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

Pretty much what Bob already said, when a 3 phase induction motor is started DOL, the initial current is the same as the locked rotor stall current, which is usually estimated to be about 6 times the rated current. So for a 17A rated motor, it will be drawing about 100 amps for a fraction of a second, you will probably see your lights dim for a short period. Once running it will drop quickly to match whatever the load requires.

When you are running from a VFD, the VFD brings up the voltage and frequency slowly, ( depending on what you set the acceleration ramp to ) and you don't get the current surge. VFD's are much smoother starting.

Ray

PS.. If you find you are tripping circuit breakers, you can get breakers with different time/current response curves to suit motor starting.

Originally Posted by BobL

I have mine on a 20s start up, it could be a lot shorter (e.g. 5s) but I'm usually in no hurry.

I was thinking about this earlier. Did we cover long ramps and splash lube in your compressor thread?

Stuart

Originally Posted by BobL

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.




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.

Thanks Bob,
I get / can imagine the single phase instant load and then rapid drop off. One question though... Assuming a 60A instant draw on startup and then a rapid drop-off - Would a 30A breaker in the dedicated circuit be OK?

With a VFD, assuming say it has a 10s start time. Im assuming 0s is OFF and 10s is full operating speed. Is this a way of managing current draw? i.e smaller current for longer as opposed to big current for shorter periods (when compared to the single phase characteristics)?

That being the case, how is torque managed? Im guessing (on the limit of my knowledge now), that low speed via a VFD = low torque. Im also guessing that as the compressor turns off with the pressure switch the compressor pump cylinders should be de-pressurised and therefore re-starting has minimal load and therefore minimum torque requirements and therefore will be OK with a slow ramp up start.

Im happy to get a sparky to do the work, just trying to get my head around the pro's & cons of single phase vs 3 phase + VFD.

R
J

Originally Posted by ventureoverland

That being the case, how is torque managed? Im guessing (on the limit of my knowledge now), that low speed via a VFD = low torque. Im also guessing that as the compressor turns off with the pressure switch the compressor pump cylinders should be de-pressurised and therefore re-starting has minimal load and therefore minimum torque requirements and therefore will be OK with a slow ramp up start.

I think you're better off thinking of torque being the same(it isnt but thats another story). Dont get torque confused with power.

The compressor will(should) have a one way valve between the cylinders and the receiver, which also vents the line from the cylinder to the receiver so that the compressor is unloaded at start up.

A question to the guys that know, what is the current like when the VSD is first turned on?

Stuart

Originally Posted by Stustoys

I was thinking about this earlier. Did we cover long ramps and splash lube in your compressor thread?

Stuart

Yes we did and that was when I decreased it from 30 to 20s and now that I feel comfortable about 20s I will drop it further.
One thing that I observed recently is that during the ramp up, the reaches about ~10A at the end of the 20s ramp up and then over the next 20 seconds or so drops back to around 9A and then rises again sort of inverse exponentially up to the full 15A or so when it switches off. I wonder if this is related to poorer lubrication over the initial 20 s?

Hi Bob,
As a WAG I'd say no. My first is its something to do with the compressor temp(the motor load would go down as the cylinders got hotter right?). But I wouldn't really know

Stuart

OK, quick update on the tank issue.

I spoke to one mob who repair compressors and tanks and they said regardless of leak size, get a new tank because the cost to re-certify would be un-ecconomical.

I also spoke to Peerless and they had a slightly different approach. Of course, safety was their prime concern but they noted that as the compressor was no longer in a business environment the tank did not need re-certifying, just welding by a certified / competent welder if the welder considered the work/weld to be safe. They suggested that a small localised hole that is not on an existing weld/seam could safely be welded up, noting that the tanks are pressure tested to many, many times more than the pressure release valve. Apparently, they re-weld tanks all the time in their repair arm of the business using certified & competent welders and the expensive bit is the re-certifying.

As a course of action it was suggested that I strip the compressor, turn the tank over, cut/grind off both feet and gently grind back to metal and see what the actual damage is. At this point a certified / competent welder could fully assess the damage and decide to weld or not.

In the meantime I am also looking at new tank costs and also asking Peerless to look for any carton damaged tanks that they can no longer sell as new. If there is little difference in price between another tank and repairing mine, I'll go for a new tank.


Jon

The responses are about what I would have expected but I would still try and look inside the tank even before getting it welded. If it is too rusty it wont be worth doing.

Originally Posted by BobL

The responses are about what I would have expected but I would still try and look inside the tank even before getting it welded. If it is too rusty it wont be worth doing.

Agree. I welded on an old Ingersoll-Rand tank but:

It was to replace a buggered drain fitting (threads not rust).

The tank had been painted inside & was pristine except for a good coating of oil emulsion which washed out fine.

The tank thickness was at least 2X what more modern ones are.

I once passed all my pressure vessel welding tickets though it was a long time ago so I had a few clues.

I do a lot of welding and have the range of electrodes for the job - in this case I used E4111 deep penetration rods and weld-on fittings not crappy hardware store pipe fittings. That weld is not going to fail at 1000 psi let alone 120 psi.

Wouldn't do it for anyone else though unless the tank was to be hydro-tested to the specified overpressure afterwards.

The trouble with rust is, you can really be chasing your tail. Unless it was very localised, I wouldn't bother. Anything bigger than a pinhole is going to require cutting out, forming and butt welding in place a new piece of steel and knowing you're getting 100% penetration so the weld is as strong as the parent material. This assuming the tank is just plain mild steel - if it's a high strength low alloy material, I wouldn't touch it.

