View Full Version : RPC noise

07-23-2008, 08:57 AM
i see an RPC in my future and have been reading with interest the current threads. There is a lot of complaints about how noisy they are, noisier than the machines they drive etc. I don't get it - aren't they basically a free spinning motor? How could that be noisier than a transmission, spindle & bearings etc. Motors not connected to anything mechanical don't seem that loud so I'm looking for perspective - how bad are they and what causes all the noise? Do most try to reduce it? rubber feet, insulated box etc?

07-23-2008, 09:07 AM
I have a suspicion that RPCs are almost invariably created using "reclaimed" motors. Since they are usually multi-HP, they are ex-industrial, and perhaps not "at their best." This likely means growling bearings or similar complaints. As you say, they are a free-spinning motor, so they dont get an overhaul and they are noisy. Its a lot easier to complain than repair! Duffy

07-23-2008, 09:17 AM

A rotary converter does use a free spinning (unloaded) 3 phase motor as a rotary transformer,
when I opened the case on the one I bought new, I was a bit disappointed to see a cheap looking, probably Chinese motor sitting there.

A friend of mine also had a 3hp converter with a decent Brookes Crompton motor
British made and very quiet

Mine on the other hand is noisy with the motor whining sometimes ( place bad joke here )

I looked at swapping the motor, then quickly put the cover back on as the amount of wires going in and out of the motor scared me :D

As to the operation of the converter, no problems at all, starts and runs all the machines, even a little bandsaw welder, two phase, just noisy.

The plan is to remote locate it with a r/c socket and contactor.


07-23-2008, 09:34 AM
Before me and my partner got out own shop we were renting a corner of a steel building from a carpenter, he had a big 4 axis gantry router with 3 rotaries, 5 foot by 12 foot, that he made himself, it was nice.

Anyways, we used his 25hp RPC and he had it bolted to a shelf that was welded to the building, it turned the building into a giant speaker/amplifier for any little noise that came out of it. There was also a ton of fan noise, and in the small unenclosed building, with no insulation to absorb the noise, it was miserable.

Running a 10 and a 20hp now, while waiting for 3 phase. They stay outside in a dog kennel. I could live with the 10 being inside, but the fan noise from the 20hp is just too much.

07-23-2008, 09:38 AM
I built my RPC from a used motor I bought from a building demolition salvage company. The converter worked great but was noisy, getting worse as time went along. Finally I tore the motor apart, the grease in the bearings was fried into a hard crisp mess. Cleaned the bearings in solvent, blew them clean and dry with air, repacked with very high quality pretty red #2 lube, reassembled the motor and now have very quiet RPC. JIM

07-23-2008, 10:15 AM
I haven't used mine recently, but I made an RPC many years ago using a big old sleeve-bearing motor, on the recommendation of a local motor shop. They guy there said basically to use the biggest, oldest motor you can find, with plain bearings and lots of iron and copper. Since it runs unloaded, it's pretty quiet. Even so, it is not inaudible.

But if possible, I'd try to figure out a way to put the converter either in and adjacent room, an outside shed, or an enclosure.

07-23-2008, 10:17 AM
I have a 7.5hp rpc. Mine was inside a 1 car well insulated shop. The noise was pretty loud, not terrible but just grating. The frequency is high enough to annoy. My mill is cnc so turning off the rpc between cuts is painful (re-boot, re-home, re-zero)

The other issue is heat. In winter the waste heat was welcome but in summer it turned my shop into an oven. Noticeable temp increase... easy 5-10 deg (my shop is tiny)

I built a box to try to muffle the noise. Worked pretty good but I wound up cooking a few caps. Had pretty good ventilation too. stood on 2 2x4's so the bottom was open and the top had 4 4" hole saw cuts. Not enough I guess.

Now it sits outside under a roof. MUCH better.

07-23-2008, 10:19 AM
I bought a 7.5 hp unit from American Rotary, and it's quieter than my mill's motor. I have it sitting on a wooden platform, on a piece of sprint car tire tread, so that helps dampen any vibration and noise.

Montezuma, IA

07-23-2008, 11:17 AM
I built mine from what turned out to be a new surplus Hitachi motor that I got for the bargain price of $25. As such, I expect the bearings are good. Mine is a TEFC motor, and they have an external fan. I think almost all the noise is from that....spinning at 3750 RPM and mounted a half inch from the back of the motor housing and inside a shell designed to direct the air over the motor. It makes for a good bit of wind noise.

