Minimum VFD speed for specific application

Strostkovy · Oct 29, 2018

I need an air compressor for my apartment and don't want it to be cycling on and off at full power being extremely noisy when I only have a little bit or periodic air use, especially at night.

I figured it would be a fun project to make a servo air compressor, that cranks the compressor just as fast as it needs to.

Long story short, the treadmill motor I expected to work is even junkier than I thought, but I can get a decent deal on a 3hp 1175 RPM motor and single phase to 3 phase Hitachi VFD with sensorless vector control and analog inputs.

I have already designed the PD servo loop and it has minimum speed cut out and cut in, and a maximum ramp up speed so it should be VFD friendly. The control system also operates a fan for the ODP motor, VFD, and TEC air dryer, along with controlling the TEC power to stay at the right temperature.

That's the plan anyway.

The compressor is stated to draw 2HP at 9.9 CFM and 150 psi and 1050 RPM, but I think it will take more than that. Pulley ratio is 4:3 motor to compressor, so I will stay a bit below the compressor's max speed and will probably keep the servo loop set at 100psi or less. The compressor bypasses as much as it pumps at around 30 rpm for 40 psi and likely 60 rpm at 90psi, so there is no point in running that low.

Bottom line is: Is it practical to run a 1200 RPM motor at ~80 RPM and still reliably get at least 70% rated torque out of it continuously? I think I've covered all of my bases but want to run it by other people.

9.9 CFM Twin Cylinder Air Compressor | Belt Driven Compressors | Air Compressors & Vacuum Pumps | Air & Pneumatics | www.surpluscenter.com
3 HP 1175 2/4 VAC 3PH GENERAL ELECTRIC MOTOR 184T | GENERAL ELECTRIC | Brands | www.surpluscenter.com
3 HP Hitachi Variable Frequency Motor Drive WJ2-22SF-CP | Variable Frequency Drives | 3 Phase AC Motors | Electric Motors | www.surpluscenter.com

This is more expensive than my DC treadmill motor attempt, but paying for a VFD is plenty worth it to avoid having to build a fairly high current IR compensated drive and worry about smoking out undersized motors.

DDoug · Oct 29, 2018

Is the compressor head splash lube ?

thermite · Oct 29, 2018

Strostkovy said:
I need an air compressor for my apartment and don't want it to be cycling on and off at full power being extremely noisy when I only have a little bit or periodic air use, especially at night.

I figured it would be a fun project to make a servo air compressor, that cranks the compressor just as fast as it needs to.

Long story short, the treadmill motor I expected to work is even junkier than I thought, but I can get a decent deal on a 3hp 1175 RPM motor and single phase to 3 phase Hitachi VFD with sensorless vector control and analog inputs.

I have already designed the PD servo loop and it has minimum speed cut out and cut in, and a maximum ramp up speed so it should be VFD friendly. The control system also operates a fan for the ODP motor, VFD, and TEC air dryer, along with controlling the TEC power to stay at the right temperature.

That's the plan anyway.

The compressor is stated to draw 2HP at 9.9 CFM and 150 psi and 1050 RPM, but I think it will take more than that. Pulley ratio is 4:3 motor to compressor, so I will stay a bit below the compressor's max speed and will probably keep the servo loop set at 100psi or less. The compressor bypasses as much as it pumps at around 30 rpm for 40 psi and likely 60 rpm at 90psi, so there is no point in running that low.

Bottom line is: Is it practical to run a 1200 RPM motor at ~80 RPM and still reliably get at least 70% rated torque out of it continuously? I think I've covered all of my bases but want to run it by other people.

9.9 CFM Twin Cylinder Air Compressor | Belt Driven Compressors | Air Compressors & Vacuum Pumps | Air & Pneumatics | www.surpluscenter.com
3 HP 1175 2/4 VAC 3PH GENERAL ELECTRIC MOTOR 184T | GENERAL ELECTRIC | Brands | www.surpluscenter.com
3 HP Hitachi Variable Frequency Motor Drive WJ2-22SF-CP | Variable Frequency Drives | 3 Phase AC Motors | Electric Motors | www.surpluscenter.com

This is more expensive than my DC treadmill motor attempt, but paying for a VFD is plenty worth it to avoid having to build a fairly high current IR compensated drive and worry about smoking out undersized motors.

