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Thread: 3 phase compressor
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27th Mar 2019, 04:44 PM #1Most Valued Member
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3 phase compressor
Partially an electrical question, with a bit of compressed air thrown in....
Looking to get a decent compressor for my home workshop. By decent I mean something that is going to last 20+ years with minimal maintenance, and at least 12CFM. I've only got single phase in the workshop, but good cabling with a 40A breaker. Will be a fixed, plumbed in installation. Its for intermittent air tool usage, small blasting cabinet, and the odd bit of spray painting.
Only really need 100psi max.
I've survived the last 25 years on a cobbled together ~6CFM contraption with about a 25L receiver
Obviously I can get a 3HP single phase unit and just plug it straight in, but I'm also considering a larger (up to 4kw motor) 3 phase one, and running it off a inverter/VFD. Obviously the motor needs to be able to be converted to 240v 3-phase
Stumbled across a decent ~30CFM one with 7.5kw motor, so here's the questions:
- If I reduce the pump speed it will reduce the flow rate - does it also reduce the power consumption relatively proportionally (ie if its 7.5kw for 30cfm it will only be about half that at 15cfm)?
- Is it likely to still pump to 100psi, or do the compressor heads need to operate at full speed to achieve a decent pressure?
I'm thinking that it might effectively give me a variable flow compressor. Slow it down to say 10CFM normally, but have the ability to wind it up to whatever my power supply can handle when I need the extra flow (will still be less than its 30cfm rating).
Crazy idea??
Steve
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27th Mar 2019, 05:49 PM #2Member: Blue and white apron brigade
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I'm not sure about what others have done but I must be one of the few people who have run a compressor on a VFD.
In short - not easy - In long, very difficult and liable to burn out your motor.
BIG problem one - most 3Phase compressor motors are hard wired as 415V ∆ so cannot be modified to run as 240V ∆. The alternative to ∆ wiring is Y but if you convert a 415V ∆ to Y you will need 690V to make it develop full power. If you run a 415V ∆ on 240V the motor will run BUT you will get less than half power at 50Hz and proportionately less at lower speeds. A 7.5kW compressor motor will almost certainly be wired as ∆ so this means you have to find a suitable HP 415V wired as Y that can be converted.
Problem 2. Some compressor motors will pull more power than their full rating to compress the last 20 or so PSI. Most motors can handle this provided its only for a short period. However, if you start out with an under rated motor it simply won't manage those last few PSI. Yes you can stop before the last 20 or so PSI but you will be still straining the motor to get to that limit. Even to compress to 90 PSI requires 2/3 - 3/4 of full power. You cannot say - "I will run it slower using a VFD" because then the motor has proportionately less power.
Problem 3. After problem 2, the next is the "restart to recharge" where a certain amount of starting torque is required to get the pump started. If you run the motor say at half speed its will nominally have less than half power motor so it it may not even start the recharge. This is the biggest unknown. A 7.5kW compressor is likely to have a high starting torque that needs the full grunt of a 7.5kW motor to restart.
I started out with a 4HP 3P 415V wired as ∆. I thought it might be unlikely to succeed but I tried it anyway. You can read about some of my adventures with it here.
//metalworkforums.com/f65/t1844...fd#post1771988
What is not posted in that thread is how I tried out about half a dozen motors and burnt out a 5HP motor (fortunately it only cost me $35) after it survived a few months of use. However, I did not learn my lesson and I burnt out another that cost me $50 in the few months following that.
At that point I gave up an bought a new 4HP single phase motor and that has been running for the last 4 years without a hitch.
The 5HP VFD still works - it seems to have protect itself though all these shenanigans.
From what I can tell there is no way to easily predict what will happen for a given size compressor, motor and VFD running under non-standard conditions - you basically have to suck it and see. This becomes expensive.
I'm sure it would be possible to run an X kW compressor using an Inverter type VFD and a properly wired motor. But you will be dicing with burnt motor problems trying to do it with anything less than a X kW motor
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27th Mar 2019, 06:37 PM #3Most Valued Member
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Thanks Bob - good info. I'll put away the variable flow idea. You're spot on with most of the motors being 415v delta wired too.
If I'm understanding correctly, what you're saying about the motors pulling more than their rated power briefly before cutoff is due to the torque increase from the pressure? Thinking along the lines of gearing down a larger flow compressor would likely get around that particular issue - ie it should be practical to run a pump rated for 30CFM at 20CFM providing whatever is driving it has a. the power to deliver the 20CFM and b. sufficient torque to take it to cutoff pressure without burning the motor out. Most compressors seem to be 2 pole motors, so fitting a 4 pole might be a solution there.
I'll have to look more into the "start to recharge". I know on a compressor where there's no form of unloading valve the head valves will basically have tank pressure on them and cause hard starting, but I was under the impression that most compressors did dump the air between the tank check valve and pump at cutoff.
My memory from 30+ years ago when my father-in-law got me a "load genie" to fit between the pump and tank that it became an easy startup and the compressor was pretty much up to speed by the time the pressure built up and started to deliver to the tank.
