Synchronous compressor motor fails to get up to speed 1

[oftenlost]

Surplus 250 HP, Westinghouse synchronous, 514 RPM, 440 volts, 261 AMPS FLA. Procured and inspected by service shop, good megger, surge test, rotor drop test and amortisseur appears to be original. Lead markings, 2 each of 1,2 and 3. XLE compressor with rotor on compressor crank
Installed and start attempted. Original GE 14 ohm resistor across rings. Motor accelerates to 250 to 260 RPM and hangs there. Start current 600 AMPS. Compressor unloader checked OK. Start resistor taken up in steps 18, 21 and then 26 with no apparent effect on speed. No arcing at rings or amortisseur. Talked to used motor vendor who indicated that he removed the motor personally from an idled plant site and had used the compressor for other purposes.

 

[itsmoked]

Interesting that the unit hangs at 1/2 speed.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[oftenlost]

My thoughts exactly. Too slow to be where I would expect overload and if the motor had been rewound for 550 volts, I would have expected less LRA. Torque does not appear to be the problem as changing the resistor should have had some effect. Could be rewound for different poles but why on a synchronous? Induced voltage across the rings on start operate about as expected with 360 volts with 21 ohm resistor and 390 VAC with 26 ohms.

 

[DaveScott]

oftenlost, I know very little about synchronous machines, except basic theory. Please excuse my comments if they are obvious to you: sometimes an innocent question can form a chain of thought.
I read a little and noticed that, during starting, the texts mention that the field winding is not energised, allowing the amortisseur winding to bring the speed up to about 97% and then finally the field winding is energised to lock in.
Since during starting, the motor is acting as an induction machine, the information seems to point a finger at either a) the number of poles has been changed, or b) stator connection mis-wiring.

My reference:

http://www.kilowattclassroom.com/Archive/SyncMotors.pdf

I hope this helps.

 

[jraef]

The resistor has nothing to do with starting torque on a synchronous machine. It is there to prevent a buildup of field voltage that can happen when starting (hence the name "Field Discharge Resistor"). You are probably mixing it up with a different type of slip ring motor, the Wound Rotor Induction Motor, where the resistor has everything to do with the starting torque. Don't deviate from the original resistor rating or you may cause damage.

When starting, a Synchronous motor uses the amortisseur winding, which acts exactly the same as an Induction motor. It should take the motor to about 97% of full speed by itself. Only THEN is the Field Discharge Resistor taken out of the circuit and the Filed is applied to lock the rotor into synchronous speed.

So DaveScott's a) or b) are the most likely culprits in your problem. It almost seems as though you have the windings connected in Wye when they should be in Delta. A motor wound for Delta and connected in Wye will give you 33% torque and 33% starting current. So if normal stating current was 600%, Wye starting current would be 200%, so 261A x 2 = 522, getting close to 600A.

If you have the original Westinghouse serial numbers and nameplate info, you might try calling Teco-Westinghouse Motor Co. in Round Rock, Texas and see if anyone there can dig up the original drawings and specs on it. They will likely charge you for them, but that would be a good thing to have.

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

 

[oftenlost]

Thank you for your inputs. If I knew what was wrong, I would have fixed it so I am searching for inspiration.
I agree with most of what you have said, and some synchronous machines seem to have the start resistor value set up strictly to keep induced voltage below a safe level during start but much has been written of the selection of FDR for torque optimization. A couple of links included below. I do not believe that this is the case here as the resistor has most effect near synchronous speed and this machine only gets up to roughly 50%. The motor is hooked up 1-1 to L1, 2-2 to L2 and 3-3 to L3 so unless there was some internal misconnection, I do not see a Y here.

http://www.geindustrial.com/publibrary/checkout/38652.30055.22278.50388/PDF/spm-a7_3.pdf

http://findarticles.com/p/articles/mi_qa3726/is_199711/ai_n8771142

 

[edison123]

With a synchronous machine, a mismatch between the stator and rotor poles wouldn't cut it. Did you physically check whether the no. of poles match the speed ?

