Motor Failure - Reciprocating compressor on VFD 1

[nhcf]

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.

 

[waross]

Valid points Keith.
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

 

[jraef]

Side issue: the PowerFlex 400 drive is an HVAC drive, they are only rated for Variable Torque loads, which under no circumstances includes reciprocal compressors. VT rated means the drive can only handle OLs of 110% for 30 seconds, 150% for only 2 seconds. So what will happen is that in attempting to deal with the current surges in a compressor, the drive will either trip off line or someone got around that my programming its Current Limit feature to 110%. But the Current Limit feature on that drive will artificially override the speed command in order to keep current from exceeding its rating. So the motor may have been subjected to having it's speed curtailed for longer than it was intended to in a compressor application because lowering the speed of a compressor means it will take longer to re-charge a tank than normal. That then may have exposed it to repeated and long term thermal stress, because often times a compressor motor is sized based on a specific duty cycle, including rest periods, factored into the CFM rating they give you, but assuming full speed. lowering the speed means it may never get a rest and if the motor was not over sized for that duty cycle, may explain the failures. This is all just more evidence that someone was likely thinking of this as an energy saving move, which was n erroneous concept from it's inception.


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

 

[moltenmetal]

GroovyGuy- not saying it can't be done with proper engineering or that it's always wrong- but as you know, there's more to it than just motor selection! Of course if you can use a screw compressor, that gets rid of many of these problems.

 

[GroovyGuy]

jraef: You are correct. This ASD is designed for variable-torque loads only (apparently). With most ASD OEMS, they give you the option of derating any ASD from a VT application down to a CT application. I glanced thru the PowerFlex 400 manual, and they don't mention his ever. I wonder if this is a marketing ploy, as the PowwerFLex 400 is touted as a low-cost ASD. In my mind you should be able to use any ASD for CT applications as long as it has the required current rating. It sounds like the incorrect ASD might have been provided here, but it is difficult to say if this was a significant factor in the motor failure.

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)

 

[crshears]

With the right unloader valves and control system installed a recip compressor is capable of stepped rates of compressed air production [from full capacity right down to zero] while running at near-constant full speed, therefore without any adverse effect on either flywheel fan cooling or lubrication problems. I'm not saying I'd dismiss the use of VFD/ASD drives from recip duty outright, but I'd certainly give it a long and searching look, perhaps just applying some power factor correction if required.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[nhcf]

Thanks for all of the good information and experiences.

- 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.

 

[GroovyGuy]

Hi nhcf,
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)

 

[nhcf]

Followup to an earlier question - the VFD is AB 22C-D208A103 drive. Nameplate 150 HP, 208 continuous amps, 228.8 60-second ovld amps.