I can't explain what it is, but I have found the same issue to be true in my case....I expect things to go silent when I turn off the machine I am running and somehow even the quiet idler messes with my minds expectations. I suspect that in a commercial shop, everyone expects that the "noise floor" is high enough that something like this would be completely un-noticed. I have a TV and radio in my shop and somehow my expectations are different.

Like I posted in another thread, however, I remote mounted the motor under a work bench in a corner of the room with the motor sitting on a piece of rubber on the concrete floor and got rid of a lot of the noise.


07-23-2008, 11:52 AM
my RPC is built from an old 5HP compressor motor and i have it mounted to a cheap Horror Freight two-wheeled dolly so i can move it around. it sounds like any other old electric motor just sitting there spinning free. i'm not sure what motors these guys are using that they are calling them "loud". a small Dremel tool running is louder than my RPC.

andy b.

07-23-2008, 12:01 PM
Mine is not louder than a small dremel tool running. Edit-- I take that back...it does move more air so there is more air noise. I guess that's my point, however. I would find it annoying to work over the noise of a Dremel tool running all the time. I could not care less when another machine is running....its the fact that the noise is occuring and nothing is happening. Its like leaving the vaccuum cleaner running in your family room while you watch TV. I know its nuts. I would probably just adjust if I worked in a factory where most certainly the background noise level is high.

In my shop, however, the "norm" is near absolute silence or just a TV or radio on...its somehow different than a motor just doing nothing making a relatively quiet dull roar. I am sure its worse for the guys with a motor with bearing noise etc. Motor bearings are not that expensive for more typical size motors so that may be a good investment.

I think its the particular spectrum of the fan noise too....it masks a lot and that means I don't hear my cell phone, the radio, etc. Its sort of white noise in that sense. Like I said, though, I reduced the issue substantially by burying the motor in the corner of the shop behind some stuff.


07-23-2008, 01:56 PM
When I bought my mill, lathe, and surface grinder, I bought a RPC "Kit" from the dealer I purchased my machines from. The kit cost me $150 and he threw in a 5HP motor for free. It is a real industrial, made in USA, ball bearing motor and is so quiet that I sometimes forget to turn it off when I get done working. I mounted the motor and box on a frame that I attached to some pressure treated 2 X 10 scrap. It sits on the concrete floor next to the I beam upright that is part of the frame of my metal building. The breaker is clamped to the leg of the I beam and wires to the machines run up the frame and out to the machines. The whole system has worked great and I wouldn't trade it for anything. That said, the only thing I can imagine that would cause the noise would have to be bearings and fans. My motor gives off a low frequency hum that is typical of a motor idling, but it is not that noticeable if you are not listening for it.

07-23-2008, 04:02 PM
My home made unit is quiet. I picked up a used GE 5 hp motor. Replaced the bearings. It makes a lot less noise that my shop fan that is a necessity he in Florida in the summer. I too sometimes forget to shut it off.

07-23-2008, 04:50 PM
My 10hp one I built is far quieter than any of my machines. I have accidentally left it running in the shop over night.

You guys that have high pitched whines- my guess is you did something wrong and your capacitance values are way out of whack, or your starting circuit stays latched to long. It should not whine, it should not have any high frequency component.

As for motors, I agree, get a good one, or rebuild a surplus one. I think this is likely the biggest issue most people with noise. Just worn out bearings and sad tired motors. I was really shocked the first time I came on here and saw people saying, RPCs are noisy, I have never heard much out of mine.

07-23-2008, 05:30 PM
I have a 5 hp GE motor and it is no louder than the mill motor or the lathe it powers. I too have left it on because it was not loud and when I turned the shop lights out the pilot lamp on the wall told me to turn it off.

I would suppose those that are loud have some kind of problem. It's just a motor and should not be loud.

07-23-2008, 05:54 PM
My 5hp motor has brand new bearings. The old ones rusted due to a flood which came up about 8 inches in the shop. This was the only casualty. But it still makes too much noise for me. While I am running other machines, its no problem, but when they are off, it is annoying. I put rubber feet on it and that helps some, but does not solve the problem.

