From: John De Armond
Subject: Re: A tale of 2 generators
Date: Tue, 09 May 2000 05:59:00 EDT
Lon VanOstran wrote:
> Scene: Brother's cabin in the north woods. No electricity. Onan Emerald
> 5K (lp, 4 years old) generator. (actually rated at 120v and 40 amps
> which computes to 4800 watts)
> Problem: 3 HP air compressor which works fine in lower MI with Edison
> type electricity. Cold, it works *once* to pump up, but when preasure
> drops and motor tries to start it lugs, motor becomes very hot, and
> breaker then pops at cabin breaker panel.
This makes me think the air compressor's air unloader isn't
working. The unloader allows the compressor to start without load
so that a cheaper motor/smaller branch can be used. Look for an
aluminum air line running to the pressure switch. This line should
vent to air when the switch trips off. It should close when the
switch turns back on. If you're trying to manually turn the
compressor on when it still has some air in the tank, then this is
sure to be the problem. Most pressure switches have a metal lever
to manually turn the compressor off. The arm will also activate the
unloader valve. Turn the compressor off with this valve before
trying to start it on the generator.
If your compressor really is 3 hp (and not just 3 spec writer HP),
then you're really on the edge for that generator anyway. I suspect
that it is not 3 HP because a true 3 hp motor will have a hard time
starting on a 20 amp, 120 volt branch. My wheeled 2 hp Speedaire
industrial compressor will just barely start on my 4kw Yamaha
generator on 240 volts.
> Cabin is wired to code, complete with 12-2 w/ground wiring. Garage 150'
> from cabin with it's own 60 amp breaker box. 6-Gauge underground cable
> to garage.
> I went to our campsite 300 yards from cabin and retrieved Good Governor.
> 121v 61hz in cabin. 124v +62hz in garage. We shut off every other draw
> on the generator and tried again. The Good Governor showed a voltage
> drop to about 56 when the motor tried to start until the breaker popped
> We hauled the compressor to my motorhome where I have an Onan Marquis
> (gas, new) 5k generator. (again, 120v and 40 amp which translates to
> 4800 w)
> Energy management system gave us lots more info than we could get at the
> cabin. Upon firing up the generator, I see a load of 5 amps (fridge and
> converter) which I left alone and plugged in the compressor. Load jumped
> to 43 amps for about 1 second or less and then dropped to 24 amps and
> stayed. We cycled the compressor several times with the same exact
> results. Perfect performance to my way of thinking.
If it really is pulling 24 amps, then it couldn't run on a 20 amp
branch. You'd have to have a 30 amp dedicated plug. I wonder if
they really did make a compressor that requires a special branch or
if the MH panel is in error. What does the motor nameplate say?
> Clearly my brother's generator is suffering from a lack of available
> amps since the volts are there.
> 1.Does anyone have suggestions regarding the poor performance of
> brother's generator?
> 2.Probably not since the volts are there, but: Could this be caused by
> running his exhaust underground via 2" pipe and bringing it up about 25
> feet from the cabin in the back yard? (tractor type flap on the end of
> pipe to prevent water infiltration)
> 3. Is lp generator less effective than gas?
What does the frequency do? If the frequency is staying on target,
then the engine has enough oomph.
What you're seeing really isn't that odd. Small generators just
don't have the iron and copper to supply huge inrush. If you need
to make a given generator run a given compressor, there are some
tricks to try.
First thing to try is to try adding some capacitance to the starting
side. More capacitance will increase the torque and will usually
reduce the inrush current. We use that trick in the refrigeration
field to make compressors start on low voltage. Try approximately
doubling the start capacitance.
A large run cap across the main terminals of the motor will help
with the very low power factor when starting. In other words, it
will supply the reactive VARs that the generator can't. I'd start
with about a 160 microfarad motor RUN (different from a start cap).
Having another motor running, particularly if it has a heavy rotor
and/or flywheel, will also help. When the compressor comes on, the
other motor will act as an induction generator and supply power to
the starting motor. This is the trick I use to get my restaurant's
walk-in freezer compressor to start on my emergency generator. An
old motor is a lot cheaper than a new generator :-). Motor should
be larger than the compressor motor. The bigger the better.
Hope that gives you some ideas. To get any more detailed, we'd have
to instrument the thing and see in detail what's happening.