If you are in an apartment, must have "quiet", and need only a small amount of air, how about a Nitrogen bottle and regulator? Or a portable tank of the sort tire service vehicles sometimes use, charged at the shop and toted home? I used to carry one in the car when I was flying in case the rented aircraft had need of a tire topped-up. They are not heavy.

Another off-the shelf and near-as-dammit soundless source is the tribe that luxury motorcars use for air suspension. Not a LOT of air, but capable of 200 PSIG. Another source those sold for medical office use, for artists' air brushes, and the like. Air-brake and air-shock compressors are not all equally noisy.

This is not a wheel as needs reinvented.

PLENTY of places quiet compressors and vacuum pumps have been needed and supplied over the past hundred years without having to part-out a stealthy-quiet "boomer" or attack submarine.

DDoug · Oct 29, 2018

I use a bottle of Carbon Dioxide (gasp !)
Being a liquid, you get more in the same space, as opposed
to a simple pressure tank.

I have used it to run my impact wrench, blow gun, air nailers, etc.

Pathogen · Oct 29, 2018

If you are going to displace the air in your apartment with bottled gas; CO2, Nitrogen, whatever, don't kill yourself and others. If you don't mind killing yourself, get your paperwork in order as a kindness to others. Also, if that gas bottle falls over and breaks the neck off, you may be collected by Homeland security and be whisked away to a secret prison where you will be waterboarded and other nasty things.

Strostkovy · Oct 29, 2018

I currently use a air tank I lug around. I do have a CO2 bottle I could use, but that's not much more practical than walking down to the large compressor.

I only need to be quiet because I don't like loud machines. There isn't anyone nearby to bother.

For small air requirement I have an axial compressor, which is obnoxious on its own.

This is going to both be a tool and a fun little project, so I'm not too worried about spending time on it.

The idea is to have a compressor that is as quiet as reasonably possible, which ramps up to loud as needed.

Does anybody have any input on the VFD side of things?

thermite · Oct 29, 2018

Strostkovy said:
Does anybody have any input on the VFD side of things?

Waste of money for a DIY compressor where you don't have to chase already-established needs of some prior Pilgrim's off-the-shelf product.

You control the entire design, "clean sheet of paper", yah?

Just design for steady-state single-phase or DC operation, use higher storage pressures and/or a bigger tank. You can have more than one compressor manifolded to a single tank. One up, two back? Gang smaller store-boughts, make no more "parts" than you MUST make, and put your own brain-power into the valving (also store-bought) and load cycling con-trols?

It's only ignorant AIR. Playin' wityourself to elevate it to the level of gender re-assignment surgery on a submarine hull.

DDoug · Oct 29, 2018

Pathogen said:
If you are going to displace the air in your apartment with bottled gas; CO2, Nitrogen, whatever, don't kill yourself and others. If you don't mind killing yourself, get your paperwork in order as a kindness to others. Also, if that gas bottle falls over and breaks the neck off, you may be collected by Homeland security and be whisked away to a secret prison where you will be waterboarded and other nasty things.

We can hope can't we ?......

Jesus cheerist, just use proper bottle handling procedures
and you'll be fine.

Strostkovy · Oct 29, 2018

thermite said:
Waste of money for a DIY compressor where you don't have to chase already-established needs of some prior Pilgrim's off-the-shelf product.

You control the entire design, "clean sheet of paper", yah?

Just design for steady-state single-phase or DC operation, use higher storage pressures and/or a bigger tank. You can have more than one compressor manifolded to a single tank. One up, two back? Gang smaller store-boughts, make no more "parts" than you MUST make, and put your own brain-power into the valving (also store-bought) and load cycling con-trols?

It's only ignorant AIR. Playin' wityourself to elevate it to the level of gender re-assignment surgery on a submarine hull.

This entire compressor looks like it will cost around $800. I can't do much better than that for a continuous duty 9cfm @ 90psi compressor with built in air dryer and 5 second pump up speed no matter how cheap the compressors that I gang together are. Also, it's quiet and ideally would have lighter surge loading.

Its tank is only what is needed to supply air as the compressor gets up to speed, which is so far looking like a small coil of copper tubing.

The original plan was DC but I couldn't find any suitably priced, suitably sized, not beat to crap DC motors.

CalG · Oct 29, 2018

Strostkovy said:
I currently use a air tank I lug around. I do have a CO2 bottle I could use, but that's not much more practical than walking down to the large compressor.

