Thread: 5.5HP 3Phase motor on Single Phase with a VFD ????

Please don't pressurise it again until you figured it out - I'd like to be able to correspond with you again one day....
First thing I would do is take out the 2" bung and put a torch inside, along with a mirror to see the bung end.
I would also try and poke something pointy into where the leak appears to be coming from. That allows you to jsee the rust and judge the metal thickness where it leaks. It may just be a burnt-through weld with an inclusion or something - or it's stuffed.....
After any repair, I'd have it hydrostatically tested befor presurising it with air again.
I found a 4kW (5HP) single phase VFD you might like to consider for your motor: http://www.ebay.com.au/itm/111454228395 I corresponded with the seller and it does produce 17A output, which is 4kW. It seems a pretty good price to me.

Hi, I was told at tech that welding pressure vessels like this is illegal, it creates local stresses which can cause major failure. When the tanks are made the welds are stress relieved by heating the whole tank and allowing it to cool slowly. I would get the tank checked professionally for your own safety, the leak could be a very small pin hole in a weld which may not be an issue if the surrounding material is OK.
Mm.

I'd say $250 is still a good deal but it looks like you are going to have to do a lot of leg work to get it operational and safe.

You cant't leave it as is because any leak in a tank, no matter how small, is a just a down hill trip to a dead compressor. The compressor will have to cycle more often and that means more wear and more water condensation which means more corrosion and then more leaks. It looks like that tank simply wasn't vented often enough.

Unfortunately this is the leak you can see, and where there's one that's and indication that there are others ready to happen sooner or later.

Unless you are a certified pressure tank welder I wouldn't be doing my own welding repairs or pressure testing on a compressor tank.
These things can cause severe injury and even death if not treated with respect.
I suggest getting someone that is familiar with this sort of thing to look at it.
At our mens shed we bought a refurbished 160L compressor that had been re-certified by a compressor servicing mob.
With small tanks they usually don't bother and just say "get another tank" but your tank is probably big enough to be worth getting them to take a look at.

However,, before you do that I'd suggest getting hold of a inspection camera that you can poke into one of the tank's openings (vent valve is one place) and see what's going inside.
Like these
http://www.amazon.com/Wall-Inspectio.../dp/B003629WRQ
I think you can hire these.
If the tank has lots of lumps of scaly rust I wouldn't even bother taking it to a professional.
Also look all over the outside very closely - use head magnifiers, for other rust spots and do what Joe says

I think it's still a steal at $250. You might have to spend a little to get it properly fixed, but well worth it.

I've had my peerless compressor for 40 years, never missed a beat. It's a bit smaller than yours.

Ray

Is there a test date on the cylinder?

Originally Posted by Stustoys

Is there a test date on the cylinder?

Hi.
The 110L tank has a plate on it with a date of 30/07/07.




Thx
Jon

Originally Posted by jhovel

Please don't pressurise it again until you figured it out - I'd like to be able to correspond with you again one day....
First thing I would do is take out the 2" bung and put a torch inside, along with a mirror to see the bung end.
I would also try and poke something pointy into where the leak appears to be coming from. That allows you to jsee the rust and judge the metal thickness where it leaks. It may just be a burnt-through weld with an inclusion or something - or it's stuffed.....
After any repair, I'd have it hydrostatically tested befor presurising it with air again.
I found a 4kW (5HP) single phase VFD you might like to consider for your motor: http://www.ebay.com.au/itm/111454228395 I corresponded with the seller and it does produce 17A output, which is 4kW. It seems a pretty good price to me.

Thanks Joe,
Noted. I'll take out the plug later today and have a look inside. Then do some ringing around this coming week so see who can inspect / test / repair.

The 4kW VFD seems like very good money - I won't pull the trigger on that one though until I know the state of the tank.

With regards the 3 phase with VFD / single phase debate - Any ideas as to which would be better / more reliable long term? Im assuming the 3 phase option (if only because there are no big capacitors to fail)? A single phase 4kW motor is more expensive than a 3 phase one, but is cheaper when you factor in the VFD.


Thanks for the advice guys, very much appreciated.

Jon

Originally Posted by Stustoys

Is there a test date on the cylinder?

Hi Stuart, What's the tank size limit for mandatory inspections... I'm pretty sure mine is well under the limit requiring inspection. ( Maybe I shouldn't have asked.. )

Ray

This should be neither here nor there, but just a note to say the picture of the VFD shown by the ebay seller mentioned by Joe is not that of a 4kW HY VFD but of a 2.2kW unit.
If you magnify the image of the side badge you will clearly see this

Here is what the 4kW looks like compared to its smaller cousins - its quite a bit bigger

Originally Posted by RayG

Hi Stuart, What's the tank size limit for mandatory inspections... I'm pretty sure mine is well under the limit requiring inspection. ( Maybe I shouldn't have asked.. )

Ray

Hi Ray,
I think it was every 10 years. But even if that number was correct, its about 15 years old. I've no idea if its a size limit or a private/business. Though I do recall looking for one, no one could think of where the fourth compressor tank was, dawned on me some time later it was the vacuum varish.
Iike you I have a small cylinder thats 30odd years old.

