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cuemaker
04-08-2010, 11:31 AM
Why would a 5hp motor deliver more CFM that a 1.5HP motor?

Assume that pulley's, RPM etc all the same...

If the smaller motor is able to make the pump work at 1.5hp, it should be no different than a 5hp motor in my mind.

lakeside53
04-08-2010, 11:38 AM
Only if you spin the the pump faster, but they all have a max rpm....

ADGO_Racing
04-08-2010, 12:01 PM
A pump and an engine have a lot in common. They have a certain pumping efficiency. In race engines we work hard to increase the engines pumping efficiency to reduce the power required for the engine to breathe. A pump is basically the same, but it is far cheaper to just add more power than to port, polish and play with valve events. Most compressors use reed valves, and that adds a level of difficulty to making it more efficient.

It takes a certain amount of power to do anything. A fan moves more air (within reason) by adding more HP to it. A fan does not necessarily move more air because it is turning faster. A pump does not necessarily move more air because it is turning faster. They both have an efficiency range, designed around a certain RPM range, but adding more power will move more air. There are calculations to figure it all out.

larry_g
04-08-2010, 12:01 PM
I'm a bit unsure of what your asking here? Are you intending to remove a 1.5hp motor from a compressor and replace it with a 5hp? Then no increase in air will be observed.

The amount of air delivered is a function of the pump displacement (piston pump) times the RPM's. If the 1.5 hp motor will sustain the RPM's then going to a bigger motor will gain you nothing unless something else changes.

lg
no neat sig line

JCHannum
04-08-2010, 12:14 PM
Air compressors are rated in CFM at some pressure. A compressor will deliver a given CFM of free air at a given RPM regardless of HP, but as pressure increases the work required increases and the HP requirements increase to maintain the same CFM.

Mcgyver
04-08-2010, 01:20 PM
If the smaller motor is able to make the pump work at 1.5hp, it should be no different than a 5hp motor in my mind.

doesn't take much to turn the pump over initially, but listen to how it starts having to work as the pressure builds. Presumably the designer balanced that heavy load and duty cycle and safety factor and the answer was 5hp......if 1.5 will fill the tank in time and not burn out and meet the duty cycle requirement, well then, the the 5 was over spec'd :)

cuemaker
04-08-2010, 01:21 PM
Well, its getting time to replace my motor on the compressor as the current one is getting to hot and tripping itself off (its really old, had burnt windings when I cleaned it out after I purchase the compressor)...

I was trying to figure just how big a motor I needed and in doing google stuff, lots of different things I found said to increase CFM, increase the HP...

I am currently happy with the CFM my little 1.5hp is giving. I am able to run my sandblaster, the only problem I am having is after 5 or 6 cylces in an hour the motor gets to hot and trips the thermal protection.

ADGO_Racing
04-08-2010, 01:31 PM
A 2 HP or 3 HP motor certainly will not harm anything. It may give you a longer life cycle. I don't think I would go to a 5 HP, as the mount and tank may not be designed for the additional load.

Bruce Griffing
04-08-2010, 01:48 PM
If the motor is overheating, it can be due to built up dust and dirt in the motor. It is worthwhile to clean the motor internally and see if the overheating persists.

ulav8r
04-08-2010, 02:54 PM
The old motor is rated at 1.5 hp and barely works. You probably need more. Two would be better, might even be optimum. Three would be all you could use near efficiently, anything larger would be a waste of energy.

Just be sure that the replacement motor is accurately rated. The old 1.5 hp motor was likely a true 1.5, where a new compressor rated 2 hp would likely be 3/4 hp or less.

Black_Moons
04-08-2010, 03:40 PM
You only get more SFM out of a given compressor with more HP if you incress the RPM as well, ie incress the load

a 5hp 'compressor' produces more air then a 2hp one because the compression cylinder(s) is usally designed to compress 2.5x as much air per revolution, hence consumes 2.5x as much horsepower.

You likey do not want to run your current compressor at any higher RPM's then its currently running or IT can overheat/wear out prematurely.

IE: more HP only makes more air when mated with the correctly sized compressor. a motors HP rating is just when it overloads/cooks. It will keep spining untill it you draw that much HP and make it overheat/stall.

oldtiffie
04-08-2010, 03:46 PM
Check your unloader valve. Its purpose is to see that motor starts under minimum load.

cuemaker
04-08-2010, 03:53 PM
Check your unloader valve. Its purpose is to see that motor starts under minimum load.

Unloader valve works...but I do have some belt slippage and a small air leak at the junction of the tube and unloader valve. All of which means I will go through and tighten up the system to ensure I am getting the most of the effort being applied...

davidh
04-08-2010, 05:51 PM
and how are the valves in the compressor pump itself ? ? ?

Black_Moons
04-08-2010, 06:22 PM
On a related note, you can reduce the HP used by a compressor by simpley lowering the RPM (smaller motor pully) but less CFM will result.

Also, while maybe not a big factor in efficency per say, if your compressor sounds 'clacky', it could benifit from a modren all teflon proppet style check valve. I installed one in my slow old clacky 2hp compressor and its so quite now you can't even hear it running inside (it lives outside in a small alcove)
I leave it setup to pump up whenever and the neighbors never complain about it kicking in at random hours during the night (especialy if I leave too much hose/tool pluged in with those leaky quickconnects)

darryl
04-08-2010, 10:29 PM
Lots of factors involved. If rpm is the same for either motor, and the pulley ratios are the same, then the higher powered motor doesn't work as hard as the lower powered one, and doesn't draw 5 hp either, although it may be capable of it. If the lower powered motor gets the pump up to speed and is capable of keeping the rpms up there while the pressure builds to it's maximum, then that's all the compressor needs. This isn't to say that the 1.5 hp motor is going to like it- it could be barely adequate for the job and maybe a 2 horse would be a better choice.

