Alternative Power sources - 4 ton AC, how big a generator?

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4 ton AC, how big a generator?

Robert Morein

2005-08-02, 10:21 pm

To reliably start a 4 ton unit, how large a generator is required?
Does anyone have any practical experience sizing this?

The specs for my unit are:
Compressor 23.1 amps, ampacity 30.4, breaker = 40 amps, locked rotor 105
amps, V=240

Blower 5.6 amps, 240 volts

GeekBoy

2005-08-02, 10:21 pm

"Robert Morein" <nowhere@nowhere.com> wrote in message
news:KZmdnci8Aq6Vk23fRVn-sg@giganews.com...
> To reliably start a 4 ton unit, how large a generator is required?
> Does anyone have any practical experience sizing this?
>
> The specs for my unit are:
> Compressor 23.1 amps, ampacity 30.4, breaker = 40 amps, locked rotor 105
> amps, V=240
>
> Blower 5.6 amps, 240 volts
>
>

Use the equation P.I.E.

(P)Power [Watts] = (I) Current [Amps] x (E) Energy [Volts]

To take into consideration of the 40 amp breaker

P = 40 x 240

P = 9600 Watts of generation to run the AC

Robert Morein

2005-08-03, 12:21 am

"GeekBoy" <nobody@spamcop.net> wrote in message
news:1123XXXX28.e46254fb2ffcfdb4843c287cb17355e9@teranews...
>
> "Robert Morein" <nowhere@nowhere.com> wrote in message
> news:KZmdnci8Aq6Vk23fRVn-sg@giganews.com...
>
> Use the equation P.I.E.
>
Actually, I know that

It doesn't take into account the starting surge.

GeekBoy

2005-08-03, 4:21 am

"Robert Morein" <nowhere@nowhere.com> wrote in message
news:2NKdnbM7Qfykq23fRVn-gw@giganews.com...
>
> "GeekBoy" <nobody@spamcop.net> wrote in message
> news:1123XXXX28.e46254fb2ffcfdb4843c287cb17355e9@teranews...
> Actually, I know that

> It doesn't take into account the starting surge.

But generators do. They are usually rated for the continous wattage.

>
>

Hank McCall

2005-08-03, 1:21 pm

GeekBoy wrote:
> "Robert Morein" <nowhere@nowhere.com> wrote in message
> news:2NKdnbM7Qfykq23fRVn-gw@giganews.com...
>
>
>
> But generators do. They are usually rated for the continous wattage.
>
>
>
>
The numbers say that the compressor takes a surge of 105 amps at startup
( could be up to 10 seconds) and runs on 23.1A. The blower needs 5.6A.
Guestimate a surge of 20A for a much shorter duration.
240*(105 + 20) = 30,000 watt surge at startup
240*(23.1 + 5.6) = 6,888 watt running power

Surge and continuous ratings are normally given for compressors

SQLit

2005-08-03, 2:21 pm

> It doesn't take into account the starting surge.

Your right about the starting surge. I had a 5 ton that nameplated at just
under ~30 amps. Actually it never drew over 24 amps but that is another
story. I never put a genny on it cause I needed at least a 25 kw to do the
job. I knew if I had the ac going I would also have everything else running
as well.

We do not know the type of starting your particular compressor has. IE
across the line or soft start. Assuming a standard reciprocal compressor I
would want a 25 kw to be reliable.
A soft start on the compressor would help the starting current, make it
lower.
Seek local manufactures advice for softstart applications.

Do some checking into the ratings and time factors of gennys. Most standby
sets are not rated for continuous duty nor are they rated for heavy surges.
If your still considering this I would look into a 1800 rpm water cooled
diesel with frequency controls. Stay away from the 3600 rpm " construction
" noise makers.
http://www.northerntool.com/webapp/...oreId=6970&N=36

Even if you got the starting curves from the manufacture your probably going
to find that your stuck up in the 20-25 kw range. Have you considered the
amount of fuel this is going to take?