Hi Jon,
As far being reliable I'd think the single phase motor, yeah you might need a new cap every 20years or so, but they are easy to get. A vsd would have many more points of failure(especially a cheap one), cost more and take longer to get. Having said that I've replaced 3 motor caps and had 0 vsd failures, though I know of others with vsds that needed repair.

Originally Posted by Stustoys

Hi Ray,
I think it was every 10 years. But even if that number was correct, its about 15 years old. I've no idea if its a size limit or a private/business..

According to Worksafe Victoria, the cut off point for mandatory inspections is 100MPaL so, 1MPa is 1000 KPa which is about 150 psi, so greater than a 100L tank at 150 psi would require periodic inspections.

Seeing as the tank under question is 110 Liters, that means if the compressor can generate more than about 140 psi, then it's over the 100MPaL limit.

See http://www.worksafe.vic.gov.au/__dat..._receivers.pdf for more fascinating details..

Ray

Originally Posted by RayG

According to Worksafe Victoria, the cut off point for mandatory inspections is 100MPaL so, 1MPa is 1000 KPa which is about 150 psi, so greater than a 100L tank at 150 psi would require periodic inspections.
Seeing as the tank under question is 110 Liters, that means if the compressor can generate more than about 140 psi, then it's over the 100MPaL limit.
See http://www.worksafe.vic.gov.au/__dat..._receivers.pdf for more fascinating details..
Ray

Thanks for posting that link Ray.
Interesting to see they recommend the compressor be fixed firmly to the floor - I wonder how many folks with big compressors have done that?

Hate to nit pick but a more exact number for a 110 L tank is 133 psi.

I wonder if that inspection criteria 's why I see a lot of 120 L tanks limited to 120 PSI, and 100 L tanks to 145 psi?

A very interesting read. Now, if Ive got it right and with regards to a 110L tank...

1mPa = 1000kPa
1kpa = 0.14503774psi

In order for me to NOT need a periodic inspection my max pressure would need to be:
-->100,000kPa/110 = 909kPa
--> = 909*0.14503774
--> = 132psi (as Bob suggested)

However, my compressor' max operating pressure is 145psi so based on the above logic, I have:

-->145psi / 0.14503774 = 1000kPa
-->110L * 1000 = 110,000kPaL
--> = 110mPaL

And therefore exceed the VIC 100mPaL. Which is all well and good but Im in NSW (Just Google'd it for NSW and the requirement is the same).

Whichever way I look at it though, I still need to get the tank looked at and end up with a re-certified tank - at least that way I know its one less thing for me to have to worry about.

With regards taking the end plug out of the tank to have a look inside... not a chance! That thing is uber tight!

Gonna call Peerless tomorrow and speak to their technical boffins and see what they say. Hopefully they can recommend somewhere fairly local to me to have a look at it.

R
J

Originally Posted by Stustoys

Hi Jon,
As far being reliable I'd think the single phase motor, yeah you might need a new cap every 20years or so, but they are easy to get. A vsd would have many more points of failure(especially a cheap one), cost more and take longer to get. Having said that I've replaced 3 motor caps and had 0 vsd failures, though I know of others with vsds that needed repair.

I am assuming that a single phase 4kW (Chinese) motor would wire straight into a suitably rated ON/OFF switch that was on a dedicated 20A supply?


The single phase motor I came across was from Krotec here in Sydney - they import them from China. I wonder about reliability and spares availability.

The 3 phase motor I found was a Teco one (again off the shelf in Sydney), so I would expect to be very reliable.


With the single phase motors, do they take the full 17A / 18A immediately when they start up, or only when working at max pressure? Is the current draw pattern any different to that of a 3 phase hooked up via a VFD?

Originally Posted by ventureoverland

I am assuming that a single phase 4kW (Chinese) motor would wire straight into a suitably rated ON/OFF switch that was on a dedicated 20A supply?

Yep.
I would recommend using a No-Volt switch in case the power fails without you knowing about it and it starts again when the power comes one and you happen to be doing something to it.

With the single phase motors, do they take the full 17A / 18A immediately when they start up, or only when working at max pressure? Is the current draw pattern any different to that of a 3 phase hooked up via a VFD?

A single phase motor connected to a compressor that is rated at 17/18A will take a lot more than that to start up.

For example, on start up the 3HP on my TS draws over 30A and the 3HP on the DC draws about 60A but the dedicated breakers can cope with these high start up current for the couple of seconds these motor takes to start. A 4kW motor on a compressor will be at least 60A on start up but then it will drop rapidly to maybe 7-8A and increase up to the max from there.

With the VFD you have to specifically program the start you want, i.e. instantaneously through to 100s of seconds if you want to.
The default or factory setting is usually an instantaneous start
I have mine on a 20s start up, it could be a lot shorter (e.g. 5s) but I'm usually in no hurry.