare a good specification document because there are some areas that manufacturers can take shortcuts.

What is your application? Is this a compressor train that runs 10 years without shutting down, or an extruder that starts and stops many times per week? What is your reliability target for this machine; how long can you be down for repair until major production losses occur?

Incorporate any commonalities into this machine that may exist in a future spare machine such as shaft height, terminal box location, etc. Search surplusrecord.com to see what's available now to get an idea.

Consider installation and removal of the motor at its installed location for repairs. If possible, leave room or provisions for heavy lift equipment (cranes, etc.) to be sited near the motor for repair/replacement.

If you don't have access to API-541 (American Petroleum Institute guideline for large induction motors), I would recommend getting it as a reference and potentially a specification basis.

If this machine is very critical to operations, I would recommend heat run and winding immersion tests; both witnessed. Expect the stator to fail the immersion test at least once and incorporate this (rewinding or building a new stator) into the delivery schedule with your internal procurment/project department.

These motors are pretty low maintenance, but good online monitoring is important. Some important things to monitor are internal air temperature, winding temperature, bearing temperature, vibration, current signature analysis, and partial discharge (especially above 10kV).

Protection is also important, but this also depends on what operational constraints you have. In some processes, the motor is sacrificial, most others it is not.

If there is a lot of fines in the area (powder, dust, dirt, etc.) then consider getting a TEWAC or TEAAC enclosure. We have had very good results with these enclosures with reliability significantly better than WP-II style enclosures within our large motor population.

I'm not a motor expert, but we do have a fair number of very large machines in my area. Let me know if there is anything specific you would like to ask. Most of the information above is based on the assumption this will be a newly manufactured motor.

 

[itsmoked]

Laplacian.... nice rundown.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[arcflash99]

The motor is approx. 20 years old. It had been 10 years since it was last semt out for reconditioning. We had some reliability issues with the field wiring and motor controls as this job was put in on the cheap. We averaged 4-5 starts per year for the last few years. ~2600 amps for ~15 seconds.

We have had sucess indentifying motor issues with CSA. We use a tester from a company called PdMA.

 

[edison123]

Laplacian

Good post indeed deserving a star.

What is a "winding immersion test" ?

 

[itsmoked]

I was wondering too, sounds um... ugly.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Laplacian]

The winding immersion test is where full voltage is applied to the stator alone while either submerged or sprayed with water. Manufacturers don't particularly care for this test and sometimes take exception to it, or stipulate that the customer pays for additional windings if required. It also forces a higher level of attention to detail and quality control during manufacture.

 

[edison123]

Thx Laplacian.

I can understand OEMs' taking objection to this rather stringent test. The IEEE/IEC standards I use (for rewinds) don't call for such a test.

Unless it is a submersible motor (which is invariably LV and low HP), I don't see the point of such a test.

Do you happen to have a reference standard for this test ?

 

[electricpete]

NEMA MG-1 is the reference standard for the submersion test. It is not a required test but the details of the test are there for the user to specify at his option.

While the test can be a little intimidating for the supplier, it can also be a little intimidating for the buyer. I personally would be nervious about putting a "brand new" critical motor into my plant which had already been operated flooded. My preference would be to test take some sample coils and clamp steel plates to the slot sections to simulate the actual motor. Then put those sample coils in the tank at the same time as the actual stator. Then do sumbersion test on the coils. If they fail, the stator has to be rewound (condition of the contract).

It may not be quite as thorough as testing the whole motor since you don't have any connections which may be a weak point in the insulation. But at least you don't have any concern about lingering damage or contaminants which may exist from the test.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Here is an excerpt from NEMA MG-1 (2003), Section 20 (Large Motors):

