Motor Failure - Reciprocating compressor on VFD
Motor Failure - Reciprocating compressor on VFD
(OP)
Does anyone have good reference material on application of VFD on reciprocating compressor loads.
Existing facility experienced a motor insulation failure on a 150 HP 480 VAC compressor. It is not clear how much engineering was applied to the initial conversion of the compressor to VFD control, so I am trying to familiarize myself with the various considerations, to confirm whether improper controls/configuration could have contributed to the failure.
Existing facility experienced a motor insulation failure on a 150 HP 480 VAC compressor. It is not clear how much engineering was applied to the initial conversion of the compressor to VFD control, so I am trying to familiarize myself with the various considerations, to confirm whether improper controls/configuration could have contributed to the failure.





RE: Motor Failure - Reciprocating compressor on VFD
RE: Motor Failure - Reciprocating compressor on VFD
RE: Motor Failure - Reciprocating compressor on VFD
nhcf; VFDs put large voltage transients onto motor windings (often thousands a second) if this was "a conversion" it could be the motor's insulation system was not up to par for VFD duty - a common occurrence.
Where exactly did the insulation fail?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor Failure - Reciprocating compressor on VFD
We are still attempting to gather information on the application. The motor failure was determined to be insulation failure, although the motor was inverter-duty. There are other potential issues associated with the installation, including length of motor leads/non-vfd cable that have likely contributed. I have not applied VFD to recip load before and am reading posts suggesting mechanical issues with running at reduced speeds on VFD (lubrication, vibration) and I am trying to determine if the recip load could contribute to failure.
RE: Motor Failure - Reciprocating compressor on VFD
Don't often hear of VFD's on recip compressors. Maybe there's a reason for that?
RE: Motor Failure - Reciprocating compressor on VFD
Motor winding insulation damage is absolutely a potential issue when using VFDs, especially on older motors made with 1000V insulation. bearing EDM damage is another one, plus all the other things ScottyUK mentioned. Without the big incentive of energy savings that are not going to be true on a recip, there is little justification for using it at all. Now that the motor must be changed, it might be a good time to investigate why the VFD was used in the first place and if it was because of a perceived energy savings scheme, this might be the time to challenge the assertion.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Motor Failure - Reciprocating compressor on VFD
RE: Motor Failure - Reciprocating compressor on VFD
As Scotty says you will lose the flywheel effect and get large torque pulsations.
The torque pulsations will be accompanied by current pulsations.
The I2R heating will probably drive the RMS loading past the safe motor loading point.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Motor Failure - Reciprocating compressor on VFD
RE: Motor Failure - Reciprocating compressor on VFD
We have installed lotsa ASDs on recip compressors over the last several years. This was done as much for process reasons (ie turn-down) as for the ability to start and accelerate the compressor on weak power systems. These compressors have varied in size from a few hundred hp to 7000hp. Recently we have implemented synch-transfer on many of the larger units. For MV applications (ie 4000V thru 6600V motors) we generally use API-541 motors, although sometimes we can get away with a API-547 motor. For LV applications (ie 460V or 575V motors) we always use a IEEE-841 motor.
In most circumstances, the motor is started with the compressor unloaded.
The key to having a successful installation is getting the right motor.
I have a few questions wrt your problem;
1) What type of motor (ie is it a IEEE-841)?
2) If not an IEEE-841, is the motor Inverter-duty rated?
3) Is the motor designed for a constant-torque application over the desired down-down ratio?
4) Is the motor fitted with an external blower fan? This is almost always provided on a Constant Torque application where the turn-down is > 2:1.
5) Who spec'd and supplied the motor (ie was it the compressor OEM)?
6) What is make / model of the motor?
7) What is the length of the motor feeder cable?
8) Was the drive fitted with an output dv/dt filter?
GG
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: Motor Failure - Reciprocating compressor on VFD
I would be expecting the system to experience surging that would be overloading the motor during each compression stroke when running at a reduced speed, due to the energy storage of the flywheel depending on rpm^2. I suppose you could design the compressor to operate over a speed range with an oversized flywheel? It really doesn't sound like a great application to just throw a VFD onto.
