Steve,
The intermittent starting problem you are experiencing, is caused by the
very large locked rotor amps (LRA) pulled by the motor. Instantaneously, all
motors will pull 5 to 7 (sometimes 10!) times the normal run current in
order to start. If the supply feeding the motor, can't deliver enough
current, the motor flat won't start. The reason it starts sometimes and not
others, is probably due to other cycling loads on your supply "grid" not
being on, perhaps lighting, refrigerator, etc. at that moment in time.
There are electronic devices sold to allow "soft" motor starting in
situations like yours. What they do is electronically reduce the voltage
that is allowed to reach the motor during the start, but allow that voltage
for a longer time, therefore the watt seconds, stay the same, or
approximately the same, so the motor will start. It actually takes much
longer to start a motor with a reduced motor starter, 10 to perhaps as much
as 30 seconds. Grainger offers "reduced voltage starters" in the $1300 to
$1600 range, a very expensive cure but, they will do the job for you!!
However, you are stuck with the problem of the steady state run current!
Perhaps a more efficient motor will draw less current, but I doubt it. Your
air compressor system has to do a certain amount of real work to create "X"
number of cubic feet of compressed air. That amount of work with the
compressor you have, requires that many watts, which of course is the
measure of the work required to do that work! And that number of watts at
115 Vac draws "X" number of amps. It is straight Ohm's Law; Watts = Amps X
Volts! There are 746 Watts per HP if you are curious!
As always, NFI and YMMV.
Good luck,
Jim Bauder
'58 TR3
Scottsdale, AZ
-----Original Message-----
From: shop-talk-owner@autox.team.net
[mailto:shop-talk-owner@autox.team.net]On Behalf Of Steven W. Reilly
Sent: Monday, September 15, 2003 2:59 PM
To: shop-talk@autox.team.net
Subject: compressor motor starting problems
Hello All,
Bought a new compressor for my shop at the cottage. The compressor in
question is a good quality unit with belt driven oiled compressor. The 5hp
peak motor is rated at 15A@115V, 7.5A@220 volt (Really a 1 1/2 hp
motor?). I am having problems starting it on my Solar-electric power
system. I am wondering what can I do in the meantime in order to make it
start easier.
The inverter is a 3000 watt unit rated for 6000 surge watts. When the
compressor is started it pulls the battery voltage down to 10.5V@300Amps
for a second or 2. After it has started it pulls 160A@12.0V. Sometimes
the inverter will not start the unit. Upgrading all the battery/inverter
connections for minimal voltage drop has helped somewhat. I will replace
my batteries next year, but would like to make the motor start easier in
the meantime (and for later too).
The motor has a start capacitor of 136-163 MFD@220v and run of 50.0uf@370
V. and I am wondering if it will help increasing the start capacitor to one
in the 200-300 MFD range before I replace the batteries or decrease the
drive belt pulley diamater.
Thanks,
Steve Reilly
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