"Don Foreman" wrote in message
...
On 3 Jul 2005 03:18:17 -0700, "gtslabs" wrote:

I have had problems getting my 10 hp 2 stage compressor to run on my 10
hp rotary phase converter from Andersons. It is currently wired for
240v at 1750 rpm. Can I replace the motor with a smaller hp motor and
possibly a smaller rpm or would I also have to change some pulleys too?
I am mainly using it for a large sandblast cabinet so I don't need a
real high psi.

What are the advantages/disadvantages of using a DC motor to drive an
air compressor?

You'll want the HP if you're sandblasting.

You'd need rectifiers to run a DC motor. That's easier than a rotary
phase converter.

If you can find a 10 HP 240-volt DC motor with about the right speed,
I see no reason why it wouldn't work well. They're not real common,
but they do exist. It could work particularly well if you could
contrive a starter that applies heavier field current for starting.
This would actually reduce total starting current because heavier
field current produces more torque at lower speed for given armature
current.

It's real hard to beat DC motors for starting torque. That's why
they're used as "traction motors" on things like diesel locomotives.


Don

Whats your thought on the use of a lower HP 3 phase motor instead of a DC
motor for this compressor?
It seems obvious that the reason for the circuit breaker's opening is
because of excess current draw.
A 10 HP load on a 3 phase motor thats driven thru a rotary converter gets
very little help from the 10 HP idler. That 10 HP 3 phase motor on the
compressor is going to draw alot of current from a single phase circuit when
fully loaded. It will certainly be drawing more current than that
indicated on the motor name plate
It seems to me that the 40 amp 240 VAC single phase circuit
is marginal for the (reliable) use of any 10 HP tool load.

I dont know what kind of shop this compressor is being used at. The 240V
@ 40A indicates a home type shop. I would suggest using a 5 HP 3 phase
motor and a smaller pulley if it is desired to change the motor. That
would probably be alot simpler than installing a DC motor.

I'd also suggest that there may be no need for any pressure greater than
even 100 PSI for long term blasting. That is, the media normally lives
much longer at nozel pressures of about 60 PSI. And, the compressor will
probably run continuously when supplying air for a blast cabinet when it is
used for a minute or more.

So, if this was my problem to solve, I'd try installing a new pressure
switch. I'd try ON at 60 PSI and OFF at 100 PSI. Switches are cheap.

I know that you have alot of experience with both blast cabinets and
motors.

Jerry