4160VAC Sync Motor and Physics

[mkees]

I have 2 4160VAC 4500hp Sync motors that power reciprocating compressors. My issue is starting the electric motors, this is very inconsistent sometimes they start and sync, and sometimes they don't sync. I am trying to make the issue with management that the reason they are inconsistent is because the motors are tying to start with the compressor loaded with anywhere from 650 to 900 psi pressure. Management says that because the pressure on the compressor is equal on both sides of the compressor (suction and discharge), and a bypass valve is open tying both sides together that this is the same as having zero pressure on the compressor. I know this is wrong, because when I had to work on the motor I couldn't rotate it if there was any pressure on the compressor. If I unloaded the compressor the motor would spin freely. What I'm asking for is a basic law of physics that describes what I'm trying to prove, yes I could do the homework myself, but I'm hoping someone knows off the top of their head. Thanks

 

[GroovyGuy]

Thanx Lionel for the info,
My only experience with synch-motors has been GE & ABB units > 14,000hp. These motors were always brushless and always equipped with speed-switches. I guess that the smaller motors do not use speed-switches to initiate field application. How do these smaller brushless motors determine when to apply the field, (ie a timer, motor current)?

I wonder if the motor is not synchronizing due to a combination of too weak a field at a time when the stator voltage is also low? This might explain why sometimes the motor will synch, and sometimes it fails to synch.

Regards,
GG

 

[electricpete]

1 - The effective area on both sides is the same and there is a piston rod on one side.

The presence of rod on one side of piston suggests to me unequal pressure-bearing area by an amount pi*Rrod^2 where Rrod is rod radius. That rod-diameter circle is not exposed to system pressure on the rod side (the rod extends through the seal out to a non-pressurized area) but is exposed to system pressure on the non-rod side. Right?


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(2B)+(2B)' ?

 

[mkees]

Electricpete you are correct about the connecting rod.

Waross, please realize, I am a instrumentation and controls guy, I studied motors and wrote a final paper about them for my degree years ago. What I've seen since has been 480 3 phase with a VFD. So please don't assume that I'm using the correct terminology. We checked that (I had to look up the purpose of the amoetisseur winding) winding since the new sync controls installation.

Lionelhutz, the unloader has always been the same. If the compressor and the bypass are not returned to their starting positions/conditions the unit will not be available to start when requested.

Grooveyguy, the exciter field is energized when the motor contactor is energized. The rotating assembly determines when to apply to the motor rotor, it is rotational speed and I think frequency of the slips.

 

[electricpete]

ok, unequal area on piston means static pressure on both sides creates a force on each piston in the direction toward the connecting rod. What does that tell us…

I think in terms of torque on crankshaft it's a wash. If I try to rotate a given direction, half the pistons would be moving toward the rod end and half away from the rod end. Static pressure force toward rod is going to assist my efforts on half the pistons and opposie my effort on the other half of the pistons. Crankshaft torque effect cancels.

But that static pressure force also puts some static radial load on the crankshaft bearings (every piston is loading crankshaft bearing in same direction, no cancellation here). Friction torque in the bearing is as a first approximation proportional to radial load. For anti-frictions it's not a big effect but for sleeve bearings the static friction (breakaway torque) can be fairly high and radial load can affect it dramatically (the load helps force the oil film out of the bottom clearance while idle). Other variables that can enter into this scenario include the oil temperature and duration of shutdown.

In other words a sleeve bearing with high radial load (potentially due to static pressure) may squeeze oil the oil out of the bottom requiring very high breakaway torque during subsequent attempt to rotate or start. Especially when shutdown/stationary for a long time or under hot conditions (low viscosity)


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(2B)+(2B)' ?

 

[waross]

Pete.
It used to start and sync. They changed the control package and now it starts but won't always sync. As I understand, no changes were made to the unloader system which worked for years.

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

 

[waross]

"The rotating assembly determines when to apply to the motor rotor, it is rotational speed and I think frequency of the slips."
Was the rotating assembly changed? If so then this is where the problem may be. A new assembly may be looking for a lower slip frequency before closing compared to the old assembly.

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

 

[mkees]

There is a new rotating assembly, and the slip frequency is lower. I spoke to the manufacturer to see if the is an adjustment for the slip frequency, there's not. The question remains do I need to reduce the pressure in the compressors? Do you all think just adjusting the field current may be my best bet?

 

[waross]

It's a 4500 he motor. I am amazed that it can be turned by hand under any circumstances
Mkees;If the rotating module will not turn on the amperage is not important.
You may try explaining to the manufacturer that the module is not serviceable as is. They may be able to supply a modified version.
As I see it our options are a module that connects the field at a higher slip frequency or lower motor speed.
Or getting the motor to turn faster in induction mode.
My strong feeling is that an electrical solution may be easier than a mechanical solution.

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

 

[electricpete]

"by hand" was used interchangeably with "manual" and not intended to preclude use of strap wrenches or even cheater bars. Motors uncoupled can usually be rotated fairly easily but I'm sure it's a lot harder when coupled to that big recip compressor.


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(2B)+(2B)' ?

 

[LionelHutz]

OK, now the story is clear. You changed the rotating controls which set when the motor synchronizes and now it won't synchronize. This could be due to 2 different reasons.

The motor never reaches a high enough speed for the rotating controller to apply the field. In this case, you either need to get Hyundai to help fix the problem, likely by increasing the slip frequency, or you need to reduce the load on the compressor.

The other issue could be that the motor attempts to synchronize but fails. In this case, you need to apply more current so the motor can develop the proper pull-in torque. Personally, I would be applying full rated current to the pilot winding until after the motor is synchronized.

See how describing the details of what was done makes a big difference. The Basler parts go external to the motor so they have no control over the field application timing. Saying "Hyundai Ideal Digital Synchronizer System" didn't really help because a Google search turns up no useful information. I can find a site that hints about Hyundai providing the external electronics for a motor with power factor and current regulation but nothing specific. I could assume from this that the Hyundai part is some external thing. Except now that you have posted you upgraded the rotating rectifier I bet this is the brushless exciter mounted on the end of the motor. Right?

I misspoke before because I believed you had only replaced the controls external to the motor and you can't set the slip frequency on a brushless motor in the external controls.

 

[GroovyGuy]

Mkees,
Check out item # 29 in the attached link.

http://www.kinetics-industries.com/papers/QA-Kinetics-Excitation-030510.pdf

"Wish I didn't know now what I didn't know then." -- Bob Seger

 

[mkees]

Thanks everyone, for your help I will try all the suggestions. This has been a learning experience as I hoped it would be. Watch in the future I will keep you posted when I find the result.

 

[LionelHutz]

I know that information is copied, but there is almost no way you can attempt to start the motor with the field open and not cause damage to the field. The rotor will produce many 1000's of volts when open and this will cause the field to flash over at the weakest insulation spot.