How significant a role does a stabilizer play in starting a compressor?

[Aluminiumni]

How significant a role would a stabilizer play in starting a compressor?

We plan to buy a stabilizer for our portable ACs (mains voltage drops to 170 V at times, and also to convert 220 to 110). I would like to figure out what capacity/technology to buy, especially considering how much it might help our invertor generator to run them.

Our Generator has an economy, load dependent rpm mode, however, the serviceman said it has a max rating of 1.8 kW (full throttle mode rating: 5.5 kW). I know that starting a single 1421 W AC in economy mode sort of seems out of the question, but what if the stabilizer is sized to run all three of the (same) ACs on the mains -- could it (a certain technology and/or one further sized up) actually accomplish this?

Then there is also concern about the event of two ACs starting together in full throttle mode and whether the stabilizer can help with that. Lastly, more out of curiosity, the prospect of running all three on the generator (taking into account the event of all three starting together).

 

[schmitt trigger]

By stabilizer I believe you are talking about an AC voltage regulator? Correct?
Also that you are planing to run some air conditioner units from small portable gasoline generator? Correct?

So...if I'm understanding your questions correctly:

There are two types of AC voltage regulators...ferroresonant and autotransformer tap changer....... Stay away from the ferroresonant. It has a hard limit on the output current and WILL NOT start an air conditioner compressor. Also its regulation is highly dependent on frequency stability, something that portable generators are not very good at.

Starting two or more compressor loads from a small generator is not possible. For a couple of seconds, it draws what is called LRAs (locked rotor Amps) which is anywhere from three to five times the running amps. It is better if you build some sort of sequencer, which would prevent an AC unit attempting to start up in the event that other unit is doing the same.

And to your question...no, a stabilizer would not help, it would even the problem worse. If you exceed the generator's current overload ratings, its output voltage will drop. The stabilizer will attempt to correct for this by changing a tap, which has the effect of increasing the current load that the generator sees. The generator will either stall or its breaker will trip.

EDIT: I don't understand your generator's "economy, load dependent mode"...never seen that feature, please elaborate.

 

[dick_freebird]

It's most important to have a properly working unloader
valve, so the compressor doesn't start into a pre-existing
pressure head at stall. That's your breaker-tripping
current condition. You can add volume to the unloader-
relieved plumbing to allow the compressor to spin up
more fully before current (which follows back pressure)
comes up.

A voltage stabilizer attached to a generator will just
demand more current to make flat voltage, if input
voltage sags. And voltage sag probably means you
already ran out of current capacity. Tweaking the
generator control might be better for sag, provided
that you don't push it to instability.

Me, I think I'd look at plumbing first, then some logical
prevention of coincident starting. There's no good
reason I could see, why start could not be staggered
by a second or two; it's not like pressure switches are
that fast, accurate or tight.

 

[Aluminiumni]

Wow its been about week, I didn't even thank you guys. At the time I looked up hard start kits, and was caught up in the matter of getting started with a converter/stabilizer or two (still ain't running the ACs).

Relating to hard start kits: not much information on the net for Pakistan, though people install them here (use the term kickstart (capicitor) I think) (primarily or maybe even pretty much exclusively) for old/weak ACs. That's really strange considering that we get 8-14 hours load shedding and a whole lot of people use generators and UPSes. Inverter HVAC equipment is imported, yet (despite the concern of lacking the edge/competitiveness in this particular area) I heard one local manufacturer, Dawlence has come out with an inverter split AC. Regarding innovation, one manufacturer actually introduced **broken link removed**. I think the cooling loss estimation (normal fridges) might be when power only comes 1 hr at a time (every other hour) and (max) temp 40, 45 C.

Right now I'm wondering about a variable frequency drive -- not a motor installation, just for delivering 60 Hz instead of 50 Hz, and some seconds or so of storage for starting current. I'm surprised this stuff wasn't mentioned: I didn't know about maintaining V/f. Still thanks very much for the detailed info.

- - - Updated - - -

EDIT: I don't understand your generator's "economy, load dependent mode"...never seen that feature, please elaborate.

I found a nice article (e-mail answer) that gives us an example and information about generators and starting ACs. Now I'm not completely sure about the difference in max output in economy mode and full throttle mode for our generator.

[...]

Here's what I measure on a nice cool 70 deg night:

volts 121
Amps 11.5
KVA 1.4
KW 1.3 [wow, quite close to our ACs ]
PF .94
Inrush 63 amps

[...]