In theory, the 5 horse would be capable of running the compressor faster, which you'd do by playing with the pulley ratios, but the compressor is probably going to overheat and pumping efficiency is probably going to drop. If you wanted a faster replenishment of the air pressure and greater cfm, you'd use the larger motor for sure, but also a compressor capable of more cfm.

Then there's the start current factor. Assuming we're comparing motors that are wired for the same input voltage, the starting current is going to be much larger for a 5 hp motor than for a 1.5 or 2 horse. You might actually get 0 cfm out of the 5 horse motor because of not having a circuit capable of running it.

Everything I see about larger displacement compressors says they are not driven very fast, they are instead larger displacement to be able to deliver the volume of air demanded. This takes torque, and this is where the higher power motor is required.

In your situation, I might be safe in assuming that the 1.5 hp motor is being overloaded slightly, and going to a 2 horse would be prudent. It will take a little higher starting current, but probably won't slow as much when the air pressure has built up and is taxing the motor just before the cutout switch operates. It's possible that for this reason alone the cfm might increase slightly. Don't make the mistake of using a larger motor pulley to increase the compressor speed- even though the motor might be able to handle it, the compressor might not. You'll just be throwing away that extra energy you're using as heat, and probably damaging the compressor or at least shortening its life.

Evan
04-09-2010, 12:08 AM
There will be a difference. It is about torque ripple and it doesn't matter what type of motor it is. The small motor is going to slip a great deal more against the force of compression near the top of stroke than the large motor. Slip means it is slipping against the rotating magnetic field that makes it turn.

When that happens the dwell time as the piston nears TDC is a lot longer than with the large motor. Air will escape past the rings and you can probably easily tell the difference by the amount of air coming from the crank case port. This will effectively reduce the volumetric efficiency as well as the inlet pressure to the receiver. That multiplies the efficiency loss especially as it tries to reach higher pressures. Because of this less air is pumped and more remains in the compression chamber. That air then returns energy to the piston on the down stroke acting as a gas spring. The net result is that the rpms stay the same but less air is pumped.

lakeside53
04-09-2010, 12:31 AM
Nice theory with a ring of truth.. ;) but... exactly what are we talking in cfm gain when moving from say a 1.5hp to 2 to 3 hp motor. i.e. - Real world gains?


One other factor in moving from a smaller to larger motor - you may need a larger motor pulley to stop slipping. If so, you'l need to increase the pump pulley size to maintain desired speed. You might be o.k. with a 1.5 to 2hp move, but I'd check the NEMA minimum recommended pulley diameter if you're moving to anything larger. IIRC.. a 3hp motor has a recommended minimum diameter of 3 inches - many factors play into this though.

dr pepper
04-09-2010, 01:23 AM
Better compressors have a flywheel for that purpose, the flywheel absorbs energy from the motor while the piston is not compressing hard, and then towards tdc energy from the flywhel goes into compressing the air straightening out the torque requirement.
Cheap machines tend to be direct drive, and will be less efficient and need a larger motor.

The rotor gets its current from slippage effect in an indution motor (zero slip is 1500 rpm for a 4 pole motor on 50hz uk, or 1800 rpm for 60hz us, the motor speed will be slightly less and will decrease with load), the more slip the more transformer effect into the rotor, and the more current it gets and therefore more torque and power, if your measure the revs on your motor if its the same or higher than whats stamped on the rating plate then its probably not overloaded.

Paul Alciatore
04-09-2010, 02:18 AM
Anybody can post anything on the web, true or not.

It takes a certain amount of power, aka Horsepower, to run the compressor. The motor should be matched to the compressor's required HP. At that HP, the motor should run near it's rated speed and the compressor along with it.

As others have said, you can only increase the CFM by increasing the speed or RPM of the compressor. If that is what you want to do, you will need a motor that is both faster and that has more HP as the faster speed will generate more resistance from the compressor and hence more power is needed to maintain that faster speed. Of course, you can increase the compressor's speed via pulleys but you still need more HP. But that may not be a very good idea as the compressor is designed to run at a given speed and increasing it may easily decrease it's life.

A small increase in HP with a motor rated at the same speed would probably produce some small increase in CFM as it will produce a slight increase in speed because the higher power motor will run at a slightly higher speed with the same load.

Evan
04-09-2010, 03:15 AM
Nice theory with a ring of truth.. but... exactly what are we talking in cfm gain when moving from say a 1.5hp to 2 to 3 hp motor. i.e. - Real world gains?


That depends on the CFM vs pressure. As the pressure required goes up the CFM possible goes down and it isn't linear. For a particular setup where all things remain the same except the motor the larger motor will reach a higher pressure before it can no longer pump air into the receiver. Once the smaller motor has reached it's maximum pressure any increase over that by the larger motor is infinitely greater than the smaller motor.

At low pressures and high volumes the difference will be negligible. At high pressures and low volumes it will be significant.

dr pepper
04-09-2010, 04:02 AM
Sounds like a quote of boyles law, the volume delivered by the pump will decrease logarithmically with incresing pressure.
The compressors here on site with seperate pumps have 1440rpm motors, and that might be the norm, the next up would be a 2880 rpm motor, thats a massive increase in workload for the pump and might wreck it.
Stick with the 1.5, is it fan colled?

JCHannum
04-09-2010, 07:17 AM
Air compresors are very simple machines. Minor losses due to slippage past rings, etc aside, a given air compressor will deliver a constant CFM at a given RPM and pressure. It will consume a fixed amount of power to accomplish this. If a larger motor of the same RPM is used, it will still use the same amount of power. If a 5HP motor is used to replace a 1-1/2HP motor, the 5HP motor will still only use 1-1/2HP.