Quite a cool device if you haven't come across them. Basically a sliding valve - in non-running position pressure from the tank moves the valve piston towards the pump and opens a vent allowing any high pressure air between the pump and valve to vent. It also acts as a check valve to trap air on the tank side. When the pump is running the air delivered exceeds what the vent can cope with, pressure builds up and moves the valve towards the tank, initially closing off the vent then opening delivery ports through the valve to the tank. Simply looks like a check valve with a small vent on it and inlet/outlet fittings.
At the end of the day if there's no advantage from either a cost or performance perspective I'll likely just end up with a single phase compressor. I've got plenty of projects already so not planning to add another one without a good reason
Steve
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27th Mar 2019, 07:08 PM #4
All this messing about when a quality 3hp 1ph will do the job no worries, there are some 3.5hp units like the peerless fatboy I have(15cfm), but they are physically a fair bit larger than a standard 50lt size tank so need more space!
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28th Mar 2019, 07:48 AM #5Most Valued Member
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28th Mar 2019, 12:25 PM #6Member
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Is it possible for you to get 3 phase power connected to your workshop? This would make it easier to find a suitable industrial 3 phase compressor.
If you cannot get 3 phase power, I would not consider a 3 phase compressor, just buy a new single phase compressor with a good warranty.
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28th Mar 2019, 07:51 PM #7Gear expert in training
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At my old work we had an older version of this as a backup/on site compressor: https://www.gettoolsdirect.com.au/pe...ompressor.html
Not cheap, but also not out of China
If you want to go BIG on single phase there's this https://www.machines4u.com.au/view/a...ressor/143443/
You'll need to wire in a 15A outlet if you don't already have one.
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29th Mar 2019, 02:10 PM #8Most Valued Member
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Given that most compressors are splash feed lubrication, I would be wary of slowing operating speed too much as some bits may miss out on lube oil if there is not enough agitation of the oil to throw it up to gudgeon pins and cylinder walls etc.
Some variation should be fine, but under 50 or 60% could be dangerous territory.
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29th Mar 2019, 02:26 PM #9Most Valued Member
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Good point Karl. Definitely something I hadn't considered.
Steve
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1st Aug 2019, 11:35 PM #10Senior Member
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I have been running my Peerless 25 for years using a 3 phase convertor without any problems until my neighbour complained to Essential energy that excessive voltage was shutting down his solar panels so they reduced the supply voltage from the transformer - result, my compressor will now not start and I'm looking for a solution all of which are too expensive for this old pensioner - all my other equip. runs O.K i.e. Beaver mill, Takisawa lathe,hoist etc. solutions suggested are Honda power-single ph.,- or
head pressure unloader valve (by Peerless@$500) and after extensive monitoring of voltage supply by Ess E and changing phase on transformer no luck. I have no knowledge of your " head genie" can you please elaborate - I am not electrically savvy but together with my neighbour who is an all rounder we removed an M8 allen screw from the centre of the cylinder heads and the compressor started as it is not under load - our thoughts are to remove the heads -open out the M8 to say M12 with a 8 mm. hole up the guts and manifold it with a small ball valve - open valve ,start motor and close valve when motor reaches max. RPM - not convenient but cheap - no real solutions from Peerless
Any thoughts please gentlemen???
I
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2nd Aug 2019, 11:49 AM #11Most Valued Member
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Did you google "head genie" or "load genie"?
All sounds a little strange
All the single phase compressors I recall seeing have a "load genie" fitted, I'm pretty sure most 3 phase compressors would as well. Maybe you have one that's blocked?(though if you don't have one, fitting one sure sounds like it would fix the problem)
I wonder what Peerless wants to sell you for $500!
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2nd Aug 2019, 11:47 PM #12Most Valued Member
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Alf you need a valve on the tank like this https://www.ebay.com.au/itm/Brass-3-...MAAOSw0Rdc-JE4 your compressor should have one, the small pipe may be disconnected.
An air line is fitted to the small thread on that fitting and the other end of the air line is normally fitted into the compressor switch, when the compressor switches off it dumps the air in the air line between the compressor head and the tank and this reduces the start up load, the valve stops air from venting back through the compressor and the smaller air dump pipe.
I think you could probably fit one of these to that vent/dump fitting on the valve, 1 wire to neutral and the other to one of the phase's, https://www.ebay.com.au/itm/2-Way-So...frcectupt=true
Check with the seller that it is open when no power and closed at start up.
cheers, shed
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3rd Aug 2019, 02:01 AM #13
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3rd Aug 2019, 10:53 AM #14Senior Member
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12th Aug 2019, 07:21 PM #15Most Valued Member
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Alf, here's a ref with the official "Load Genie", but as others have said any form of unloader should work.
The beauty of the Load Genie is that its just an inline fit and doesn't need connection back to the pressure switch for the unloading function.
https://www.glencoairpower.com.au/wp...-Datasheet.pdf
Steve
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