Now let's contemplate what these numbers tell us. If RUNNING the AC
was the only concern, a 1500 watt generator would be more than
enough. However, that's not the issue. In order to RUN the AC, I
have to START it. The issue is whether the generator can supply the
inrush KVAs required to start the compressor. From the above, 121
volts * 63 amps = 7623 volt-amps or 7.6 KVA. (This is worst case
since in reality, the line - even shore power - will sag a bit under
inrush conditions resulting in fewer KVA being required but worst
case is best in these analyses.)

[...]

Apparently it would take an 8 KVA generator to start my AC. That
can't be true since it runs just fine on my Onan 3kw AJ generator.
How? Each generator has a surge rating that is much higher than its
continuous rating. For conventional generators, the instantaneous
surge rating is determined by how much iron and copper there is in
the stator, how strong the field is and how much flywheel effect
there is on the rotating parts. Heavier = better!! After the first
few cycles of inrush, the motor starting capability depends in large
measure on how fast the governor reacts. For inverter generators
such as the Honda EU series, the main limiting factor is how much
current the inverter can supply before going into current-limit mode
and secondarily on how fast the engine can spool up to feed the
inverter. A TYPICAL inrush capacity is 3 times the continuous
rating.

Taking that number, my 3 KVA onan should be able to supply 3 * 3 = 9
KVA surge. Since it easily starts my AC, the evidence is that it
indeed can supply that much current. One can observe the governor
slam the throttle wide open almost instantly when the AC kicks on,
followed by tapering back to about half throttle to run.

This analysis has been pretty straightforward to this point. The
complication arises in that not all generators can supply 3X
starting current. Lightweight and/or cheap generators usually
can't. Iron and copper and rotating mass are minimized to reduce
cost and weight and it kills the inrush capability. For example, I
have a 4 KVA yamaha generator that will not start my 2 HP portable
air compressor even though the inrush is less than 3X the
generator's nominal rating. Just can't supply the umph to make it
happen!

[...]

https://yarchive.net/car/rv/generator_sizing.html

 

[schmitt trigger]

I feel your predicament. I've read that among large cities, Lahore is one of the world's hottest.

And I've also read that Pakistan is running short of electricity and blackouts are common.

So, from this and your post, I would recommend the following:

1) Use a good generator, like the analysis you've included. Even with a 100% efficient stabilizer, the starting energy has to come from the generator. Large flywheel inertia and low generator impedance are the key. There is no way around this.

2) Use the stabilizer for your EXCLUSIVELY for your TV or other electronics. The stabilizer, will have its own losses, and worst, when the input voltage drops, it attempts to compensate and the input current increases, further exacerbating generator losses due to its impedance.

3) If there is anything you can do, is to correct the power factor to be as close to unity as possible. Things are tights as they are, and you don't want the additional losses created by VARs.

 

[Aluminiumni]

I feel your predicament. I've read that among large cities, Lahore is one of the world's hottest.

And I've also read that Pakistan is running short of electricity and blackouts are common.

Feels good to know people are aware of such circumstances being present (also in other countries to whatever extent). Makes me wonder why people buy pure electric cars (except of course, people on the verge of going totally off-grid). I feel investing in basic things like solar cookers and solar (water or home) heaters would be so much more fruitful.

) Use a good generator, like the analysis you've included. Even with a 100% efficient stabilizer, the starting energy has to come from the generator. Large flywheel inertia and low generator impedance are the key. There is no way around this.

Actually, I heard one UPS manufacturer here say that the capacitors in a VFD could take care of the starting concern. I might possibly start a thread about frequency converters, and whether they can (easily?) be expanded into a UPS, maybe charge pump for charging the battery, and having the ability to connect to solar.

2) Use the stabilizer for your EXCLUSIVELY for your TV or other electronics. The stabilizer, will have its own losses, and worst, when the input voltage drops, it attempts to compensate and the input current increases, further exacerbating generator losses due to its impedance.

Hey that's interesting.

1. No-stabilizer voltage sag has to be compensated.

2. But the sagging is compounded due to increased current draw, and all of it has to be compensated.

3. And now, considering stabilizer losses, further compounded sagging has to be compensated.

I guess it would be sort of a non-issue if the frequency converter might be handling most of the starting. There's also the prospect of the the frequency converter having its own step down. In that case I'd have to consider not buying a stabilizer for pure mains use.

3) If there is anything you can do, is to correct the power factor to be as close to unity as possible. Things are tights as they are, and you don't want the additional losses created by VARs.

The thing on my mind is the PF of frequency converters. We still have to make an initial search to see what's actually available. I guess that, if we happen to find a versed FC manufacture willing to do a custom design, I might start the thread I mentioned above and also ask concerning improving the PF.