The only way to increase CFM at a given pressure is to increase the RPMs, this will also require more HP.

cuemaker
04-09-2010, 08:23 AM
I am very glad that my inner sense of logic is intact based on all your answers........

If anything, I might be slowing down the RPM.. With the current pulley setup and RPM I am running anywhere from 1000 to 1100rpm..I am thinking that I want it to be 750 is, maybe more...The pulley on the fly wheel is 6.75 in diameter, dont give me much room to work...

Harvey Melvin Richards
04-09-2010, 10:26 AM
We have 2 rotary screw compressors. Both have the same screws, one is 15hp the other 25hp. As you might guess, the 25hp is capable of much greater CFM.

rock_breaker
04-09-2010, 11:08 AM
There has been no mention of what the pump is doing -- does it have a problem after running for a time ?

Motor efficiency does degenerate over time . Can you check the amperage draw at full load ? Obviously this should be at or below the rating on the data plate.

cuemaker
04-09-2010, 11:23 AM
There is no problems with the pump....

Just talked to Worthington, I found someone who was willing to talk to me with out me paying them a $50 to order a manual for it...

Max RPM for the pump is 1750... He said suggest HP is 5hp, but that seems a bit over done given its for home shop use and only 120psi...He thought 3hp and 1150 rpm sounded like a sweet spot after I gave him the bore and stroke info..

ulav8r
04-09-2010, 03:06 PM
If Worthington says the pump can be run at 1750, then it should do that. The 3 horse at 1150 might be adequate for a small blasting cabinet that is only used occasionally. Once you have used a sand/bead blaster a little you may decide your compressor is too small.

1.5 hp at the current speed should be enough for a small paint sprayer, a die grinder, or blowing chips into hard to reach areas. For larger air tools the 3 horse solution will barely be adequate.

Toolguy
04-09-2010, 04:10 PM
The one thing I haven't seen mentioned on here yet is that a larger motor is required for a larger pump with bigger bore and stroke. The larger pump is mainly the way to get more CFM. In this case, the bigger motor generates more volume of air because of the pump it drives.

larry_g
04-09-2010, 04:18 PM
There is no problems with the pump....

Just talked to Worthington, I found someone who was willing to talk to me with out me paying them a $50 to order a manual for it...

Max RPM for the pump is 1750... He said suggest HP is 5hp, but that seems a bit over done given its for home shop use and only 120psi...He thought 3hp and 1150 rpm sounded like a sweet spot after I gave him the bore and stroke info..

Ok then the proper information to make informed decisions with. If you have a pump rated at 5hp and were driving it with a 1.5 motor then of course more motor will make more air and you can approch maximum capabilities of the pump head. If you are going to drive the head with less than optium motor HP then you will have to select the proper ratio of your drive to not overload the motor current draw. If I were in your shoes and had to drive this pump with a 3 hp motor then I would select a drive ratio to run the pump at ~1000 RPM (3/5*1750) and monitor the current draw. Adjust the drive ratio till the motor is pulling near max amps just before the pressure limit kicks out.

lg
no neat sig line

cuemaker
04-09-2010, 05:20 PM
Ok then the proper information to make informed decisions with. If you have a pump rated at 5hp and were driving it with a 1.5 motor then of course more motor will make more air and you can approch maximum capabilities of the pump head. If you are going to drive the head with less than optium motor HP then you will have to select the proper ratio of your drive to not overload the motor current draw. If I were in your shoes and had to drive this pump with a 3 hp motor then I would select a drive ratio to run the pump at ~1000 RPM (3/5*1750) and monitor the current draw. Adjust the drive ratio till the motor is pulling near max amps just before the pressure limit kicks out.

lg
no neat sig line

Well, based on the many other threads posted, CFM is not relative to HP. A 1.5hp that turns the pump is the same a 5hp..

My current setup runs my sandblaster just fine, or any other tool that I have tried with it. Its just that with my current motor, it shuts down after about 6 cycles due to over heating.... the motor is at least 30yrs old and has a burnt winding.....

And I am not trying to up my CFM, I am just trying to understand if there really is a relationship that more HP equals more CFM.

As for what I am think I am going to do, I have a nice Alis Chalmers 5hp 1740rpm, but 3 phase motor...With a static phase converter, i should get 3.33hp out of it...Not the best solution, but certainly the cheapest..

larry_g
04-09-2010, 07:56 PM
Well, based on the many other threads posted, CFM is not relative to HP. A 1.5hp that turns the pump is the same a 5hp..



And I am not trying to up my CFM, I am just trying to understand if there really is a relationship that more HP equals more CFM.

As for what I am think I am going to do, I have a nice Alis Chalmers 5hp 1740rpm, but 3 phase motor...With a static phase converter, i should get 3.33hp out of it...Not the best solution, but certainly the cheapest..
I'll bet that your 1.5hp motor was not spinning the pump at 1750 rpm or putting out the air that pump is capable of. No problem. You need to do what I said to optomize the setup you have. You set the system up so that you are getting the most from it. I'm not sure what the static converter is capable of in terms of starting torque, and running current. But I still believe that measuring the motor current and gearing the system to get the most out of it is the way to go.

lg
no neat sig line

darryl
04-09-2010, 09:37 PM
Seems like it's getting complicated where it shouldn't. You have two motors, one a 1.5 horse, the other a 5 horse- both rated for the same rpm. If both of them turned the pump at that same rpm, there will be no difference in air output. There is the effect that Evan brought up about torque ripple- the lesser rated motors armature will slow somewhat more every time a pressure pulse is being forced through the reed valve- but I'd have to suggest that the lesser motor will be running more slowly in general, and that means that the pump will not turn as many times per minute as it would be doing with a larger motor. But a 5 horse won't be turning significantly faster than a 2 horse in that same application. The difference is fairly small anyway, nothing that makes or breaks the situation regarding air delivery. If you really wanted to improve your compressor, would you be willing to settle for 5%, or 10%? If you really have a problem with too little air, you'd probably want at least a 50% improvement or more- putting a 5 horse motor on the same compressor isn't going to do the job, all it will do besides probably giving you the 5 or 10% is require a higher starting current.