 

[oldfieldguy]

I'd think you want to take a good look at the amortisseur winding. It provides the starting torque, acting as a squirrel-cage winding. High impedance due to old connections will limit the current it can develop, and therefore, the torque. "No arcing" doesn't necessarily equate to the needed low impedance.

old field guy

 

[DaveScott]

Hey, it's rotating the right way, isn't it?

 

[rbulsara]

Has anyone ever witnessed this motor working correctly?

Even surplus or used equipment should be witness tested before finalizing the purchase.

 

[LionelHutz]

600A is the current when it reaches 260rpm? I would expect to see between 500A and 900A when the motor reaches full speed before the DC is applied.

Measure the frequency across the field resistor if you want to know how close to full speed it is.

 

[jraef]

oftenlost,
Please don't take offense, we are trying to help. But PLEASE get off the idea that there is a "start resistor", there is not. Nothing in either of your posted articles disputes that fact either. Don't confuse "pull-in" with starting when discussing synchronous motors, they are not the same.

Moving on, keep in mind there IS a solution, we just need to find it. So step one, we need to more accurately define the problem. There are really only 4 possible things (3 probable) that can affect motor speed, which by a process of elimination we can narrow down to one in which to spend our time.

1) The line frequency is less than what you expect. I think we can eliminate that one straight away.

2) The number of motor poles is greater than what you expect, i.e. sometime in the past the motor was rewound, but someone changed the speed and failed to notate that on the nameplate. Or there is a legitimate connection issue at hand, someone relabeled the wire leads without drawings to know what they are, or something like that.

3) The output torque is less than what you expect, i.e. a voltage, connection or winding damage problem.

4) The load is greater than what you expect, i.e. the unloader is not working as expected. Another possibility that you were not 100% clear on is whether or not THIS motor was connected to THIS compressor. For now let's continue to assume so.

Here is how I would approach it. Starting from the bottom, you said earlier that "the unloader checked OK." Start here because this is the easiest to double or triple check and is necessary for the next step. Some issues I have come across with unloaders on compressors before:
Did you assume the valve was working because you heard the solenoid click? That doesn't always mean the plunger or diapragm moved! Could there be an obstruction? Corrosion? Can you remove the unloader and bench test it? Can you just remove it and try restarting, open to atmosphere?

If the unloader is assured to be working, next is to see if the motor speed problem is related to torque loss or not. Disable the Field Application Relay so that you cannot pull in for this test. We need to apply a load to this motor and one way to do that is to load it artificially. Before moving on with this, make sure there are no other potentially damaging consequences of doing that. If so, another possibility is to rig up a way to partially load the compressor by using a manual valve added into the unloader system somehow. Either way, please keep in mind this needs to be a very short test. Start it again and get it to where it stops accelerating as before. Then manually load the compressor. If the motor stalls or significantly slows down further, then the problem is related to the amortisseur windings not developing enough torque.

If it is a problem with the number of poles, it should remain at the same relative speed, (plus or minus 5% or so) but current should jump a lot.

Once you narrow the problem down to one of these 3 issues, we can help you further delve into the possible causes behind them.

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

 

[edison123]

One way to check whether this is a torque issue is to decouple the compressor from the motor and run the motor in open shaft mode. If the speed is still the same as before, then the name-plate speed (or wherever you got the rated speed from)is wrong.

In a synchronous machine, you cannot just rewind for another speed without physically changing the poles, which is next to impossible given the cost.

 

[waross]

Did I understand you that the rotor is mounted on the compressor shaft? I would check the number of poles on both the rotor and the stator. You may have the wrong rotor installed.
respectfully

 

[oftenlost]

jraef,
No offence taken. I appreciate all the input.
Waross,
This is a Ingersol XLE compressor where the rotor and stator are mounted on the compressor. There is no rotor shaft. The compressor shop claims to have removed the compressor and motor as a unit.
Edison,
Cannot uncouple as rotor is mounted on compressor shaft.

