On Mon, 18 Jun 2007 13:49:59 -0700, "RST Engineering"
wrote:

I thought I had this compressor power thing down to a pretty good science
until they started screwing around with "rated watts" and "peak power" and
all that crap that makes their compressor look really good until you go to
use it.

Back when we were using "real" horsepower I used a figure of 750 (to make it
easy to calculate, I believe 746 is the actual number) watts per horsepower
and an efficiency factor of 85% so that a one horse motor would take 860
watts to do the actual work.

But then you multiplied that times two for "starting" wattage for a couple
of seconds to give 1725 watts under start and then times three for starting
under some volume of air left in the compressor reservoir or about 2600
watts.

That presses my 2200 watt continuous duty (2800 watts peak) fairly close to
the load limit, but certainly gives a margin for error that seems
reasonable.

Before I tell elebendy bazillion Kitplanes readers that the Harbor Freight
Subaru 2200 watt generator will drive the Sears 1 horse compressor (and even
worse, buy the Sears compressor only to not have it work), will somebody
please do a reality check on me for horsepower/watts for this lashup.

(Note ... convert watts to amperes by dividing watts by 120 volts).
860w=7a 1725w=14a 2600w=21a

Sears and Harbor Freight are damned near clueless about this sort of stuff.
If anybody has a source I can reference for running/starting/starting under
load for air compressors it would be well received.

I'd LIKE to buy a 3/4 horse compressor but they go from the kiddie's 1/4
horse toy straight to one horse with darned little in between.

Jim

Jim,

Living off-grid, and generating most of my own power from renewables, I am
somewhat familiar with your issue. Although your approach may make
theoretical sense, in the real world (as you found out) it frequently
doesn't work.

The startup surge for an electric motor is given by a parameter called
"locked rotor amps" (means pretty much what it says). This can either be
measured (with a clamp-on ammeter), or derived from a letter on the
nameplate of the actual motor multiplied by the running amps.

A compressor is one of the more difficult devices to start. Without
specific data, I would figure starting amps to be at least five times the
nameplate amperage rating. Fugedabout trying to convert nameplate HP to
startup surge. It might be less, or not.

So far as your generator is concerned, it is likely that the ratings assume
a power factor of 1. But an induction motor, such as is in your
compressor, will have a power factor considerably less than 1. This
further increases the amount of "real power" the generator must supply.
And may also explain why the nameplate amperage rating seems higher than
what you predict by using HP and an assumed efficiency.

I happen to have a Sears 1HP compressor (1.5HP Peak). Mine has a nameplate
rating of 10.5A @ 120V. I could not see/locate the nameplate on the motor
itself, so I figured a 52.5A startup surge (5X). My inverter has a 78A
peak capacity (46A continuous) so I figured things would work -- and they
have.

Your 2800W peak generator translates to 23.3A at 120VAC. If you have the
same Sears compressor as I do, I'm not surprised that the generator will
have a problem starting it.

Another issue that comes up with compressors has to do with flat spots on
the rotor as they age. This causes them to draw the locked rotor amp
startup current for a longer period of time when new. This can also cause
an otherwise adequately sized system to "blow" on startup, occasionally.


--ron