Were it me, I'd find out first if the 1.5 horse motor is losing too much rpm because of the loading effect of the compressor. If that turns out to be the case, I'd go with a larger motor, but a 2 or 3 horse, not a 5.

There was a spec given, 1440 rpm. That might be what a reasonably matched motor might slow to as air pressure comes up to maximum, but the other figure, 1150, is too slow for a 1750 rpm motor under load. If you put a 3 horse motor rated at 1150 rpm on that compressor, you will lose air delivery figures, not gain. With a 3 horse rated at 1750, you will gain a little, but not a lot as I explained earlier.

If you want more air, same pressure- you need a larger compressor, complete with a higher power motor.

lakeside53
04-09-2010, 10:11 PM
well said!

Farbmeister
04-09-2010, 10:34 PM
First, unless you rig up a 220V/25-30A line you will *NEVER* get 5HP out of a wall socket.

750W in a HP (ok, 745 and change)

110V * 15A = 1650W / 750 = 2.2 MAX HP in a 100% perfect world. You might be eeking out 1.5 HP off a wall socket if the motor is any good

220V * 30A = 6600W / 750 = 8.8 MAX HP ~ 5-6 HP in a quality motor.

As for more cfm's, you might look for a two stage pump (I don't recall many 2 stagers running off 1-2HP with 20+CFM at 90psi.

There are all kinds of trade offs you can make.. more CFM for less PSI is a common one. Few tools need 90PSI. Most need CFM (paint, media blasting, drills/sanders). Large receiver to allow the pump to catch up, large lines for reduced line loss.

darryl
04-10-2010, 01:05 AM
I've been able to routinely get a pulse of current of about 60 amps off a 110v, 15 amp breakered outlet. This works out to 8.85 horsepower- for one second. :) These figures would be higher if I was closer to the breaker panel.

I'm not arguing for or against the capacity of a typical feed to a wall socket, but I know that many microwave ovens purposely draw up to about 20 amps for a minute or so from the 110 socket, before they revert to a lower current draw for the remainder of the time period punched in. That works out to about 2.95 hp, or say 3 horse. I don't recommend to run a 3 horse motor off 110, though you could. It's kind of like trying to run a refrigerator from 12v using an inverter. You CAN, but obviously it's not a preferred method.

I don't know if there's a rule of thumb regarding at what point you should switch to 220 to run a motor, but even a 3/4 horse motor responds better to running on 220.

J Tiers
04-10-2010, 10:25 AM
The whole power issue is really quite simple.......... What is one HP? 33000 foot lb per minute. Power is expressed here in force x distance moved per unit time. Metric units same-same, just different names and numbers etc.

A compressor moves a piston (or some equivalent) a distance per stroke in a certain time, against a certain effective back pressure.

Therefore you have all the elements...... force, distance and time. You can convert that to power required per CFM at pressure, or whatever, but the derivation is from force x distance /time.

You can easily determine the minimum possible power that is required to supply "X" number of strokes per minute in CFM at whatever pressure is wanted based on a lossless system.

To that you have to add the losses, heat, friction, leaks, "throttling", etc to get to a "real" system, and they may be harder to calculate. But the absolute minimum lossless power is easily known.

Result when you include losses is the actual power needed.

So you can easily see that more power is needed if you:

1) increase rpm...... more strokes per minute, so more distance per unit time, same force, more power needed

2) increase pressure.... so more force on piston, same distance and time..... more power needed

3) increase piston size..... same PSI, but more square inches area, so more total force, same distance and time........ more power needed

larry_g
04-10-2010, 11:34 AM
It utterly amazes me that so many answers have nothing to do with what the OP is trying to achieve. The poor OP has a compressor head that is spec'ed to run at 1750 rpm and be driven with a 5 hp motor. He has been running it with a 1.5hp and burned the motor up, he has a 3 hp motor he wants to drive it with. To do this he will have to drive the pump at some slower speed to prevent overcurrenting the motor. three fifths of 1750 = ~1000 rpm at the pump. He must select pulleys that give that PUMP speed. The rated speed of the motor will be what determines the pulley sizes.

All the specs and theory in the above few posts do nothing to get the OP running with what he has to work with.

lakeside53
04-10-2010, 11:51 AM
As for replacing the 1.5hp motor with a 5hp motor via a static converter... the motor does not "put out" hp - it develops HP in response to a load and within it's physical limits will be quite happy to try to develop 5+ hp though only 2 windings, and create smoke. Be very careful with static converters... nothing limits the "output" of the converter other than your breaker panel and the load. A motor starter with thermal or other trips can be put in the circuit to provide protection.

J Tiers
04-10-2010, 12:02 PM
All the specs and theory in the above few posts do nothing to get the OP running with what he has to work with.


Bull.....

The title of the thread is "why does motor HP affect the CFM of a compressor"

1) the theory is exactly on-topic for the thread title.

2) it gives a qualitative understanding of what is going on, to allow the OP to make tradeoffs and understand what is going to happen

3) is shows exactly why replacing a motor which is already 'good enough" with a larger motor has no particular advantage.