From: John De Armond
Subject: Re: Autoformers
Date: Tue, 23 May 2000 03:28:59 EDT
> Thanx for your comments. Anything in the $2K area is out of reach for us. How
> can so many parks (maybe 60+% of those we've gone to) be so far below standard
> and yet everyone still seems to use the substandard electrical current? Are we
> being unduly concerned?
> Generally, the lower readings will be at or near 115 V and then go below that
> level when you turn the A/C on. It would seem that a gain of a set 10% would
> take care of most of our situations.
Your AC should start on 115 volts. It should start as low as
105-109 volts. If it isn't, you need to run down the cause.
One cause could be a defective start capacitor (if it has one) or
starting relay (again, if it has one). Need to check that out
Another cause may be excessive voltage drop during the starting
inrush. You need to hook your RV to a good, solid power source and
then take a digital voltmeter with peak/hold and track down the
starting voltage at each step. (the peak/hold will store the peak
and the valley voltage during the test interval) First, check the
voltage at the compressor terminals. If the voltage is OK there,
then you need to look elsewhere for the problem. Hook up the
voltmeter, activate the peak/hold function and start the unit. If
the voltage at the compressor stays above about 105 volts during
starting, it is OK. If not, work back. Test the voltage at the
unit's terminals. A high resistance thermostat contact or selector
switch contact could drop enough voltage to cause the problem. If
the voltage is OK there, go back to the breaker panel. If the
voltage is still low here, it could be a bad power cord. Does your
power plug get warm when the AC is on? If so, there's voltage drop
You could have a tight compressor. Not a frequent occurrence but it
does happen. This manifests itself as difficult starting, high
starting current and perhaps a bit higher run current. The start
current should be less than the locked rotor current (LRA) listed on
the nameplate. If it's at or near the LRA, then either the
compressor is tight or the start device is defective. Takes an
amp-clamp with peak/hold to see this.
If the voltage at the compressor is marginal and/or the compressor
is tight, a possible solution is a hard start pack. This is a
capacitor and relay assembly that is designed to aid undervoltaged
or tight compressors start. Commonly known by the trademark
"KickStarter". Available at HVAC supply stores. Installation is
straightforward - just hook across the compressor terminals. The
kickstarter has the added benefit of reducing the starting current.
From: John De Armond
Subject: Re: Airconditioner startup surge currents?
Date: Wed, 12 Mar 2003 22:33:09 -0500
On Fri, 07 Mar 2003 14:31:18 -0500, GaryO <> wrote:
>What are the typical startup surge currents for a Dometic Duo-Therm
>BriskAir AC (model 57912.621)?
>The voltage in the campground here measures typically 116V, and the AC
>current nominally changes from about 2 A (fan only) to 8 - 9 A when
>the compressor starts, and the voltage dips to 114V. No problem.
>However, I'm seeing some occasions where the current jumps to 45A, the
>AC fan slows, the compressor hums and fails to start. The line
>voltage during these incidents drops to 102 (at power pole) - 104V (in
>RV) during this starting surge. I'm on a 30A circuit.
The peak starting current will be a little less than the locked rotor current
or LRA. the LRA of the compressor is expressed on the nameplate. You can
pull the cover off and look if you really want to know. there will be an
"LRA" with a number beside it on the nameplate. I'm going to guess about 50
You can calculate the impedance of the campground supply from the information
you have provided. The formula is:
impedance = delta voltage/delta current. Your voltage drops from 116 volts to
114 volts when you draw, let's average your numbers and say, 8.5 amps. the
delta voltage is 2 volts and the delta amps is 8.5 so 2/8.5 = 0.235 ohms.
This quite "soft". Typically I'd expect the impedance at the end of you RV
power cable to be 0.03 or less.
To check our math lets use the same formula to compute the starting voltage
drop problem. If your voltage drops from 114 to 104 during starting and the
LRA is about 50 amps, then 10/50 = 0.2 ohms. Given the estimated numbers,
this is quite good agreement. Since the voltage at the pole and the RV is
practically the same, one may assume that the CG wiring is where the excess
impedance (softness) is.
Your AC is obviously on the edge. I know that mine will start at 104 volts
but probably won't at 100. I imagine your voltage varies that much as the
load on the whole park changes so sometimes your AC will start and sometimes
>I initially though that the campground's power line was a bit 'soft',
>but now I'm not so sure. I suspect that the compressor is attempting
>to start under a load, and drawing the extreme current. The travel
>trailer is only 19' and the small size may be contributing to short
>cycling of the AC and causing this problem. The AC is a non-ducted
>model with the standard roof vent mounting.