I only need to be quiet because I don't like loud machines. There isn't anyone nearby to bother.

For small air requirement I have an axial compressor, which is obnoxious on its own.

This is going to both be a tool and a fun little project, so I'm not too worried about spending time on it.

The idea is to have a compressor that is as quiet as reasonably possible, which ramps up to loud as needed.

Does anybody have any input on the VFD side of things?

I have my 7 HP CH compressor on a VFD. I wouldn't have it any other way.

I love the soft start up. But the compressor runs at full rated frequency for lubrication reasons.
I could dial the rpms back somewhat based on the chart of recommended pulley sizes and motor horse power.

I suggest you get your VFD at least double capacity to the motor HP, set it up without pressure control loop, use a 5 second ramp up time, and see how everything works at 50% max (30hz). If you are satisfied, run it there. Then tell us how it works, because we are only guessing. YOU ARE THE GROUND BREAKER!

Now designed from scratch, with an oil less compressor and a motor matched to the requirement...
Many on this forum can speak from authority and experience.

Strostkovy · Oct 29, 2018

CalG said:
I have my 7 HP CH compressor on a VFD. I wouldn't have it any other way.

I love the soft start up. But the compressor runs at full rated frequency for lubrication reasons.
I could dial the rpms back somewhat based on the chart of recommended pulley sizes and motor horse power.

I suggest you get your VFD at least double capacity to the motor HP, set it up without pressure control loop, use a 5 second ramp up time, and see how everything works at 50% max (30hz). If you are satisfied, run it there. Then tell us how it works, because we are only guessing. YOU ARE THE GROUND BREAKER!

Now designed from scratch, with an oil less compressor and a motor matched to the requirement...
Many on this forum can speak from authority and experience.

The motor and VFD are sized about double what I anticipate max requirement will be.

Do you use a solenoid as a blowoff so the soft start occurs unloaded?

I may put a little impeller in the oil pan to splash the cylinders. That could take some dialing in though.

thermite · Oct 29, 2018

Strostkovy said:
This entire compressor looks like it will cost around $800. I can't do much better than that for a continuous duty 9cfm @ 90psi compressor with built in air dryer and 5 second pump up speed no matter how cheap the compressors that I gang together are. Also, it's quiet and ideally would have lighter surge loading.

Its tank is only what is needed to supply air as the compressor gets up to speed, which is so far looking like a small coil of copper tubing.

The original plan was DC but I couldn't find any suitably priced, suitably sized, not beat to crap DC motors.

I'd guess my average cost at $100 per HP, 180 VDC wound. Most were professionally rebuilt, commutators, bearings, brushes renewed, new leads and re-varnish - then put on some major industrial plant's shelves as spares.

The leads indicate they were never reconnected before they shuttered that facility and ERC bought it, building and all, to salvage incrementally.

One is a NOS T.B Woods badged / Baldor-Mexico built. All the other are Reliance RPM III.. plus a few OEM 240 VDC wound Monarch 10EE motors.

Oversizing could work a treat. They only draw what they are loaded to. With 180 VDC windings, NEMA 4X housed KB-Penta are fine direct off the line. Starting torque vs AC brings grins.

DOWNSIDE and CAVEAT: Brushes spark. Sawdust can be explosively flammable. You'd want "brushless" and PM ELSE "explosion proof" models if spraying solvents or sanding woods.

I think I would simply run a sized-up air line up from the shop instead of messing with a one-off compressor at all.

Disclosure: Easy for me to say. I am already DEAF!

rolf · Oct 29, 2018

Sometimes I ran the 3 phase motor of my lathe with a VFD. Minimum frequency is around 15 ... 20 % of the nominal frequency with smooth running and a decent torque. But the motor is getting warm!! Take care that you don't run an electromagnetic switch with the same source. So in your scenario it would be180 ... 240 rpm.

When I was living in an apartment I used old compressors from a fridge (250 ... 300W), which you are getting for free. It takes its time but up to 8 at ( 115 PSI ) is possible. You could just adapt it to your big compressor to fill it with air and even use the pressure switch for cut off.

Strostkovy · Oct 29, 2018

thermite said:
I'd guess my average cost at $100 per HP, 180 VDC wound. Most were professionally rebuilt, commutators, bearings, brushes renewed, new leads and re-varnish - then put on some major industrial plant's shelves as spares.