It seems that would be directly in the center of the target....

cuemaker
04-10-2010, 01:03 PM
I stand by the title of my thread..as I have thought I have also expressed in various postings in here also..But I do accept that I more than likely muddied the waters by posting other info and items in here..

So, to reiterate..

I am not looking to increase CFM....I really wanted to know why I found on many other websites increasing HP increased CFM. It has always made sense to me if 1.5hp is doing it given all specs same, 5hp dont make it better except the motor will last longer maybe.

I understand Larry's point... stick with what the manufacturer says..and i might as I have found some 5hp single phase motors for a decent price.. But if I could get away with a static converter and using my 5hp 3phase motor while losing a 1/3hp in the process, that makes over 3hp to power the pump..

My current motor has gotta be at least 25yrs old and had burnt windings when I got it. Here is a link to a thread about the motor..http://bbs.homeshopmachinist.net/showthread.php?t=37252

During that thread, I cleaned it out and replaced the bearings with some NSK's..

So I muddied the thread with other crap.. and I am sorry.. I like to jump form thing to thing as my mind goes there...

ulav8r
04-10-2010, 01:20 PM
Jerry is right, some have not thought their answers through properly.

The 1.5 horse motor runs the compressor at a lower speed because of the pulley sizes installed. It probably did that for many years based on the condition it was in before Cuemaker cleaned it up. Worthington gave Cuemaker the speeds required for 3 and 5 hp to give maximum air delivery. The speed is adjusted by changing the pulley size, not by overloading the motor thereby reducing the rpm. That would quickly burn up the motor. A new 1.5 hp motor with a 1.25 duty rating would probably last 20-40 years running the pump at its current speed. Going to a larger motor and higher pump speed would reduce the pump life because of increased wear at the higher speeds.

Since the compressor satisfies Cuemakers' requirements at its current output a good 1.5 or not quite so good 2 hp would give the longest pump life. If he has a cheaper motor on hand( the 5 HP + a phase converter) he could run that at the current speed to maintain the life of the pump but would have increased operating(electricity usage) because of the large motor.

Cuemaker now knows that the air delivered is a function of three things, size(displacement) of the air pump, the speed of the pump, and the horsepower delivered.

larry_g
04-10-2010, 02:59 PM
Bull.....

The title of the thread is "why does motor HP affect the CFM of a compressor"

1) the theory is exactly on-topic for the thread title.

2) it gives a qualitative understanding of what is going on, to allow the OP to make tradeoffs and understand what is going to happen

3) is shows exactly why replacing a motor which is already 'good enough" with a larger motor has no particular advantage.

It seems that would be directly in the center of the target....
Yea but your shooting at the wrong target..
The OP has finally asked the correct question in post 31
"As for what I am think I am going to do, I have a nice Alis Chalmers 5hp 1740rpm, but 3 phase motor...With a static phase converter, i should get 3.33hp out of it...Not the best solution, but certainly the cheapest.."

That is what needs answering here, and what I intended to do. All the crap your posting is correct but does nothing to solve the problem the OP is trying to solve. I would think that you would try to get beyond the title and help to solve the real problem this guy has. I've given him the answer on what to do with the ratios, can you give the problems and solutions he is facing with the static converter and what he should do for current limiters?

lg
no neat sig line

JCHannum
04-10-2010, 04:21 PM
What is not known, and being only guessed at is exactly what is the compressor actually doing with the current set up? We know the motor is a 1725 RPM, and rated at 1.5HP and the compressor is turning at approximately 1000 RPM. What we don't know is the receiver pressure and the compressor's actual power requirements at that pressure and RPM.

Looking at the motor, it is an older, heavy cast iron motor. While it is rated at 1.5HP, if a higher load is placed on it, it will produce the required power, but will overheat in a very short time and kick the circuit breaker. Sound familiar? This is confirmed by the Worthington rep suggesting a 3HP motor.

There is nothing to lose by going to a 5HP motor if one is available. If the current pulley ratio is maintained, the compressor will still only need the power it is consuming now, which is probably more than 1.5HP but less than 5HP. There will only be a minimal penalty in power factor and your electric bill will not be affected. The advantage is that at some time in the future, when your air requirements increase, a simple pulley change is all that will be needed.

If the 5HP three phase motor is what is available, give it a try, but do check the amperage it is drawing at full load. If the 1.5 HP motor is still connected and running, an amperage check of that can also give you information as to what the compressor's requirements are.

cuemaker
04-10-2010, 06:19 PM
Well, I put a fluke T5-1000 on the the motor and if I did it right, its drawing a steady 15.8amps.....lots more than my little motor is supposed to do..No wonder its getting hot!

Based on a pulley/rpm calc I found, I am running between 1250 and 1300rpm depending on how exactly you measure the pulleys.

New question.

While I am NOT going to do the following, the question is just for my education....would my motor draw or need less amps if I increased the pump rpm?

JCHannum
04-10-2010, 06:58 PM
Increasing the RPM will increase HP requirements.

Your motor is doing something a bit over 2HP, so you should replace it with a 3HP minimum if you anticipate the same performance from your compressor.

larry_g
04-10-2010, 08:23 PM
Well, I put a fluke T5-1000 on the the motor and if I did it right, its drawing a steady 15.8amps.....lots more than my little motor is supposed to do..No wonder its getting hot!

Based on a pulley/rpm calc I found, I am running between 1250 and 1300rpm depending on how exactly you measure the pulleys.

New question.

While I am NOT going to do the following, the question is just for my education....would my motor draw or need less amps if I increased the pump rpm?

Are we correct in assuming that the rpm you quoted is the pump rpm?