20.18 MACHINE WITH SEALED WINDINGS—CONFORMANCE TESTS
An alternating-current squirrel-cage machine with sealed windings shall be capable of passing the following tests:
20.18.1 Test for Stator Which Can Be Submerged
After the stator winding is completed, join all leads together leaving enough length to avoid creepage to terminals and perform the following tests in the sequence indicated:
a. The sealed stator shall be tested while all insulated parts are submerged in a tank of water containing a wetting agent. The wetting agent shall be non-ionic and shall be added in a proportion sufficient to reduce the surface tension of water to a value of 31 dyn/cm (3.1 microN/m) or less at 25°C.
b. Using 500 volts direct-current, take a 10-minute insulation resistance measurement. The insulation resistance value shall not be less than the minimum recommended in IEEE Std 43.
(Insulation resistance in megohms ? machine rated kilovolts plus 1.)
c. Subject the winding to a 60-hertz high-potential test of 1.15 times the rated line-to-line rms voltage for 1 minute. Water must be at ground potential during this test.
d. Using 500 volts direct-current, take a 1 minute insulation resistance measurement. The insulation
resistance value shall be not less than the minimum recommended in IEEE Std 43. (Insulation
resistance in megohms ? machine rated kilovolts plus 1.)
e. Remove winding from water, rinse if necessary, dry, and apply other tests as may be required.
20.18.2 Test for Stator Which Cannot Be Submerged
When the wound stator, because of its size or for some other reason, cannot be submerged, the tests
shall be performed as follows:
a. Spray windings thoroughly for one-half hour with water containing a wetting agent. The wetting
agent shall be non-ionic and shall be added in a proportion sufficient to reduce the surface tension
of water to a value of 31 dyn/cm (3.1µN/m) or less at 25°C.
b. Using 500 volts direct-current, take a 10-minute insulation resistance measurement. The
insulation resistance value shall not be less than the minimum recommended in IEEE Std 43.
(Insulation resistance in megohms > machine rated kilovolts plus 1.)
c. Subject the winding to a 60-hertz high-potential test of 1.15 times the rated line-to-line rms voltage
for 1 minute.
d. Using 500 volts direct-current, take a 1-minute insulation resistance measurement. The insulation
resistance value shall be not less than the minimum recommended in IEEE Std 43. (Insulation
resistance in megohms ? machine rated kilovolts plus 1.)
e. Rinse winding if necessary, dry, and apply other tests as may be required.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

The purpose of the test is supposed to be to verify that you have a very moisture resistant winding. I know one nuke plant that requires that test for all their outdoor critical open (WP2) motors.


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Interesting excerpt above - they mention minimum insulation resistance of kv+1... that was removed from IEEE43 a long time ago. Now it's at least 100 megaohms temperature corrected 1-minute I.R. and we expect much higher.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[sms]

I have cared for a number of motors of this size, and I am a rotating machinery guy, not electrical. I will bow to Pete's opinion on the electrical. Mechanically, these motors tend to be journal bearing machines which are pretty reliable, but motor frames are getting pretty flimsy which means on a 2 pole machine, especially on 60 Hz power you are very likely going to have a motor that runs above the first critical. In the old days this would not be the case, but today it is almost guarenteed that a new 3600 rpm motor above about 5000 Hp is going to be running above the first critical. As a result you need to include transient (runup/rundown) vibration in your shop acceptance testing, with a requirement of 15% seperation margin between the critical and running speed, as well as limiting the amplification factor to less than 4 or 5.

You also need to specify XY proximity probes, and you need to carfully consider the coupling type, including prestretch if using a discpack style. The flimsy frame also means that it is important to do a good job of designing the foundation and sole plates, and installing the motor. A twisted motor frame will deform the stater assemble, screw up air gaps, resulting in a motor that runs rough.

Oil seals on the bearing housing are also important, but most big motors use a Renk bearing assembly and oil leakage into the motor housing is not much of a problem. Motor cooling however can be, I was involved with a 10,000 Hp air cooled motor that had a tendancy to suck in rain water, resulting in a explosion as it shorted phase to phase while being started during a rain storm.

My two cents for what it is worth....

-The future's so bright I gotta wear shades!

 

[oftenlost]

Amen to those who advocate proper base security. I had a 4 pole 19500 HP unit mounted on a substantial concrete foundation erected on a concrete floor over a swampy condition occasioned by a water pipe leak. The motor, when running, caused the floor to sink .050" or more and the hydraulic pressure during shutdown periods lifting the motor and base assembly back to approximately original alignment readings.