RE: Motor Failure - Reciprocating compressor on VFD
RE: Motor Failure - Reciprocating compressor on VFD
- The failed motor was WEG, 'crusher-duty' motor, inverter duty but I don't see IEEE 841 referenced. The compressor mfr/oem was not involved in the VFD integration. VFD is an AB Powerflex 400, lead lengths are approx. 200 ft, single conductors in conduit. No load side filters.
My sense is this is a misapplication of the VFD control, but still chasing some other aspects of the install.
RE: Motor Failure - Reciprocating compressor on VFD
nhcf: WEG is the quality of motor that you get when you don't spec something better (ie Toshiba, Siemens, Baldor).
A couple of things to watch for;
1) Turn down ratio (if > than 2:1, you will require an external blower motor)
2) Output dv/dt filters; (always a good idea on any drive, regardless of the cable length)
3) Special drive cables (waste of money if you have dv/dt filters)
4) Ensure that the motor OEM is aware of the application (ie constant-torque recip-compressor, turn-down ratio, operating speed range, type of drive, etc)
GG
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: Motor Failure - Reciprocating compressor on VFD
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Motor Failure - Reciprocating compressor on VFD
Often the difference of a hundred RPM can preclude all start-stops of the compressor (short-cycling). This is way better for the motor than the torque ripple inherent in a few hundred RPM off of nominal.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor Failure - Reciprocating compressor on VFD
RE: Motor Failure - Reciprocating compressor on VFD
Also, see my last post....
RE: Motor Failure - Reciprocating compressor on VFD
The following flow-chart is an example of the decision making process that one of our Clients will use to determine the selection process between a API-541 and API-547 motor. [ BTW this Client has hundreds of compressors installed, and installs at least a dozen (or more) per year. All are on ASDs. Most are API-541 motors. ]
As you can well imagine, with this flow-chart, not many motors end up being a API-547 motor. Recently they have veered away from this flow-chart if there is a 100% spare installed.
The API-541 motor is considerably more costly than a API-547 motor, more so depending upon the level of testing required.
GG
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: Motor Failure - Reciprocating compressor on VFD
Myself, I would prefer to unload the compressor rather than start and stop it or run it slow.
With a VFD in place, you can start unloaded with very little stress on the motor and then run the compressor at full speed.
Basically an expensive soft starter.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Motor Failure - Reciprocating compressor on VFD
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Motor Failure - Reciprocating compressor on VFD
RE: Motor Failure - Reciprocating compressor on VFD
Perhaps the OP could give us the complete catalog number of the ASD as well as the full-load NP current for the 150hp 460V motor.
Also: How old is this installation? Was the ASD installed as an after-thought?
GG
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: Motor Failure - Reciprocating compressor on VFD
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Motor Failure - Reciprocating compressor on VFD
- Don't have the exact VFD model number - but it is 150 HP (i.e. it is not oversized in attempt to deal with Constant Torque load)
- Motor nameplate FLA = 170A
The compressor history is interesting -
- compressor 30+ years old, original mfr no longer in business. nameplate says its rated for 500 rpm with approx. 120 hP mech load.
- at some point in order to increase capacity, owner modified the sheaves to increase speed to 590 rpm. mechanical evaluation was performed by 3rd party. no changes made to motor.
- at some more recent point, the VFD was added - normal operation 60 - 100% speed, based on the new, 590 rpm operating speed.
- the motor that recently failed was not the original motor, but still not clear how long it had been in service.
Seems likely that the original change to the system should have changed the motor hp. Then the VFD potentially made the situation worse.
RE: Motor Failure - Reciprocating compressor on VFD
BTW the failed motor was likely too small for this application when the ASD was added and the sheave sizes were changed. The motor should have been replaced with a 200hp NP rated motor. (ie 120 brake hp x 590/500) = 142 brake hp). Allowing for some losses thru the belt-drive system ,etc and you are almost certainly > 150hp.
Having a too small ASD likely exasperated the problem.
Please let us know what you final findings are.
GG
"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)
RE: Motor Failure - Reciprocating compressor on VFD