 

[schmitt trigger]

Actually, I heard one UPS manufacturer here say that the capacitors in a VFD could take care of the starting concern. I might possibly start a thread about frequency converters, and whether they can (easily?) be expanded into a UPS, maybe charge pump for charging the battery, and having the ability to connect to solar.


The thing on my mind is the PF of frequency converters. We still have to make an initial search to see what's actually available. I guess that, if we happen to find a versed FC manufacture willing to do a custom design, I might start the thread I mentioned above and also ask concerning improving the PF.

After I had posted my reply, I realized that probably a better advice from me to you would have been to get yourself an inverter-type AC. From what I've read, they are VFDs that soft-start the compressor, thus avoiding the large inrush currents, and the better ones are also power factor corrected to better than 96%.......Well, that is, if you have not bought your AC already.

 

[Aluminiumni]

[...]Well, that is, if you have not bought your AC already.

Yup, already had them (though boxed) when I started this thread. One factor was that we heard that dual hose portable ACs work better than splits in extremely hot weather.

Isn't really the whole/real picture, but I didn't really get to investigating the efficiency of inverter ACs. I just, sort of, did a confirmation that 9.5 EER might be good. I searched for high EERs and among small units got EER above 9.5, 10 rarely. I saw that a few actually achieved EER around 14 by combining an evaporative cooler. I started entering numbers like 13, and after coming up with no results for room sized ACs (I don't remember clearly) concluded that 9-11 was near the maximum for room sized ACs.

I guess I just didn't run high enough to enter into extremely high inverter effeciency models. Actually, maybe its just hard to search for EER. Searching for '"inverter AC" EER' right now, I'm just getting average results (the text below the links) for 7 pages, except for only one exception on page 3.

I think I might ask about a small. high efficiency inverter model, maybe even 500 W, on another forum. One cool thing would be to use it with a UPS all the time. Two options for the night I guess (other than unscheduled outages):

1. No AC used in the outage. A good sort of buffering set up to even out the temperature/fan cooling effect.

2. Small inverter AC also runs in the outage.

3000 VA Stabilizer Capacity Halving at 110 V Concern:

After a break of rainy and cloudy days, we finally got a stabilizer with adjustable voltage, however, its running way too hot. According to the inbuilt Ammeter it should be able to handle the load with real ease (8*220 < 3000), but what if its facing this load at half capacity (same thickness wiring for 110 V)? This would mean that it might be running at slight over capacity.

Its designed to do this for maybe several minutes, which would explain why it hasn't broken down yet, but also why it accumulates way too much heat. For the sake of maybe getting a vague idea, here is the overloading specification of a VFD:

Over-load capacity
150% rated current 1 minute, 180% rated current 10 seconds

I don't even have a problem in accepting that a 1500 VA stabilizer could start our AC without difficulty. I think the compressor pretty much ran all the time. If there was an outage or we manually turned it off, I think it might have gotten time to normalize, and just didn't face multiple seconds of inrush. Oh, and 50 Hz would make starting easier I guess. One poster at hvac-talk.com wrote:

Regarding starting a compressor, while the inrush current seen at startup may be high, it's very brief. Most digital ammeters can't pick it up fast enough.

We're thinking of replacing the 3 kVA model and getting three 5 kVA ones. Still it would be satisfying to confirm whether certain Stabilizer designs use the same wire thickness for 110 V giving halved capacity.

Oh, I just found this in an add, would like to know if its correct:

DUTY CYCLE
Conventional type stabilizers are suitable for 30-40% load on continuous duty cycle. Our automatic voltage stabilizers are Heavy Duty & Industrial type and are suitable for 100% load continuous duty cycle. For running load of 200 KVA, generally 400 KVA / 500 KVA conventional type stabilizer is installed while [Power] make 200 / 250 KVA is suitable.

 

[FvM]

As far as I understand, you are still chasing after using voltage stabilizers with your ACs (either conventional motor or inverter based).

I expect that the point said about conventional AC compressors also applies for inverter based: a voltage stabilizer doesn't help to run them from a generator.

 

[Aluminiumni]

As far as I understand, you are still chasing after using voltage stabilizers with your ACs (either conventional motor or inverter based).

I expect that the point said about conventional AC compressors also applies for inverter based: a voltage stabilizer doesn't help to run them from a generator.

Hey thanks, I just realized the part with the heading should be a new thread. i couldn't easily find topics about how hot a stabilizers run or the matter of capacity halving at 110, so a separate thread might be useful for others too.

Could a moderator allow this please? I'll wait for approval -- I'd like to post the thread myself with some modifications, and I could edit the post above too.