We used to run chillers on generators at work. I took care of the Cat 3516's
and their 1800kw generators. We had 2 chillers and 2 generators. We each
generator could start a chiller by its self. But the cost in break downs was
terrible. So we started both generators synced them and then started one of
the chillers. We had controls so we could hold the compressor motor at just
under loading speed. Then we would start the second machine. Then release
one for loading and then the other. These girly girls used 112 gallons of
diesel at full load each.
Best of luck in your decision

Robert Morein

2005-08-03, 4:21 pm

"SQLit" <sqlit@qwest.net> wrote in message
news:Gm6Ie.15$5P1.637@news.uswest.net...
>
> "Robert Morein" <nowhere@nowhere.com> wrote in message
> news:2NKdnbM7Qfykq23fRVn-gw@giganews.com...
> 105
>
> Your right about the starting surge. I had a 5 ton that nameplated at just
> under ~30 amps. Actually it never drew over 24 amps but that is another
> story. I never put a genny on it cause I needed at least a 25 kw to do the
> job. I knew if I had the ac going I would also have everything else
running
> as well.
>
> We do not know the type of starting your particular compressor has. IE
> across the line or soft start. Assuming a standard reciprocal compressor I
> would want a 25 kw to be reliable.
> A soft start on the compressor would help the starting current, make it
> lower.
> Seek local manufactures advice for softstart applications.
>
> Do some checking into the ratings and time factors of gennys. Most standby
> sets are not rated for continuous duty nor are they rated for heavy
surges.
> If your still considering this I would look into a 1800 rpm water cooled
> diesel with frequency controls. Stay away from the 3600 rpm "
construction
> " noise makers.
>
http://www.northerntool.com/webapp/...oreId=6970&N=36
>
> Even if you got the starting curves from the manufacture your probably
going
> to find that your stuck up in the 20-25 kw range. Have you considered the
> amount of fuel this is going to take?
>
> We used to run chillers on generators at work. I took care of the Cat
3516's
> and their 1800kw generators. We had 2 chillers and 2 generators. We each
> generator could start a chiller by its self. But the cost in break downs
was
> terrible. So we started both generators synced them and then started one
of
> the chillers. We had controls so we could hold the compressor motor at
just
> under loading speed. Then we would start the second machine. Then release
> one for loading and then the other. These girly girls used 112 gallons of
> diesel at full load each.
> Best of luck in your decision
>
Thanks, that's exactly the advice I was looking for. Maybe a Generac, sized
by the manufacturer according to the installer's survey, would work.
Otherwise, it sounds like a diode-blowing endeavor.

GeekBoy

2005-08-04, 1:21 am

"Hank McCall" <h.mccall@comcast.net> wrote in message
news:rICdndkSF6Zud23fRVn-1w@comcast.com...
> GeekBoy wrote:
> The numbers say that the compressor takes a surge of 105 amps at startup
> ( could be up to 10 seconds) and runs on 23.1A. The blower needs 5.6A.
> Guestimate a surge of 20A for a much shorter duration.
> 240*(105 + 20) = 30,000 watt surge at startup
> 240*(23.1 + 5.6) = 6,888 watt running power
>
> Surge and continuous ratings are normally given for compressors

I really doubt that his 40 amp break would take 105 amps for 10 seconds.
I would feel fractions of a second would be more like it.

