Technical Merits of Running AC Compressor at 208 vs 230 volts 2

[JohnMcNutt]

I have what I think is a rather unique application. A VFD running a Copeland scroll compressor, 5 tons, three phase, 208/230 dual voltage. The VFD is oversized, but running from a single phase supply, so I am curious if changing the output voltage to the compressor can vary in any meaningful way the extra load placed by the rectifiers and capacitors in the power supply from running in this configuration. Or, will the power output be so functionally equivalent that the input will not be much affected?

 

[itsmoked]

I would do everything possible for the compressor's benefit not the VFD's. You really want to send that compressor 230 if at all possible. The higher the voltage the lower the current thru the windings and the cooler the motor will run, and the cooler the VFD will run. Both those devices get heated by the current running thru them not the voltage. Both their lifetimes are functions of their operating temperatures.

If you only have sucky 208 available. You're stuck, unless, you use a boost transformer which I would do in a N.Y. minute.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Oh, to directly answer the question:

Technical Merits of Running AC Compressor at 208 vs 230 volts

There are only de-merits for running at 208.

That compressor is made for 230V. They re-label it stating that it can also run at 208V which in reality is the standard lower limit of where most 230V motors can barely get away with running. This puts the motor right at its thermal limit. (Low voltage=>high current=>high temperature). Then a hot day, large load, and low voltage from the power company,(which always follows the first two), shoves the motor over the edge of the thermal cliff.

Again a motor is usually rated 230 +/-10% but a motor labeled 230/208 is really a motor that's 208 +20%/-0% which is not labeled in the same 'spirit' as a 230V.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Hi Keith.
No question that as the voltage drops the HP that can be developed safely drops. However my understanding of dual rated motors is that the motor is basically rated at 200 Volts or 208 volts. However the motor is designed and built with enough iron so that it will run on 230 Volts without danger of saturating.
Another way of looking at it is that a 5 HP motor is designed to produce 6 or 7 HP at 230 Volts. At 208 Volts the motor will still have the capacity to develop 5 HP without overheating.
The voltage rating limits will be 208V-10%:230V+10%.
Dual voltage rating has become common for refrigeration equipment, given the millions or brazillions of commercial units running at 208 Volts in malls and shopping centers across North America.
One other merit worth mentioning is the refrigeration efficiency of hermetic and semi hermetic units.
Motor cooling in hermetic and semi hermetic compressors is effected by the returning refrigerant vapour. The hotter the motor is running the more superheat is added to the returning vapour. This drops the refrigeration efficiency somewhat. Lower efficiency means higher operating costs.
Another good reason to run at 230 Volts rather than 208 Volts given the choice.
Your single phase supply will probably be 240 Volts. 230 Volt motors are generally applied on 240 Volt systems.
A question for jraef: Is it safe to program the VFD for 240 Volt output?

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

 

[jraef]

Bingo, beat me to it. If it is a single phase only supply, it will be 240V (nominal). For the reasons already given by Waross and itsmoked, you want to program the VFD to deliver 230V (not 240 by the way, because motors are designed per Utilization Voltage standards, which will be 230V). Dual Voltahe rated motors like that are actually a compromise using a 220V design (with extra iron as mentioned), because the over/under ratings at 220V cover a little more of the possible low-end of a 208V system, acknowledging that 208V systems are generally smaller facilities with less voltage drop internally, so -5% from 208V is more likely than -10%. But that is moot here, you have 240V. Use 230V to your motor.

There is a good discussion of this in a set of white papers called "The Cowern Papers" written by an Engineer at Baldor Motors. It was written prior to Baldor being bought by ABB and maybe because his copyright didn't extend to them, they are no longer provided though the Baldor website. But if you do a web search for "Cowern Papers" there are still several large distributors that have them on their websites. In one of those, Bill Cowern explains why 208/230 V motors are fine at (as I recall) 10HP and below, but above that it is much more important to stay true to the actual nameplate voltage.

"Will work for (the memory of) salami"

 

[jraef]

Oops, I meant Edward Cowern...

http://www.woltersmotors.com/pdf/cowern papers.pdf

The part I referred to above is on page 54, and yes, it was 10HP and under. He doesn't say that the design compromise was to use 220V, but I heard that from him in person years ago at an EASA meeting.

"Will work for (the memory of) salami"

 

[waross]

Cowern papers;

http://www.baldor.com/pdf/manuals/PR2525.pdf

Still on the site, but almost impossible to find if you don't have it bookmarked.
See page 53 and following pages.

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

 

[itsmoked]

Hi Bill.
I'm not sure I can believe the 200V motor theory... That's probably how they SHOULD be made or maybe some are but the number of 208/230V refrigeration compressor motors I've seen failed simply because they were running consistently on 207, 206, 205,..200V empirically demonstrates to me that these motors couldn't have been 200V units.

Great. Having both species exist out there reminds me of the General Motors ignition switch debacle currently in play. (Bad switches and redesigned good switches both existing with the same part number.)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

May I add a few grains of confusion? OK, here:

There are a couple of other things to consider, too. The slip and the Power factor - or rather the magnetizing current. Depending on which one is the worse 'problem', one may chose to run at a higher or lower voltage.

Slip is never good. If the motor is running at reduced voltage, you will have increased slip and the rotor will run hotter. Motor current may be lower and sometimes the two effects compensate each other. It depends a lot on what rotor type you have. NEMA D design is the worst in this case, but I do not think that you have one of those.

Running at lower voltage decreases magnetizing current. That decreases total motor current and is generally a good thing, if the increased slip can be tolerated. Even if the motor is a broad voltage motor (more iron than normally needed) the magnetizing current increases when motor voltage is high. High magnetizing current = low Power Factor.

Mix that with the other facts presented in this thread and you will probably not be any wiser. The problem usually is that not many of the factors that govern the outcome of the evaluation are very well known. Most judgements are therefore based on experience, a few facts and sometimes some prejudice.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[JohnMcNutt]

This has been very informative for me. I think my course of action will be to do nothing and leave it at 230v.

But I would like to learn a little more if I can.

I'm not sure I can believe the 200V motor theory... That's probably how they SHOULD be made or maybe some are but the number of 208/230V refrigeration compressor motors I've seen failed simply because they were running consistently on 207, 206, 205,..200V empirically demonstrates to me that these motors couldn't have been 200V units.

This unit has a minimum stated voltage of 187. It seems all of the ones I have seen do. But I have not seen them all.

I suppose you would predict that if the units you mention were run at 188 volts they would die an untimely death even sooner.

My gut feeling has always been to keep the voltage on the upper side of the scale.

 

[LionelHutz]

I'd suggest you try a lower voltage and see what happens. On a number of occasions I have seen the motor current go down with a lower voltage programmed into the VFD despite what the theories about what should happen.