outback's and motor starting and MREF [RE-wrenches]

Wed Jun 22 08:15:34 PDT 2005

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Following up on Ray and Christopher's thread on motor start loads, I will
pass along something I clipped off the Outback user forum posted by what
seems to be a pretty experienced HVAC guy. I plan on contacting him to run
through the capacitor kluge... we are trying to start a big AC compressor
with a pair of 3648s....  the DC voltage collapsed and the mate errored and
we were ready to hit the road to we told the customer to sit tight till we
came back to add the PV...

MREF was a wild time... great to meet many of you and plenty of new folks -
as worn out as we all are from the fun we are equally charged up from the
"mission" of renewable energy and the positivity of the folks involved.
Great to be hangin out late at night drinkin a keg of Oklahoma flavored beed
with Bergey, Southwest and Abundant wind guys totally relaxed and
non-competitive.  The NABCEP tent had local microbrews flowing from kegs
while the brand X tent was handing out macro-brews ("sure... we have BOTH
kinds of beer, bud and bud light").  Consensus decrees that the highlight of
the event was a certain outback mx-60 inventor teaching the awesome
slopgrass band to play a song about shaving cream with his ukulele at an
early mornin unofficial party in the red tent.....  My guys say they will
never be able to hang an mx-60 without crackin a smile...

For a brighter energy future,

Geoff Greenfield
NABCEP Certified Energy Practitioner

THIRD SUN SOLAR & WIND POWER Ltd.
340 West State Street
Athens, OH 45701

www.third-sun.com
(740) 597-3111

This is the sniped piece from the forum:

Air conditioning compressors are notorious for sucking the life force from
an electrical system. Its purpose is to pump refrigerant vapor from the
suction side to the discharge side. My R-22 system today has about 69 psig
on the cold side and about 295 psig at the condensor (the outside unit in
95F). It's typical here to install a time delay after shutdown so that the
unit is not cycled back on immediately. The time delay allows the vapor to
equalize. One cause of hard starting is cycling the unit, which makes the
compressor fight against high pressure to start.

Assuming that a delay is installed, the other common accessory is a hard
start kit, which is essentially a capacitor for phasing.

My unit is also a 3 ton, but with a compressor RLA of 16.6A and LRA of 86A.
I'll neglect the fan and inside fan currents. 36000 BTU/hr / 16.6A*240V
gives me a ratio of about 9; tons to electrical power. More efficient units
would go to 10, 11.

>From running load of 16.6A to locked rotor of 86 is about a 5.2 ratio.
Typically this ratio runs from 5.5 to 7.5. However, a compressor start
current is not the same as its LRA. Compressor start current could be 12X
the run current.

Diesel generators, with a sufficiently sized generator head, will permit
brief overloads on motor starting with corresponding voltage dip. Recovery
is by mechanical inertia and fuel demand. Inverter/battery banks may be less
forgiving if current limited.

Compressor start and increased current demand causes an IR voltage drop on
the lines supplying current to the compressor. This means that even less
voltage and current is available to start the compressor.

The Home Power Magazine number 84, Aug/Sep 2001, "New Tara in Georgia", page
28, describes the difficulty in southern climes. After being assured that
the locked rotor current was 65A at 240V, measurements showed that the
actual draw current on startup was 132A. That installation was fixed by
installing a two-stage compressor in order to limit startup draw current.
Most small installations would not have that option.

Hope this gives some hints.

Best regards
Jim L.

Ray and Wrenches -

Here are my comments with the questions included:

>>1) Can you stack three inverters with a transformer for 3 wire single
phase 220? ( a customer wanted to save some money)

You can wire three inverters in a 120/240 VAC split phase configuration
- two on Leg 1 and one on leg 2 is how I've done it.  This mostly
increases the starting surge power capability - the X-240
autotransformer still does its thing and makes it all work out OK.

>>2) What is the maximum pump HP that can be run with a pair of Outback
VFX 3648s. I have two customers each with deep well 3 Hp submersibles.
My Franklin motor book shows for a 3 HP pump a maximum current of 17Amps
(230v) but a locked rotor current of 83 amps, meanwhile their suggested
breaker size is 40 amps. The Outback 1Ms surge is 70 amps, the 100Ms
surge is 50 amps, the 5 sec surge is 41.6 amps. Sure seems like a pair
of the Outbacks would run a 3 Hp pump, but could overload occasionally
with other loads on.

We have had a number of systems reported to us which are running 3 HP
pumps on two OutBack 24 or 48 vdc inverters without issues.  I would not
suggest going to a larger pump without adding more inverters. The X-240
autotransformer is highly recommended when pushing the inverters hard
like this. The real variable is what else might be running when the pump
starts.

The locked rotor amps is a worst case scenario - which is why we like to
reference it.  In the real world the pump requires less amps to start
(unless you have really high mineral content and the pump sits off for
long time periods). I typically see about 2/3rds of what the book says
with a meter set to the 1 millisecond peak max setting on my trusty
Fluke 87 meter with an AC clamp probe.

Even if the pump needs more than the OutBack inverter is able to provide
- we don't just shut off like some older inverter designs - we actually
provide the 70 amps maximum and allow the output voltage to droop a
little - often this is hard to measure in the field because it usually
only happens for part of a cycle or for a few cycles.  This typically
will not cause any issue with other loads operating at the same time
such as computers, TVs etc.

Christopher Freitas
OutBack Power Systems
cfreitas at outbackpower.com
19009 62nd Ave NE
Arlington WA 98223 USA
Tel 360 435 6030
Fax 360 435 6019
www.outbackpower.com

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