Robert Morein

2005-08-04, 1:21 am

"GeekBoy" <nobody@spamcop.net> wrote in message
news:1123125878.07bbeb00ac128af72f146f152c0e3c44@teranews...
>
> "Hank McCall" <h.mccall@comcast.net> wrote in message
> news:rICdndkSF6Zud23fRVn-1w@comcast.com...
>
>
> I really doubt that his 40 amp break would take 105 amps for 10 seconds.
> I would feel fractions of a second would be more like it.
>
You're probably right, because the rotor is never going to lock in a
properly operating system.
But there is the real possibility of damage to either the compressor or the
generator if not sized correctly.
The answer to this has to come from someone who has specific knowledge of
the issues involved.
For example, if the A/C is improperly cycled, by someone twisting the
thermostat, the compressor could freeze from hydrostatic lock. If the
generator is undersized, it could fail to supply enough current to trip the
breaker.
All kinds of design flaws appear when designs are stressed to the limit.
Yet the generator should not be larger than necessary, because that hurts
both economy and, for a diesel, health of the engine.

Tim

2005-08-05, 7:21 am

10KW of quality genny will just do it. We use a 3 ton on a trailer for spot
cooling with a Honda 10Kw. The Cheap 10KW we had wouldn't start it, just
trip it's breakers. The Honda starts 5 ton condensing units on new installs
regularly.
"Robert Morein" <nowhere@nowhere.com> wrote in message
news:KZmdnci8Aq6Vk23fRVn-sg@giganews.com...
> To reliably start a 4 ton unit, how large a generator is required?
> Does anyone have any practical experience sizing this?
>
> The specs for my unit are:
> Compressor 23.1 amps, ampacity 30.4, breaker = 40 amps, locked rotor 105
> amps, V=240
>
> Blower 5.6 amps, 240 volts
>
>

RF Dude

2005-08-07, 2:21 am

Check KOHLER. They have a whole house unit that is 12 kW (model 12RES) that
actually has that claim in it about starting a 4 tonne A/C. Of course, you
might not be able to have anything else connected. Kohler sells most of its
generators standard with a Permanent Magnet exciter. This helps the
alternator produce large currents for short periods of time.

BTW, all generators (and transformers) are rated in kVA, not kW. OK, in
single phase the machine is usually rated with 1.0 PF so kW and kVA is the
same. kW relates to engine horsepower. kVA is the alternator current
rating. That locked rotor Amps when starting a compressor is likely 0.2 PF
(very inductive) for a few cycles. I usually oversize my alternators since
it is not expensive to add copper. For instance, I'll put a 50 kVA
alternator on an engine that would normally come with a 40 kVA version for
this reason. But I also have to deal with other harmonic issues.

Good Luck.

Sylvan Butler

2005-08-07, 4:21 am

On Wed, 3 Aug 2005 20:25:15 -0700, GeekBoy <nobody@spamcop.net> wrote:
> I really doubt that his 40 amp break would take 105 amps for 10 seconds.

True.

> I would feel fractions of a second would be more like it.

Maybe. But a few seconds is more likely. Depends on the breaker
(somewhat the individual unit, but mostly the design specs).

My irrigation pump draws 100-110amps on start according to my peak
reading clamp-on meter. Usually only for 1-2 seconds. On a 30amp
breaker (stupid previous owner) it was fine 8 or 9 times out of 10. On
a 40amp it was fine nearly all of the time. But I didn't have a spare
40amp (and I like my wife's cooking) so it is happily living on a 50amp
(max the NEC and my wires allow). My compressor is fine on the 30amp.

sdb

--
Wanted: Omnibook 800 & accessories, cheap, working or not
sdbuse1 on mailhost bigfoot.com

Hank McCall

2005-08-07, 12:21 pm

Sylvan Butler wrote:
> On Wed, 3 Aug 2005 20:25:15 -0700, GeekBoy <nobody@spamcop.net> wrote:
>
>
>
> True.
>
>
>
>
> Maybe. But a few seconds is more likely. Depends on the breaker
> (somewhat the individual unit, but mostly the design specs).
>
> My irrigation pump draws 100-110amps on start according to my peak
> reading clamp-on meter. Usually only for 1-2 seconds. On a 30amp
> breaker (stupid previous owner) it was fine 8 or 9 times out of 10. On
> a 40amp it was fine nearly all of the time. But I didn't have a spare
> 40amp (and I like my wife's cooking) so it is happily living on a 50amp
> (max the NEC and my wires allow). My compressor is fine on the 30amp.
>
> sdb
>
to clarify my previous input of a possible 10 second surge current.
Imagine the generator auto starting after a grid failure, and trying to
pick up an air conditioner with a full head of pressure on the
compressor high side. If you've ever heard it, you'll understand my
answer. Hank McCall

