Turn Down VFD Frequency to Hermetic Scroll Compressors to Reduce Load Slightly? 1

[JohnMcNutt]

Hi all, thanks for the forum. I have been reading them for many years but this is my first post.

The facility in question has a two stage heat pump unit. The power to the site is single phase. When they installed it they installed a VFD to power it, but whoever spec'ed the VFD forgot to take into account the derating of the DC power supply of the unit for single phase input.

The fault code it throws is for excessive DC bus ripple which makes me think that it is an insufficient capacitor and/or rectifiers in the DC power supply to smooth out the single phase input. It is an A-B Powerflex 400 with a rating of around 60a output, 240v. A-B gave the OK to enter some magic code to disable the fault code but as a consequence they require that the output overcurrent be set to around 42A. Trouble is, this unit draws something like 45A when fully loaded on a hot day.

I want to know if there would be a problem in reducing the frequency of the drive to get the current below the threshold. I am not sure how the compressors would react to this. They are hermetic scroll units. The indoor motor is a simple induction motor, belt drive. All else is single phase and not driven by the VFD.

A better solution would probably be to upgrade the power supply on the VFD but I am not sure if this would be feasible, but I am willing to look into it.

I have other plans to drop the second stage temporarily to prevent overload but I won't bore you with the details of that. I would just like to keep both stages running as much as possible.

Thank you very much.

 

[jraef]

Yes, you should be able to accomplish this, in fact I believe you can set up the VFD to do it automatically (I'll have to check) by setting a current limit that is lower than the motor FLA, and then also setting the OL response in A182 to "Reduce PWM". that way it should automatically reduce the output frequency to whatever will allow the current to remain below the current limit setting. I cannot however address how the compressor will react to having the speed reduced, that is a question for the compressor mfr.




"Will work for salami"

 

[MikeHalloran]

Another possibility you might explore is to pipe the indoor unit's condensate drain so the condensate dribbles onto the outdoor unit's condensing coil fins. This may be possible to do with gravity alone, depending on the relative elevations of the components.

Cheap window units do something similar, arranging things so the external fan blades dip into a pool of condensate and throw it up toward the condensing coils.

It should reduce the peak current in hot weather. ... at least until it corrodes away a substantial portion of the condensing coil's fins, so there may be reduced life or warranty issues with the unit's manufacturer.





Mike Halloran
Pembroke Pines, FL, USA

 

[JohnMcNutt]

Thanks for the ideas. I have the manual for the VFD here and I see what you mean on "Reduce PWM". However, I don't see where there is a setting for what it is going to reduce it to. I also don't see what "Reduce Clim" (the other option) applies to. The default is to reduce both but IIRC I had set the minimum and maximum to 60 Hz both because I couldn't see at the time why it would need to be otherwise. Perhaps if I drop the main setting for minimum speed to something less than 60 Hz then the "Reduce PWM" setting will become operative?

The unit is a combined CU and air handler so I would need a condensate pump to put condnsate on the condensor, and it would probably not work real well anyway because it takes several minutes for condensate to build up to such a level to be any use in the first place, and the main problem is with initial cooling of the space. Besides out here in the West we don't have much humidity to begin with.

 

[ScottyUK]

You can likely increase the bus capacitance using external caps - most drives bring out the internal DC link to terminals. This does need to be done cautiously because it may affect other functions such as soft-charge to limit the inrush current at power on, and will increase the stress on the rectifiers as the peak current increases: there is a limit to how much capacitance can be added before it causes problems, but in practice you can add a fair bit. Rockwell won't sanction this obviously, but it may offer a way forward depending on how severe the undersizing problem is.

 

[ozmosis]

If the compressor is a Danfoss maneurop scroll compressor, they are typically good between 30hz and 90hz. Obviously starting needs to be unloaded, as I expect it is now.
I seem to recall a line reactor is advised on the PF400 if operating on a 1ph supply. This may reduce a level of DC link ripple. Hovever, jraef is the man for all things PF these days..

 

[jraef]

No requirement for a reactor on the PF400 for single phase input, but it would not hurt. This drive also has the ability to put in a DC bus inductor, there are terminals for it and instructions for connecting it, that would help even more. The only thing different about the PF400 with regards to single phase input is that the power de-rate is 65% (reduced TO 35%) with single phase input, unlike the other PF families that are de-rated by 50%. So of all the drives to have made this mistake on, this was the worst one. What this means is that your 65A drive is officially only capable of running a 23A motor, so about 7.5HP at 230V. But the factory guy you spoke with must be taking the conservative gloves off in allowing you to load it to 42A.

The way to set the Current Limit is with parameter 179. Set that to 42A, then with the other setting in 182, if the motor attempts to draw more than 42A, the drive will automatically override your commanded speed setting and lower the speed, thus shedding load. Shedding load is the only way to reduce current, because current is load is current in an AC motor. It means of course that you will not be getting full capacity from your compressor, but them's the breaks.

"Reduce Clim" in the programming for 182 would be for when you want to lower the Current Limit to whatever it takes to avoid an overload. In this case, you already know the specific Current Limit you can live with and you will want to take as much of that as possible. So don't use the "Reduce Clim" option in this case.




"Will work for salami"

 

[MikeHalloran]

One tactic that can help the condenser work better even in arid areas is to shade the condenser from the sun, e.g. with a shed roof, or tall shrubs near enough to put it in shadow and distant enough to still allow service access and airflow.



Mike Halloran
Pembroke Pines, FL, USA

 

[JohnMcNutt]

Thanks for all the help. I have been mulling this over the last few days. It sounds like the frequency reduction below 60 Hz thing is exactly what I need but I am still a little concerned about it because the manual doesn't explain it very clearly. Jraef, can you tell me for sure this is what "reduce PWM" does? The manual makes reference to PWM carrier frequency but it doesn't clearly state whether the line frequency (motor speed) will drop when carrier frequency drops. Can you confirm this for sure?

 

[ozmosis]

Reducing the PWM carrier frequency will have no impact on the output frequency, it is quite separate. Leave the carrier frequency at default.
I don't know how you want to reduce the output frequency/speed but there will be numerous methods of doing this from the keypad buttons, potentiometer, analogue input etc.
I've got to ask the question, and its not intended to be as pointed as it sounds but why have you got a VFD on this compressor if you were not varying the speed?

 

[jraef]

Huh, posts from my iPad are not showing up...

Ozmosis covered what I was going to say about the carrier frequency though, it's totally unrelated. It has to do with the audible noise that the motor makes.

The PWM they are referring to is the output frequency, i.e. the motor speed.

Patrick,
Most likely the VFD was originally there as a phase converter. Unlike other parts of the world where single phase power is generally derived from a 3 phase sorce that is available right at the power pole, our utilities will run ONLY a single phase service to many neighborhoods where there are only small installations. The cost to upgrade to 3 phase is usually prohibitive because they will make you pay for the installation of the new transformers and wires, etc. So many people have discovered that using a VFD as a phase converter, even though they don't change the speed, is a simple and efficient way to take care of it if theere is only a motor involved.


"Will work for salami"

 

[JohnMcNutt]

OK, that covers the speed reuction portion of the problem. I wanted to be double sure because the manual is vague about what "Reduce PWM" actually means.

Part 2 of the problem is to disconnect the second stage compressor if the load is too high for too long. I plan to use the T055=6 "Above Freq" relay output to drive a timer that will disconnect the second stage if the freq has to drop below a determined level for a period of time and then wait 20 minutes or so before bringing it back online. I don't think the time delays related to this parameter can be made to work because it is really a "below freq" signal that I need.