If anyone suggests welding a patch over the outside, walk away. Moisture *will* get in the lap joint, scale and expand eventually rupturing something.

That said I've never heard of an air receiver *exploding*. There's no flammable gas in there. Typically a pinhole gets bigger, the air leak increases, the tank gets tossed out. A weld seam failure and catastrophic decompression is really, really unlikely. Even aluminium SCUBA bottles don't explode, at least I've never heard of one. They make good missiles if you break the valve off of a full one, though.

PDW

Originally Posted by PDW

That said I've never heard of an air receiver *exploding*. There's no flammable gas in there. Typically a pinhole gets bigger, the air leak increases, the tank gets tossed out. A weld seam failure and catastrophic decompression is really, really unlikely. Even aluminium SCUBA bottles don't explode, at least I've never heard of one. They make good missiles if you break the valve off of a full one, though.

PDW

There are a few on YouTube

This one is security camera footage and shows the actual explosion and someone getting badly injured
https://www.youtube.com/watch?v=KVP_A7eGYxw
There are no gory close ups except some stills of the tank pieces at the send - stay to the end to see these.
This is serious stuff - yet another good reason to put compressors outside in their own little box.

Originally Posted by BobL

There are a few on YouTube

I've read that these explosions are nothing to do with rusted cylinders. Its enough oil mist getting into the cylinder to ignite. Its a nice story, though I'm not so sure. what do you think?

Stuart

I had a quick look in the Engineering Peer Reviewed literature and there is an excellent article on an investigation of a receiver explosion in the US in 1983 that crippled two workers at a tyre recapping plant.

The compressor was a big-un, 1000L, 20HP, 180psi, and it demolished one end of the plant with an explosive force estimated to be equal to 3lb of TNT!

The investigation found all the OHS precautions/testing/inspections etc was adhered to and that the cause was a faulty tack weld at one of the supporting legs.
Tank was 1/4" steel.

Here are the conclusions of the investigation.

• The controls and safety devices were functioning properly at the time of the explosion.
  
• The tensile properties of the SA-445-A steel were within the ASTM specifications for this steel.
  
• The fracture origin was a semi-elliptical, surface fatigue crack which originated at a leg tack weld.0 The tack weld, which exhibited incomplete fusion, slag includions and excessive porosity, was apoor quality weld.
  
• Leak-before-break criteria were not met because of weld embrittlement. The fracture propagated circumferentially along the tack weld rather than through the tank wall because of embrittlement of thematerial surrounding the weld. Once the fracture reached a length of several inches it propagatedlongitudinally over the length of the pressure vessel and then around the heads of the tank.

Originally Posted by BobL

I had a quick look in the Engineering Peer Reviewed literature and there is an excellent article on an investigation of a receiver explosion in the US in 1983 that crippled two workers at a tyre recapping plant.

The compressor was a big-un, 1000L, 20HP, 180psi, and it demolished one end of the plant with an explosive force estimated to be equal to 3lb of TNT!

The investigation found all the OHS precautions/testing/inspections etc was adhered to and that the cause was a faulty tack weld at one of the supporting legs.
Tank was 1/4" steel.

Here are the conclusions of the investigation.

Interesting read and the date - 1983 - indicates this isn't exactly common either otherwise there'd be a ton of incidents more contemporaneous. Maybe there are, I lack sufficient interest to look myself, but I doubt it.

I didn't say it couldn't happen, just it wasn't common and I hadn't heard of an example. That tank is a pretty big size, well outside anything someone on this forum is likely to have at home. I have a 25HP Hydrovane compressor but don't have a tank that big, and the Hydrovane compressor is limited to 120 psi.

That tank was incorrectly welded at construction leading to embrittlement and fatigue failure in service. I'd be very cautious, myself, about welding on a high strength steel alloy tank, in fact I wouldn't as here you really are getting into the need for proper weld preparation, pre-heat, post-heat and weld sequence and I've forgotten all I ever knew about how to do it properly.

This is a very different failure mode to a tank that has a rust hole, though. I've seen quite a few dead air compressor tanks at the local tip, not seen one opened up by a catastrophic pressure release.

Bottom line, you have to exercise caution with any pressure vessel. I have a fair bit to do with high pressure air as a client of mine owns a licensed filling/test station and I won't make fittings for him (nor would he ask). But welding on a mild steel air tank of modest size and pressure doesn't involve huge risk if you know what you're doing with a welder, assuming reasonable wall thickness. If it was less than 3mm wall I think I'd junk it and buy another tank.

Not that hard to hydro-test to 4X working pressure, either.

If you want to see something really hair-raising there's a video clip or 2 on www.submarineboat.com where he's constructing a pressure vessel to test equipment in. Entertaining in its own way but more an example of what not to do.

PDW

Out of interest, any ideas as to how thick a tank of this nature (110L Peerless) should be? 3mm sounds thin to me, but then again a quick calc based on length and dimensions known would suggest it cant be anymore than 4 or 5mm thick.

Taking average steel density into consideration, and approximate weights for motor and pump, 5mm thick would also be an absolute max thickness.


Clearly then a few mm of steel thickness lost to rust / corrosion soon becomes very significant!

The pressure vessel inspection guys when they come to work have an eddy current probe so that they can check thickness. For our 125l compressor it was 4mm I think. I'll second the call for getting the bung out and checking out the inside. No good patching a leak if there is another one waiting in the wings. My personal preference would be to find another tank - whether from ebay, gumtree or what ever. The compressor and motor could well be fine so could be removed and run separately from the tank. You'd get better condensate removal that way too (less rust in the new tank).

Michael