If you increase the pump RPM then you can expect a proportional increase in the current draw of the motor. That is where I came up with the 3/5x1750 to figure the speed that you could drive the pump with by going from a recomended 5 hp motor to a 3 hp motor. Reducing pump speed with pulleys is like shifting down a gear in your car to pull a hill.

lg
no neat sig line

J Tiers
04-10-2010, 11:56 PM
OK. larry's on the ignore list and you have 16A draw in a 1.5HP motor.....

IF your compressor really does take 1.5HP or less, and you put a bigger motor that is same rpm, you won't see anything different, you already have a working system...... That comes from that supposedly "useless crap" I posted..... a certain amount of "work" is needed, and if you need 1.5 HP or less, you have enough motor.

But DO you?

Did you ever post the voltage of your motor? I scanned ALL your posts, and if you said it, I missed it......

here's the deal.... 1.5HP should draw 20A at 115V single phase, or 10A at 230V single phase. 230V 3 phase it would be 5.2A. All at full load 1.5 HP output.

If the motor is lighter loaded, less than 1.5HP, it will draw less than indicated above.

So if your motor is set up for 115V, you don't need any more HP, because you aren't yet drawing full load current. In that case, your motor may have a problem, because it SHOULD be good for FLA at 1.5HP continuous, unless it is rated intermittent duty, or is from china, etc.

BUT if you are at 230 V, you are at almost 160% of FLA, and something is wrong.

if you are on 230V 3 phase, its crazy wrong, should be only 5.2 A, and you then are at 3X normal.

What voltage is the motor wired for?

What are you running it on?

What voltage is actually on the motor when it gets hot?

If the voltage drops, current tends to go up, dropping the voltage more, etc...... I kinda doubt that is your problem, since usually the motor stalls and current goes up to 60A or more as teh breaker pops. ( used to have a 1.5HP 115V compressor........ been there)

Maybe the pulley actually ON the motor is one that will require MORE than 1.5HP at the motor RPM.....

Do you KNOW that the unit as-set up is, or at least WAS a working compressor at one time?

Or is this totally unknown at this point?

lakeside53
04-10-2010, 11:59 PM
Finally, we're getting somewhere...

Ignore me, it's Saturday night and the gin is talking :D

JCHannum
04-11-2010, 12:03 AM
The motor nameplate was in a photo in the other thread, 10.6A @ 230V single phase.

http://i201.photobucket.com/albums/aa129/xringx/aircompressor018.jpg

At the RPM & pressure the compressor is trying to deliver, the motor is overloaded.

The nameplate also is for a Farm Duty motor, it is obviously a replacement, not the original motor, but what was handy at the time.

This is what prompted me to suggest clamping the motor to see what it was actually drawing.

J Tiers
04-11-2010, 12:09 AM
The motor nameplate was in a photo in the other thread, 10.6A @ 230V single phase.

At the RPM & pressure the compressor is trying to deliver, the motor is overloaded.

Sho 'nuff.....



The nameplate also is for a Farm Duty motor, it is obviously a replacement, not the original motor, but what was handy at the time.

Ok, I missed the other thread......

So right now we have a 230V 1.5HP intermittent (farm) duty motor that is being apparently asked to provide about 3 HP...... (that would be 17A at 230V)

Your choices are pretty much to to reduce the motor pulley to about 1/2 the diameter with this motor, OR get a 3 HP motor.

if you reduce the pulley to half, 1.5HP should be enough (just) to run it, but you will get about half the CFM that a 3 HP motor would give with that existing pulley.

Dunno if you have established what you NEED for CFM, and what you HAVE for CFM now.....

The farm duty motor isn't necessarily a problem...... if you don't sand blast a lot, you should have the compressor turning off and a less than 100% duty cycle, but that is only if you reduce teh motor pulley. (or increase the pump pulley)

larry_g
04-11-2010, 12:25 AM
OK. larry's on the ignore list and you have 16A draw in a 1.5HP motor.....

IF your compressor really does take 1.5HP or less, and you put a bigger motor that is same rpm, you won't see anything different, you already have a working system...... That comes from that supposedly "useless crap" I posted..... a certain amount of "work" is needed, and if you need 1.5 HP or less, you have enough motor.
?
JT
If you have any comprehension of what was said in post 27 then you wouldn't be making all this noise and asking these questions. Your knowledge of electricity is well known, but what do you know of air compressors beyond the book learning. You have taken a problem and moulded it into your expertise and seem to be ignoring the the rest of the issue here and that is the requirements of the pump head, and what the Op desires in terms of the system.

lg
no neat sig line

J Tiers
04-11-2010, 01:05 AM
JT
If you have any comprehension of what was said in post 27 then you wouldn't be making all this noise and asking these questions. Your knowledge of electricity is well known, but what do you know of air compressors beyond the book learning. You have taken a problem and moulded it into your expertise and seem to be ignoring the the rest of the issue here and that is the requirements of the pump head, and what the Op desires in terms of the system.

lg
no neat sig line

Yo larry dude noisemaker......

he's GOT an air compressor, a motor, and a choice of fixes.

Some of his choices involve spending a fair amount of money to salvage this lovely device......

it makes no sense to tell him to operate it at a higher speed, etc just because WE might want that in OUR shop...... That is just projecting OUR needs onto him.

he said (post 7 that he is perfectly happy with it as it is, CFM wise, even with the sandblaster, leaks and all (post 13)

he said before that he might just slow it down (post 24)

He is confused about the relation of CFM to power (post 31)

In post 41 he asks about a different 3 phase motor, which obviously might work as it is 5HP, and the single-phase 66% power will indeed bring it to around 3 HP, at no change to pulley, but at the cost of a $150 static converter


The head NEEDS 3 HP (which we already confirmed electrically as a sanity check), AT ITS CURRENT SPEED, which seems to be about 1200 rpm ......