>Any ideas? Fixes? Will it smoke during one of these surges?
Last question first, yes this situation poses a risk for the AC. It isn't
that the motor will necessarily be harmed by the low voltage - it probably
won't. The problem is that after so many operations the Klixon thermal
overload breaker in the compressor will weld its contacts on one of the
no-start events. When that happens, the LRA current will flow until something
gives. Hopefully the RV breaker but maybe the compressor winding.
Fixes? Best bet is to move. If you can't do that then you'll need to do
something to boost your voltage. You can buy one of those very expensive
autotransformers from $CW$ or equiv.
If you're handy you can build something very cheaply. All you really need to
do is boost your voltage about 10 volts. To do that you simply need to obtain
a 10 volt, 30 amp transformer, hook the 120 volt primary to the RV power and
hook the secondary in SERIES with the RV supply, observing the polarity so
that the voltages add.
You can get such a transformer surplus for $20 or so from places like
http://www.meci.com. Mount it in a metal box along with a switch to cut it in
and out of the circuit (30 amp DPDT switch from Lowe's, etc - about $6) and
install it near the breaker panel. When the supply voltage is low, flip the
switch to boost the voltage 10 or 12 volts. When it isn't, flip the switch
the other way and switch the transformer out of the circuit. If you're
interested in doing this I can supply you with a circuit sketch. Of course,
if you're not comfortable twisting wires together then the autotransformer
from $CW$ would be appropriate. (I suppose that disclaimer is necessary for
those who don't know how to do things but criticize those who do....)
There is another option you might want to check into. There is a gadget
available called a hard start kit or "kick starter" that attaches to the
compressor terminals and generates additional torque in the motor. This
device helps tight compressors or compressors operating on low voltage to
start. The device is available from HVAC supply stores and costs <$30. A
kick starter is a good thing in general to have on the AC because it also
lowers the peak starting current. It will let you run on lower supply voltage
or at the end of longer extension cords. It is a simple 2 wire device that
simply clips to the compressor terminals.
For voltage as low as you're reporting this would merely be a patch to cover
over the real problem. But it will be useful for other CGs with higher
voltage. Also reduces light dimming when the compressor starts.
From: John De Armond
Subject: Re: Airconditioner startup surge currents?
Date: Tue, 18 Mar 2003 02:58:45 -0500
On Sat, 15 Mar 2003 22:35:07 -0500, GaryO <> wrote:
>On Wed, 12 Mar 2003 22:33:09 -0500, Neon John
>>Last question first, yes this situation poses a risk for the AC. It isn't
>>that the motor will necessarily be harmed by the low voltage - it probably
>>won't. The problem is that after so many operations the Klixon thermal
>>overload breaker in the compressor will weld its contacts on one of the
>>no-start events. When that happens, the LRA current will flow until something
>>gives. Hopefully the RV breaker but maybe the compressor winding.
>Would you know if the Klixon breaker is accessible and replaceable? I
>don't know how long this problem has been going on - but I suspect
>some time. If replaceable, I just may replace it for peace of mind.
Almost always this round black device is located in the compressor terminal
box. Just pop the box cover and there it is. If you're lucky there will be a
Klixon number on the device. If you're unlucky it will be a house number. If
you can't find the Klixon part number, I recommend getting one directly from
the Mfr, as many times the house numbered parts are custom-tailored for the
application. I would NOT trust a cross-referenced general purpose part for
>>There is another option you might want to check into. There is a gadget
>>available called a hard start kit or "kick starter" that attaches to the
>>compressor terminals and generates additional torque in the motor. This
>>device helps tight compressors or compressors operating on low voltage to
>>start. The device is available from HVAC supply stores and costs <$30. A
>>kick starter is a good thing in general to have on the AC because it also
>>lowers the peak starting current. It will let you run on lower supply voltage
>>or at the end of longer extension cords. It is a simple 2 wire device that
>>simply clips to the compressor terminals.
>Is this the same item which Chris was referring to? A capacitor and a
yep, same thing. The original Kick Starter brand used a capacitor and a
mechanical potential relay but all the ones I've purchased in the last couple
of years have been solid state. Frankly, I like the mechanical ones better.
