DC Motor for Large Air Compressor?

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

WB ...............

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 ratings earlier.

WB ..............

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. Pete

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 adjusted.

I did not see your other post about the centrifugal unloader..... Or the tripping at higher pressures..... Thus my first reply to this question wasted my time. Pete

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.

Hi 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?

Don sez:

"... 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. :)

Bob Swinney

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

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.

HTH Pete

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 or mill.

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 etc)