07-23-2008, 06:57 PM
I've put mine on a shelf on blocks of soft rubber ...
ok for a bit ...then it starts vibrating noisily...i have to go over there and give it a shove .....stays quite for another five mins ...before it starts again ...

it's just like one of those cheap chinese aquarium air pumps ...that you can never figure .


all the best..markj

J Tiers
07-23-2008, 07:51 PM
I have an Arco Roto-phase. 1HP.

When I got it, (it was already 20+ years old) it was the noisiest thing I had ever heard in my life. I don't know how anyone stood it. It sounded like beating a metal trash can very hard with a hammer.

So I took it apart. Bearings were good, but the rotor was unbalanced, someone had knocked off a balance weight. A couple screws and washer stacks fixed that.

Then I tightened all the screws, and applied some RTV in a couple spots.

Fixed. Now it has a slight whine that may have to do with the fact it appears to be a 3450 rpm motor.

07-23-2008, 10:06 PM
I made one out of a perfectly ordinary Dayton 2 hp eBay surplus motor. It's no noisier than any other perfectly ordinary motor.

I've never been able to figure out why so many claim that RPCs are loud.

07-23-2008, 10:20 PM
You guys that have high pitched whines- my guess is you did something wrong and your capacitance values are way out of whack, or your starting circuit stays latched to long. It should not whine, it should not have any high frequency component.

THAT could be the problem for some folks. when i was getting my capacitance dialed in there was some whine to the motor. it's pretty well balanced now so you just hear that spinning motor sound (i have no idea how else to describe it).

i will say though, if you shop is normally quiet (no fans or anything running), you would probably notice my RPC running. when i'm done for the day i shut everything off and if the RPC was still running i would notice it. when i'm in there doing something i normally have at least one machine and a radio running, so the RPC just blends with the background noises.

andy b.

07-24-2008, 09:04 AM
I've put mine on a shelf on blocks of soft rubber ...
ok for a bit ...then it starts vibrating noisily...i have to go over there and give it a shove .....stays quite for another five mins ...before it starts again ...


all the best..markj

That looks like a "Boost" converter, same make as mine. It is pretty quiet but as others have said, when all is off and your'e concentrating on marking out or setting up, it really gets on the nerves.

I dont have a radio, the signal was so bad it would need an outside aerial.


07-24-2008, 09:11 AM
Yes its isomatic ..same company as boost .

its a 5.5 hp model

That arrow is pointing to this ...



When it packed up one day ...it was obvious why it ran so hot from new........I've repaired it ...now it runs quite cool.

The Isomatic guy ..Peter ...seemed un-concerned about this fault ...that could have caused a fire .

all the best.mark

07-24-2008, 09:19 AM
Nice fault!

Was it just a loose terminal or too much wire in a small joint block?

These blocks seem to melt at just above room temp!


07-24-2008, 09:28 AM
The fault was ...that the block wasn't designed for vibration .

There should not have been the cheap block there..

It should have been done using the other type of block maybe ...the ones with a clamp inside of them .

Also whilst I was there ...I should have made a cowling on the back of the motor that stretches to the vent slots ..

As it's designed ...the motor fan just wafts air around inside the cabinet ...

With a cowling ...it would have forced ventilation.

a bit of fore-thought on the design ...they could have built it with the motor sticking out of a hole in the casing.

Also...I think ...a couple of thick anti-vibration pads stuck to the casing ...the type they use on cars ...would maybe sort out the noise issue .

all the best.markj

07-24-2008, 09:40 AM
I've put mine on a shelf on blocks of soft rubber ...
ok for a bit ...then it starts vibrating noisily...i have to go over there and give it a shove .....stays quite for another five mins ...before it starts again ...

That's resonance Mark. You can eliminate that by having the motor dynamically balanced -- I got a quote for ~ $50 for a 2HP in Austin, FWIW.

A Get 'R Done solution would be to add a little mass to the system -- a piece of sticky-backed lead like golfers use on the shaft might work.

As far as the overall comments about the rotary phase converter noise, that's really an issue of the quality and balance of the motor, and the quality of the motor bearings. Use a good quality Western motor, preferably with new bearings, and it'll be a lot quieter than an Asian motor. Getting the motor balance will also greatly improve the motor vibration.

At most motor shops, the software that controls the balancing machine has a vibration scale that runs from bad to NEMA standards to precision machine to ultra-precision machine to Navy Submarine standards. How well you balance the motor is mostly dependent on how much time the tech is willing to spend d!cking with the balancing weights.