A little more background.
This motor was procured by a used equipment vendor in the midwest of US from a compressor shop on the East Coast and shipped to California. My only involvement was to have my technicians perform a quick check as a courtesy to the compressor shop. The motor was installed and a controls technician was engaged to set it up to run. He seemed familiar with syncronous starters. He measured rotor speed with a tachometer and in response to an earlier question, the compressor had previously been driven by a GE motor and the starter was the original utilized by the Ge motor as was the resistor assembly. The test was performed by my lead tech, 44 years in same shop and I only visually inspected the amortisseur which appeared intact and original. We did not check the stator connection but did drop test the fields and checked phase balance and performed a surge comparison test on the stator. I do not know the skill level of the plant electrician and as they are 2500 miles away, my only contact has been by phone and email.
The motor never gets up to speed so the FA relay never pulls in and the FD resistor stays in. The start cycle has been restricted to 30 seconds to protect the squirrel cage.
I will talk to my contacts at TECO to see if a data sheet is available. Old Westinghouse data on synchronous and large DC machines has not been easily obtained as I have had much trouble getting quotations for TECO replacements based on Westinghouse model numbers. Might be able to get phase resistances. In any event, DLRO readings between the leads might tell something about the connection.I do not know whether the local electrician is astute enough to get coil groupings but we could try if the air baffles, if installed, permit. I seem to remember that the unloaders on the XLE actually vent to the atmosphere and can be heard but it has been many years since I worked on such a system. Anyone familiar? I will talk to the California concern about loading the machine during the 30 second start run. I do not know if the machine can load up quickly enough to permit speed checking in the limited time the motor can be held at lock rotor. One question about LRA. In my experience, most synchronous machines run LRAs between 3X and 4X FLA so it would appear that locked rotor should be closer to 800 than 600. Any thoughts?

 

[jraef]

Ding ding ding!
You said I-R XLE! Sorry, I totally blitzed that until just now!

Do these look familiar?

I did a line up of 6 of these in So. California a few years back. The guy in the background is the technician I had working with me, somewhere around day 3 of this fiasco. He is standing in front of the synch motor control cabinet trying to figure out why we were not getting the starters to accelerate the motors to synch up speed. Does his puzzled look seem familiar? Two very difficult and FRUSTRATING lessons we learned that week:

1. There is nothing on the compressor or the motor that indicates the proper direction of rotation.
2. When an XLE is rotating backwards, it will not get to full speed!

Something about the way the unloader works makes it partially compress on each rotation. We could hear the unloader express air, so we thought it was unloading, but it was not fully unloading, the motor was not fully accelerating, the rest was EXACTLY as you are describing. We changes the rotation and it came right up!

BTW, props to member DaveScott who asked that question back on the 8th.

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

 

[jraef]

Damn, wish I could edit after posting. Trusted my spell checker but got too excited to proof read.

Changed, not changes in that next to last sentence.

 

[edison123]

jraef

I learnt soemthing today. A star for you.

Why don't these things have the direction of rotation clearly marked on the motor / compressor ?

 

[jraef]

Got me? I remember thinking at the time that this was very idiotic. I just happened to notice that fact one day and asked the plant electrician how he knew which was the right direction. He said he didn't! He hooked it up and if it compressed, he assumed it was correct. The compression he was seeing however was apparently that quirk of the unloader.

All of these compressors have arrows on them NOW!

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

 

[edison123]

Looking at your picture again, I see that even with a DOR mark, it is impossible to see the DOR since the whole unit is sealed.

May be they should have these phase-sequence relays that are fitted with the hoists which wouldn't allow the motor to start until it is the right sequence.

(Damn, I wish I could edit the eng-tips post - my pet peeve. Previous post - it is "something").