The leads indicate they were never reconnected before they shuttered that facility and ERC bought it, building and all, to salvage incrementally.

One is a NOS T.B Woods badged / Baldor-Mexico built. All the other are Reliance RPM III.. plus a few OEM 240 VDC wound Monarch 10EE motors.

Oversizing could work a treat. They only draw what they are loaded to. With 180 VDC windings, NEMA 4X housed KB-Penta are fine direct off the line. Starting torque vs AC brings grins.

DOWNSIDE and CAVEAT: Brushes spark. Sawdust can be explosively flammable. You'd want "brushless" and PM ELSE "explosion proof" models if spraying solvents or sanding woods.

I think I would simply run a sized-up air line up from the shop instead of messing with a one-off compressor at all.

Disclosure: Easy for me to say. I am already DEAF!

I actually thought about messaging you if you had any motors you would sell, but assuming the VFD can go to 1/10 speed reliably then I'd rather do that because it's the same price you get your DC motors for but I wouldn't have to build any drive circuitry.

An air line would be nice, but that compressor gets shut off when not in use and I also don't want to draw too much attention to my antics.

Worse case I just have to add a tank to this setup and run it normal. Worst case I trash the compressor by not letting it lubricate.

My main use for this is a powder coating gun, occasional die grinders, pneumatic cylinders, assist gas for a small laser, and eventually coolant mist.

Also, this compressor is portable.

thermite · Oct 29, 2018

Strostkovy said:
I actually thought about messaging you if you had any motors you would sell, but assuming the VFD can go to 1/10 speed reliably then I'd rather do that because it's the same price you get your DC motors for but I wouldn't have to build any drive circuitry.

An air line would be nice, but that compressor gets shut off when not in use and I also don't want to draw too much attention to my antics.

Worse case I just have to add a tank to this setup and run it normal. Worst case I trash the compressor by not letting it lubricate.

My main use for this is a powder coating gun, occasional die grinders, pneumatic cylinders, assist gas for a small laser, and eventually coolant mist.

Also, this compressor is portable.

You don't really want brush sparks in that mix of an environment, anyway.
Somewhere.. In an earlier PM thread - but probably not even 3 years back - a "quiet" compressor and - IIRC Aluminium tank - from "California Air Tools" was commented-on.

I gave it a miss as I like Aluminium for the Jaguar and the Diesel's long-term fuel storage tanks, but do NOT like it for air, long-term predictability-wise. It doesn't rust, but can do other odd things.

YMMV, YOMD

dazz · Oct 31, 2018

Hi
A VFD will give 100% torque down to zero rpm but blow by makes that a waste of time. In addition, to run full torque at less than rated rpm needs a powered fan.

Running a compressor at slow rpm is inefficient. Not a problem for occasional use.

One option is to run 2 compressors, one with 2x the capacity of the other. Run them separately or together to match the air demand. A further refinement would be to run 3 compressors with capacity ratios 1, 2 and 4. It might be cheaper and simpler than a VFD solution.

The root cause of your problem is noisy compressors. The ultimate solution would be to run a Guided Rotor Compressor. These make as much noise as a spinning disk (eg. almost none). Not a cheap solution though.

Dazz.

mksj · Oct 31, 2018

Can't really see where a 3Hp motor running a V twin compressor off a VFD would be cheap or portable, let alone the overall complexity of the design relative to the need/application. Splash lubrication compressor have a minimum speed to operate, so they will usually state on the pump the permissible speed. This is usually dictated by the pump size and Hp rating vs. the drive pulley ratio. Operating a splash lubrication compressor at 10% of rated speed doesn't cut it, and has little to do with an agitator in the oil. A lot of noise is from the internals and the air intake noise, the latter can be attenuated with a intake baffle box.

If only for occasional use you would be better off with one or two California Air compressors for the required air flow. Get a good set of filters to dry the air and a descant canister if you need very dry air.
California Air Tools 9D Ultra Quiet & Oil-Free Air Compressor

DDoug · Oct 31, 2018

You can get a very quiet compressor, but running it thru a die grinder ain't
gonna be quiet by any means.

Tony Quiring · Oct 31, 2018

Stop a bit...

Most noise on a compressor is the inlets.

Test by starting one and covering inlets and they get real quiet.

Make suitable mufflers and problem solved cheap and easy

Sent from my SAMSUNG-SM-G930A using Tapatalk

Minimum VFD speed for specific application

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