Robert Morein

2005-08-07, 2:21 pm

"Hank McCall" <h.mccall@comcast.net> wrote in message
news:HrCdnZ1TI_MjgGvfRVn-pA@comcast.com...
[snip]
> to clarify my previous input of a possible 10 second surge current.
> Imagine the generator auto starting after a grid failure, and trying to
> pick up an air conditioner with a full head of pressure on the
> compressor high side. If you've ever heard it, you'll understand my
> answer. Hank McCall

That would not happen in my particular system, because it doesn't
automatically transfer.
However, my impression is that someone fiddling with the thermostat can
cause as much damage.
The breaker should be sized to stop this.
I'm more concerned about reports that generator heads fail from this,
presumably even when protected by an appropriate breaker.

Robert Morein

2005-08-07, 2:21 pm

"RF Dude" <post@thisnewsgroup.com> wrote in message
news:DQfJe.33159$pH4.1379241@news20.bellglobal.com...
> Check KOHLER. They have a whole house unit that is 12 kW (model 12RES)
that
> actually has that claim in it about starting a 4 tonne A/C. Of course,
you
> might not be able to have anything else connected. Kohler sells most of
its
> generators standard with a Permanent Magnet exciter. This helps the
> alternator produce large currents for short periods of time.
>
> BTW, all generators (and transformers) are rated in kVA, not kW. OK, in
> single phase the machine is usually rated with 1.0 PF so kW and kVA is the
> same. kW relates to engine horsepower. kVA is the alternator current
> rating. That locked rotor Amps when starting a compressor is likely 0.2
PF
> (very inductive) for a few cycles. I usually oversize my alternators since
> it is not expensive to add copper. For instance, I'll put a 50 kVA
> alternator on an engine that would normally come with a 40 kVA version for
> this reason. But I also have to deal with other harmonic issues.
>
> Good Luck.
>
Good to know. A PM exciter sounds like an advancement.
I'm wondering when alternators with completely PM cores will replace the
current designs; they are smaller, lighter, and something like 10% more
efficient. Of course, regulation would have to be external to the generator.
They are already used in many DC generation systems.

Vaughn

2005-08-09, 12:21 am

"Hank McCall" <h.mccall@comcast.net> wrote in message
news:HrCdnZ1TI_MjgGvfRVn-pA@comcast.com...
> to clarify my previous input of a possible 10 second surge current. Imagine
> the generator auto starting after a grid failure, and trying to pick up an air
> conditioner with a full head of pressure on the compressor high side. If
> you've ever heard it, you'll understand my answer.

They make simple in-line time delay modules for AC compressors for exactly
this reason. I maintain some communications shelters that have three AC units
each. We use modules of differing time delays to ensure that the AC units 1)
Have a chance to always bleed down head pressure before restarting and, 2) come
on one at a time following a power failure / generator start.

Vaughn

Hank McCall

2005-08-09, 1:21 pm

Vaughn, that's very good advise. I should have said something like that
too. I doubt most homeowners know about them.

Vaughn wrote:
> "Hank McCall" <h.mccall@comcast.net> wrote in message
> news:HrCdnZ1TI_MjgGvfRVn-pA@comcast.com...
>
>
>
> They make simple in-line time delay modules for AC compressors for exactly
> this reason. I maintain some communications shelters that have three AC units
> each. We use modules of differing time delays to ensure that the AC units 1)
> Have a chance to always bleed down head pressure before restarting and, 2) come
> on one at a time following a power failure / generator start.
>
> Vaughn
>
>

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