IF it is going to operate at that speed, because he needs that CFM, then his choice is to get (or make) a static converter to use the 5HP motor..... netting 3 HP, which I said......

if he can live with less CFM, running the pump slower will give that with the existing motor...... SO LONG AS IT WILL OIL OK AT THAT SPEED.

I've slowed pumps that way, I put a 3/4 HP on mine instead of the 1.5 and made the motor pulley smaller....... it works fine for what I need, and it seems to oil OK.

The existing motor will run more if teh pump is slowed , but it won't be over-current. he said the motor as-is cycles 6 times an hour (post 7), so he isn't using a ton of air.... If he was really using all this one can do, it wouldn't be cycling, it would be running. So slowing it might work fine.

if teh existing motor is really dead now, (per post 7) then it gets easier, he probably gets the static converter.

Seems pretty obvious to me.....

If teh motor isn't dead, I'd do the pulley now, as it's cheaper. if no-go, I'd pop for the converter and use the 5 HP.....

dr pepper
04-11-2010, 06:51 AM
At risk of being jumped on in this now slightly heated debate, here goes.

Speaking of the slowing down and oiling issues, there are a couple, maybe more methods of oiling that I've come accross, ones where the big end has a 'scoop' which throws oil everywhere inside the crankcase, and the other on older comps is where oil is injected into the intake and lubricates the top side of the comp (well used practice for refridgeration).
I would have thought the oil splash system will still work fine at up to half speed, the oil injection method may not work well at lower speeds, a close examination is worth while to see which method it uses, some use both.

Somewhere on the net theres a table that tells you the hp req per cfm of comp heads, if you can find out the correct speed of the pump and the cfm(also data is probably on the net for this), you should be able to find out how much hp your pump needs, and then also calculate the derated power for lower speeds.

cuemaker
04-11-2010, 07:00 AM
While I am not thrilled at pitting 2 members in an arguement, I sure have learned alot.. and relearned to need to be a bit more detailed when posting because it will save on confusion...

Ok, this brings up usage I guess....

I primarily use it on the weekends... Its heavyest use is when running my little bench top sandblaster, and its just because I am working on a project now and doing a lot of sandblasting that I have noticed the problem.

Otherwise its used to blow out the shop, a die grinder, random orbital sander, Sata spray gun etc etc...

While I am seduced by running this thing at the full RPM of 1750 and 5hp for all the extra air and feeling of studly-ness, its a $350 fix and really beyond my needs at this time.

I have enough extra money to either buy a very nice 5hp motor and be done with, or get by with static converter, and maybe buy the Hobart handler 180 I know someone is selling for $400 and might be talked into $350

Only real problem I have is that I can hide a new motor from my wife, I cannot hide a welder from her....

JCHannum
04-11-2010, 07:46 AM
One other option for keeping the same motor is to reduce the receiver pressure. It was mentioned as being 120PSI in one post. If this is the case, and you can function at a lower pressure, reducing the cutoff point to something between 60 & 90 PSI will bring the overload condition down to an acceptable point. The CFM output will remain the same, motor will run more, but not be loaded as much. Since you have an ammeter, you can adjust the pressure to match the motor's rating.

A farm duty motor is not necessarily for intermittent use, but they are designed for high starting torque. It is actually a good choice for compressor duty. The tag is merely an indication that it is not the original motor and was probably installed at some point in the history of the machine.

dr pepper
04-11-2010, 10:00 AM
Thats a good idea, if you have a clamp meter, stick it on the motor supply, run the compressor until the current drawn equals that on the rating plate, and that is your max continuous operating pressure for that motor with the pulley ratios you allrerady have, if it never gets that high then your worries are over everythings ok.

J Tiers
04-11-2010, 10:07 AM
You can look and see if it has a service factor...... 1.0 or greater means continuous duty. I don't think that's an issue, though, because compressors in home use are inherently intermittent. Industrial compressors may run almost continuously. As I mentioned, the farm duty motor should work even if intermittent duty, unless it's ridiculous.

Lowering the pressure will indeed fix the issue, but you might have to lower it a lot, basically in proportion to the amperage, and that might not be practical.... can you get by on 75 or 80 PSI?

Slowing the pump gives you the same pressure but less CFM.

A combination approach may let you reduce the pump speed a bit less, so if you lowered to 100 PSI, the pulley could be a bit bigger with same motor than if it had to give you 120 PSI.

BTW, I'm not arguing with anyone, don't sweat it......... someone is trying to argue with me, though.

larry_g
04-11-2010, 01:47 PM
So Cue you have finally gotten a lot of information and the last 3 post's sum it up nicely. So what are you going to do now? One thing, in looking back through your posts it looks that in one you pictured the compressor in question. This is a 2 stage compressor I believe so the information from the factory reguarding the 5hp is also including a 175psi, or thereabouts, operating pressure. So keep that in mind when figuring how to go about derating this pump to work with a reduced HP motor.

Its all a conversion of energy. You have less HP to drive the pump with then you have to give up something, volume or pressure. Drive the pump slower and give up volume (cfm), drive it fast with a reduced pressure. Either way you will have to monitor the current of the motor to determine when you have reached the max operating parameters of the pump with the given motor you choose to drive the pump with.

lg
no neat sig line

Farbmeister
04-13-2010, 10:47 PM
Wait, is the OP thinking that he can use a 115V 20A circuit, a 5 hp phase converter on a 5hp 230V 3ph motor and get 3hp?

And a static one at that?