By the way Mark -- beautiful job on the rotary converter -- very professional!

07-24-2008, 10:19 AM
I am using a 15HP rated unit from American Rotary. The motor is a Baldor with grease fittings, and the whole system is well made and gives a nice voltage balance. The voltage where I live is high so I had to put a buck transformer at the head to drop the voltage to safe levels for mr. fanuc, so the whole system unloaded does draw about 4 amps/240 single phase input. The RPC itself is fairly quiet, but you do notice that it is on (it sits next to my Sharp CNC mill).

07-24-2008, 10:28 AM
He didn't build it Robert.

07-24-2008, 11:05 AM
As far as the overall comments about the rotary phase converter noise, that's really an issue of the quality and balance of the motor, and the quality of the motor bearings. Use a good quality Western motor, preferably with new bearings, and it'll be a lot quieter than an Asian motor. Getting the motor balance will also greatly improve the motor vibration.

I think there ends up being a couple of noise makers. Certainly bad or loose bearings could make noise...but that's not all there is to it.

The motor I have was a brand new Toshiba premium efficience EQP III motor. These have lots of iron in it to get the efficiency up and as such make a good motor for an RPC. The specs suggest pretty decent bearings:

4.8 Bearings are selected to provide a L-10 life of 40,000 hours with an external load per NEMA MG 1-14 and a L-10 life of 100,000 hours in direct coupled applications.

and the balance would seem to be good:

4.11 The motor is dynamically balanced to 0.1 inch per second (0.12 inch per second for two pole motors).

The issue in my case is that its also got a mean-mother fan. TEFC motors are inheintly a bit noisier since they have a fan that blows over the outside and if I understand correctly, another to circulate air inside to keep help get the heat to the outside where it can be removed. You can imagine some noise from a fan running inside an enclosed box.

The blades of the external fan are typically absolutely parallel to the axis of rotation which is as much pitch as they could possibly have (lots of noise). Its shoved right up against the back of the motor and then covered by a metal end-cap/shroud that directs all that air flow directly over cast-in fins on the outside of the motor. That shroud has some variety of slots in the end cap through which it must draw all that air it pushes out over the motor. That is a great source of noise as is having the fan righ up against the back of the motor.

Perhaps a TEFC motor is less than ideal from a noise perspective, but they are pretty common as a typical open drip proof motor is not so safe in machinery that makes chips and swarf. I have mine remote mounted, so I suppose I could swap it out with an open frame motor if I could find one, but I like the idea of a motor that keeps the spiders, bugs, sawdust, etc. out as my shop houses a lot of different mess-making machinery:D I built an RPC for my uncle using an open frame motor because that's what he had. It was quite a bit quieter after startup.

Like I said, I think we home shop types are used to a shop that makes no noise at idle and that makes some of us more sensitive to it. My uncle is hard of hearing and touts the advantages of that from time to time. This may be one of those cases. My guess is that if a person has some hearing loss, the higher-pitch portions of the fan noise are totally lost.


07-24-2008, 12:02 PM
I made one from a used ($75) 5HP motor that has somewhat noisy bearings in it. FWIW, the money spent (or saved for that matter) is worth the little noise I get. It's not all that bad, plus I know it's actually running ;)

I have a small pictorial off how it went together. The only thing left for me to do it to tune the voltages to within range of each other. I've been to busy doing other stuff for the moment but here it is:


I have a 7.5HP motor which is MUCH nicer sitting in the garage just waiting to be installed in place of the 5HP. Figures I would get the better motor for free AFTER I got the 5HP...

07-24-2008, 12:18 PM
Metalmeter-- I glanced quickly at your pictoral and it sparked a question: What voltage are you measuring with a single voltmeter?

The end goal normally from some documentation I have is to get potential between the "generated leg" and either of the two incoming lines to be within about 8% of the value of the two supplied legs...which requires two voltage measurements (in addition to knowing the input voltage). I bought a cheap HF inductive pickup meter so I could hop from one to the other easily in initially balancing. In actuality, I got within about 5 volts on the first attempt so there wasn't much to do.

Still, if you wanted to monitor this balance on the panel on an ongoing basis with various loads attached and under differing amounts of load, it should require two meters.