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
I wouldn't consider a DC motor at all (for a stationary installation).
I believe this situation was addressed with several solutions earlier. Did
you not see the 15 replies?
If the compressor is in good operating condition (unloaders operating
properly, motor bearings in good condition, etc), you could lower the
electrical loading by using a smaller pulley on the existing motor.
This may require shorter belt lengths, but the adjustment slots could allow
the use of your present belts.
Oh, I see your reply to the previous post now.
Gee, that's messed up.. using the pressure switch for two legs of the 3PH
(it might not even be rated for 10HP).
It might be do-able, but you should have a 3-phase contactor for your motor
(including 3 pole protection/heaters specifically chosen for your motor).
With this setup, an overload on one leg will interrupt all 3 (as it should
be set up).
Grant and someone else addressed the proper sized circuit breaker and heater
> I wouldn't consider a DC motor at all (for a stationary installation). >
> I believe this situation was addressed with several solutions earlier. Did
> you not see the 15 replies?
> If the compressor is in good operating condition (unloaders operating
> properly, motor bearings in good condition, etc), you could lower the
> electrical loading by using a smaller pulley on the existing motor.
> This may require shorter belt lengths, but the adjustment slots could allow
> the use of your present belts.
> Previous post
> >>I picked up a 10 hp compressor, dual heads, 2 stage, 120 gallon tank.
> >>The Pump has a centrifical unloader. The Motor is 3 phase 240 of which
> >>I run on my 10hp rotary phase converter wired with 8-3 With Ground and
> >>a 40 amp breaker.
> >>The compressor starts fine but the breaker trips when the compressor is
> >>near its top end around 160 psi. What are some causes for the breaker > >>to trip?
> > 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?
Can you increase the unloaded volume? This will allow the motor to spin
the compressor up farther before it hits a load. All you need to do is
add a little plumbing attatched to the compressor head before the check
valve. The more volume you add, the longer it will take before the
compressor sees a load.
If you add it in the form of a coil, it will help to pre-cool the air
for moisture control. Thus achiveing two benefits.
I would try to follow Gunner's advice and adjust the regulator to
lower pressure first. It could do the trick and be the easiest
approach. On another question, surely you can run at lower power and
lower RPM on a smaller motor, with appropriate pulley. Or, you can run
your current motor with a smaller pulley, if you can get the belt tension
You'll want the HP if you're sandblasting.
You'd need rectifiers to run a DC motor. That's easier than a rotary
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
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.
I am having trouble visualizing this. Would this not put a psi load on
the head the same as what is in the tank and therefore preload the head
where I thought I wanted to have the heads unloaded before start up?
"... It's real hard to beat DC motors for starting torque. That's why
Throw in steam engine plug here: The starting characteristic of the DC
motor was developed in quest of that of the steam engine, i.e., "maximum
torque at stall." In fact, it has been reported that much like the early
A-bomb tests, where there was some concern over the entire universe, "going
nuclear"; James Watt was very cautious not to bog his engine down, for fear
it would turn the world over. :)
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
What I was talking aobut....
The non-centrifugal unloaders use a check valve going into the tank,
and a dump valve connected to the head, controlled by the pressure
switch. As soon as the pressure is high enough, the pressure switch
opens to stop the motor, and opens the dump valve connected to the
head. So while there is enough air (idle) there is no pressure on the
head. Once the pressure is low enough the pressure switch closes to
start the motor, and closes the dump valve. The back pressure at the
head builds as the motor gets up to speed. Once the head pressure is
enough it pushes open the check valve on the way to the tank. The
amount of volume connected to the head before the check valve, will
determine how long it takes to build pressure and load the
motor/compressor. If the motor starts fast enough, it doesn't take much
volume. If the motor is starting up slower, then increasing the volume
will take more time before the compressor sees any back pressure, and
thus easier, longer startup time for the motor.
Not sure how Your centrifugal unloader is working....
If there is some volume between the compressor head and the check
valve, then increasing that amount of volume will allow more time for
the pressure to build up.
Now as long as you are adding volume between the compressor and the
check valve, you might as well make it usefull volume... Maybe a coil
would be helpfull, since that would provide additional volume, and also
help to cool the air, and thus reduce it's ability to hold moisture.
The amount of drying action for the air depeneds on how cool it gets,
so that the moisture can condense out.
As I mentioned elsewhere, check to see that in fact the comperssor head
is really unloaded before the motor tries to start up the compressor.
Your description of the problem, where the motor cannot start up the
compressor after the tank has been pumped up, points squarely at a
deffective unloader mechanism. That is exaclty what the unloader is
designed to fix.
With only singlephase power available, I'd use the biggest singlephase
motor I could find and adjust the pulley sizes accordingly. It's a
lot easier to change the motor in a compressor than it is in a lathe
Find a big old pig of a repulsion start motor. I have one sitting in
the shed here in Waterloo Ontario that I think is 5HP, 120/240 volt.
Used it on a grain mill and bale elevator - TREMENDOUIS starting
torque, but rather on the large and heavy side. (old Leyland, Century