The PTC solid state ones require a bit of time to cool off and reset. If you
try to restart too soon, the kickstarter won't have any effect. The downside
of the mechanical one is that orientation matters.
>>For voltage as low as you're reporting this would merely be a patch to cover
>>over the real problem. But it will be useful for other CGs with higher
>>voltage. Also reduces light dimming when the compressor starts.
>Light dimming? My lighting is all 12V! The battery makes for an
>excellent brownout protector. :-)
Well yeah, if your converter design leaves the battery in the circuit. Mine
does. My mother's MH's magnetek converter doesn't. When AC is applied the
relay transfers the 12 volt lighting from the battery to the rectifier.
Lights very much DO dim with (not so) heavy loads like the AC starting or
using the microwave. Annoying. Didn't know which kind you had, which is why
I mentioned it.
From: John De Armond
Subject: Re: EU 3000
Date: Mon, 23 Aug 2004 19:25:39 -0400
On Mon, 23 Aug 2004 15:16:44 -0600, Bob Giddings <email@example.com> wrote:
>Hmmm. Same altitude. Wonder what other (non political) variables
1) Your AC may be bigger than his.
2) Yours runs awhile before tripping. IOW, it's right on the edge. The
difference might just be manufacturing tolerances.
3) you may have less restrictions in the AC's air flow.
4) your condenser might have a little dirt collected.
5) your fan motor may be worn and be drawing more current. I have that
problem right now.
6) the outside temperature might be a bit higher and/or the sun might be
Those are several I can think of off the top of my head. Since it is right on
the edge of working, there are several things you might try. One, turn off
the converter and let the rig run on battery. Turn it back on when the sun
goes down. Two, try blocking some of the return air opening. This will both
unload the fan and unload the compressor a bit.
Three, see if you can position the AC switch between settings such that the
compressor runs but not the fan. Mine will do that. Let the compressor run
for a few minutes before turning on the fan. This allows the compressor to
pull down the low side from the off condition. The compressor draws more
current during this drawdown that lasts maybe a minute. Don't leave the fan
off more than about a minute, for the high side pressure will quickly rise
without condenser cooling, drawing more current again.
I use the above trick when I'm visiting someone in the heat of summer and have
only a 15 amp outlet to operate from. That will usually let me get the AC up
From: John De Armond
Subject: Re: EU 3000
Date: Mon, 23 Aug 2004 19:39:49 -0400
On Mon, 23 Aug 2004 21:51:39 GMT, Chris Bryant <firstname.lastname@example.org> wrote:
>On Mon, 23 Aug 2004 15:16:44 -0600, Bob Giddings wrote:
>> No, I'm using the 30 amp plug. The AC turns on, but I can't tell if the
>> compressor does right away. Fan's on. After about 5 minutes it gets in a
>I've been missing a bit- what air conditioner do you have? A bunch of
>Coleman models are installed on RVs without any type of start kit, plus
>DuoTherm had (for a bit) a special kit for running off a generator.
>I haven't had any experience with it, but I am investigating the
>Kickstart- http://www.kickstartoem.com/ .
>In a nutshell (and a bit simplified)- the start capacitor in an air
>conditioner provides a bit of extra current for starting. Recent models
>use a PTCR as a timing device to take the start capacitor out of the
>circuit (the PTCR conducts power until it heats up, which take a fraction
>of a second). Older models, and the Kickstart, use a relay, which keeps
>the start capacitor in the circuit untill the compressor is actually
>nearly up to speed- so if it takes an extra second, it will have the extra
Hi Chris. Glad you're back.
Notice he says his runs for several minutes before tripping - not a hard start
problem I don't think.
Kickstarters (by various companies) are great. I use them on all my
generator-driven refrigeration appliances. Most all of the boosters have now
converted to PTC devices. At least the ones I've purchased in the last couple
of years have been.
What the kick start does is supply more current to the starting winding,
generating more starting torque. This is a tradeoff between more torque and
quicker overheating of the fairly fragile start winding. The same torque
increase can be achieved by connecting a larger start cap (the kickstart is
nothing more than a start cap and a relay or PTC) across the existing one.
The reason not to do this is that if the compressor does NOT start then the
added start winding current can burn out the winding before the overload
trips. The Kick starter is designed to drop out after a second to prevent
this from happening.