GAH???? :eek: :eek:

Damn the laws of physics! Full speed ahead! I cannot see that producing anything more than the current 1.5hp-ish he has now.

Piston pumps like slow, high displacement cylinders. If you want the same motor to do the same work you gotta give up either PSI or CFM. You cannot have the same motor give BOTH higher CFM and PSI without additional work (ie hp). None of this takes the pump itself into account.. the timing/size/type of valves can also limit the max speed (and psi/cfm) so no matter how many hp you put on it it will reach its maximum and you get no further output.

A new pump is not much. Size it to your motor and have what CFM/psi range you need. I bought an Eaton 3 lung pump and a 5ph (actual) motor (220V 30A) and it runs all day long (1750rpm) and nothing gets warm (other than the air).

Your best be to retain any value of your current set up is to get a higher duty cycle 220V motor and run it at the stock pump RPM.

The gains of speeding up/slowing down are so out of your control (by the pumps design) that any returns you see are far outweighed by the costs to implement them.

J Tiers
04-13-2010, 11:43 PM
Wait, is the OP thinking that he can use a 115V 20A circuit, a 5 hp phase converter on a 5hp 230V 3ph motor and get 3hp?

And a static one at that?

GAH???? :eek: :eek:


Not actually.....


The motor nameplate was in a photo in the other thread, 10.6A @ 230V single phase.

Farbmeister
04-15-2010, 12:49 AM
Than where is this 1.5 HP motor? 10A 230V = 2+hp (230*10)/750=3.1 (no losses) PLUS he says its running hot?

The valves and the intake/exhaust design will be the determining factor. You could spin that pump as fast as you want but the air will only move so fast, as dictated by the type/size of the valves in use.

Unless you move to a screw type, or a multi stage, more RPM != more CFM/PSI

If you have a single stage, single piston there really isn't much to the pump you can do to make it 'better'.

J Tiers
04-15-2010, 08:44 AM
Than where is this 1.5 HP motor? 10A 230V = 2+hp (230*10)/750=3.1 (no losses) PLUS he says its running hot?



Simple... 1.5HP motor, 3 HP load....... no workee good..

lakeside53
04-15-2010, 11:29 AM
He said the NAME PLATE was 10.6A @230... That one heck of an inefficient 1.5hp...;)

J Tiers
04-15-2010, 08:37 PM
He said the NAME PLATE was 10.6A @230... That one heck of an inefficient 1.5hp...;)

Nope, its right at what the usual allowance of current for a 1.5HP single phase motor is given as.... I have several different electricians books of useful data (except Ugly's) and that is right in there as teh number for most......

The efficiency isn't that bad.........around 80% depending on what you figure the power factor as. Small motors are not generally efficient.

230 x 10.6 = 2440VA 2440 x 0.55 = 1340W input. 1340/746= 1.8HP and 1.5/1.8 = 0.83, or 83%

The books figure 10A, which would be a few percent better.....

Farbmeister
04-15-2010, 10:29 PM
Why are you using a 55% reduction to the supply to get watts?

Then another PF reduction from the motor?

Are you saying something is dropping 45% of the power in the cabling?

J Tiers
04-15-2010, 10:57 PM
Why are you using a 55% reduction to the supply to get watts?

Then another PF reduction from the motor?

Are you saying something is dropping 45% of the power in the cabling?

Nay, sir, the 0.55 IS the PF... if you prefer 0.6 I can stand that too.. but not much higher, in most motors, even at full load.

The PF is the difference between "apparent power", the multiplication of volts x amps, and the real power, which is volts x amps x cos(phase angle), where the phase angle is that between the volts and amps... The power factor is the cos(phase angle).

In a resistor the current and voltage are in phase, in a perfect inductor or capacitor they are 90 degrees out of phase .

In the resistor, 100% of the apparent power is real power.

In either a perfect capacitor or inductor, NONE of the apparent power is 'real" power.

So we take the volts * amps, and multiply by power factor to get real watt power. That is input power. Divide by 746 to convert to HP, and take the ratio of output power to input power to obtain the efficiency.

No smoke, no mirrors, all out in the open.

Farbmeister
04-16-2010, 12:14 AM
Throw a couple labels in for clarity.. your explanation helps.

cuemaker
04-16-2010, 06:30 PM
Here is an update..

I purchased a static phase converter PAM-900HD for $168...Hooked up my 3phase 5hp motor with a 5" pulley...

Its now pulling 15.6 to 16.0 amps while running (upon start, it jumps to 50 or 60amps....)

I assume I lost 1/3hp due to the static phase converter and the motor is working a 3.33hp

My pump should be running at just under 1300 RPM...

I will be using the sandblast cabinet tomorrow... I will be interested if I see a difference.. should be a lot quieter without all the past belt squeal...

My belts flop a bit...but I cant put any more force by hand when tightening the motor down.. I dont have any squeal or slippage I think.... Leave good enough alone or work on tightening the belts?

Thanks to all for help....

Arcane
04-16-2010, 08:33 PM
My belts flop a bit...but I cant put any more force by hand when tightening the motor down.. I dont have any squeal or slippage I think.... Leave good enough alone or work on tightening the belts?
Thanks to all for help....


Cuemaker, years ago (decades actually) I was in the same position as you, trying to get decent tension on my air compressor belt and keep the alignment correct. The factory slots allowed too much twist and I needed a third hand to get it all tightened down correctly so what I ended up doing was making a tensioner out of a small turnbuckle. I welded a couple of bent rods on each end so they formed a 'Y' with the body. I slipped that into the pulley grooves between the motor and compressor pulleys and with hand pressure only I was able to spread them apart, and it would hold the tension while I did the alignment as I tightened down the bolts.