Phase converter for compressor

[Joe Jensen]

I have an opportunity to buy a used Quincy QT5 comressor that seems to be in good shape, but I can't see it run as it's 3PH and the owner has no 3PH. How much do I need to spend for a compact phase converter for a 5PH compressor. Should I swap the motor instead? Given my nature, I would not hook and unhook a phase converter every time I needed the compressor, so this converter would only be for the compressor. Is a static converter ok for this? How much current will it draw for a 5PH 1PH?

[CPeter James]

Qunicy is a top of heap brand name. I would not run an air compressor on a phase converter as the start up load is extremely high, even with an unloader, the starting friction is way up there. If you can buy it right (Under $500) and then pick up a motor from say Northern Tools or somewhere for not too much, then you might be OK. I think you are getting too much money into this and might want to look at an IR Type 30 or something like it for not much more and have a new unit.

CPeter

[David G Baker]

I agree with CPeter. If all else is in very good condition and the price is not to high a new motor is probably a better idea than a phase converter.

[Bruce Page]

I have always indorsed the static type phase converter but I don’t think they would work on a compressor. I think the load would sit too long and blow the internal capacitors inside the converter.

[Greg Funk]

According to www.phaseconvertor.com you would need a model 6000-11kw phase convertor which sells for $1,095. Unless you planned to have other 3 phase gear you are probably better off getting a 230V, 5HP single phase motor. If you don't buy it new make sure it is rated for starting compressors as they generally require high starting torque.

Greg

[Rob Russell]

Sigh,

First - I'm curious as to whether or not any of the previous posters actually have any 3-phase equipment or understand how static convertors vs. rotary convertors work. I'm not trying to be nasty, but there are a lot of opinions posted about 3-phase that don't properly lay out all the facts, from folks who have read stuff on forums and propogate it.

For example, if a machine is setup for 3-phase and you replace the motor with a similar HP single-phase, 240v motor - are you going to think about checking the the control wiring or make sure that the motor-starter is appropriately sized?

Do you really understand how a static convertor works? There are some static convertors that just get the motor started and then cutout, which gives about a 50% net HP rating. There are also static convertors that get the motor started but also include run capacitors to help balance the phases and those are the ones that higher efficiency ratings.

= = = = = = = = = = = = = = = = =

I suggest you read the material at Rick Christopherson's site and then come back to ask some more questions.

Before folks ask, yes I have 3-phase equipment. I was building a multi-idler, remote-controlled rotary phase convertor (RPC) when I needed to stop and put the schematic on something other than a piece of notebook paper. My RPC was designed with push-button stop/start from multiple stations, auto-trips for overcurrent and load-balanced capacitors tied to the idler motors, all that good stuff. Attached is a picture of the control panel for the RPC when I stopped building it. I had the opportunity to buy a PhasePerfect electronic phase convertor at a great price, which gives me true 3-phase power with none of the isses of a "wild leg" that comes off of a rotary convertor.

Rob

[Greg Funk]

Rob,

Your post illustrates why, if the OP only plans to run one piece of 3PH gear, that it is probably less expensive and simpler to just switch to a single phase motor. Quincy sells the QT-5 compressor with 3PH or 1PH 5HP motors so it's not like it will be a difficult switch.

Greg

[Rob Russell]

Rob,

Your post illustrates why, if the OP only plans to run one piece of 3PH gear, that it is probably less expensive and simpler to just switch to a single phase motor. Quincy sells the QT-5 compressor with 3PH or 1PH 5HP motors so it's not like it will be a difficult switch.

Greg

I apparently didn't do a good enough job of explaining myself.

You can't just switch out a motor without looking at the rest of the wiring for the device.

[Rick Christopherson]

There are some issues with phase converters on compressors. The first is that the motor starts under load, and the second is that the motor starts automatically. Because the motor starts under load and operates at full-load most of the time, a static converter is not going to be a good situation.

A rotary converter will start and run the motor just fine. Because the motor starts automatically, the rotary converter needs to be left running unless some additional control circuitry is added. With an on-delay relay, you could have the compressor's control turn on the converter, and then a split second later, the delay relay would turn on the compressor.

A better alternative is a variable frequency drive, or electronic phase converter. For the VFD, you would have to oversize the VFD to account for the single-phase input when the VFD normally wants 3-phase. The drawback to these is that you can't build one yourself.

[M Toupin]

I apparently didn't do a good enough job of explaining myself.

You can't just switch out a motor without looking at the rest of the wiring for the device.

Now Rob, you know you'll have two extra phases worth of parts left over We sell off those extra 2 phases worth of parts and it'll all be a wash

Mike

[Joe Jensen]

Thanks for the responses. I did some looking and I think I can get a new 5HP 1PH Leason delivered for between $300-400. I am sure I would have to do something about the mag starter. It might be possible to change some elements in the starter, not sure. The rest should be simple. The compressor is $500...joe

Update, I just found this posting online claiming that most mag starters for 3PH work for 1Ph too. I need to find an electrician.
http://www.hotrodders.com/forum/new-...elp-75695.html

[M Toupin]

A motor swap will be the most cost effective unless you intend to get into more 3ph machines. For example, a VFD would run you in the $800 neighborhood minimum. The starter on the machine now will probably work with single phase, though you'll need to do a bit of rewiring. You'll also have to replace the heaters in all likelihood. It's not difficult stuff, but you'll need to educate yourself on 3ph and starter technology. OWWM.com has a decent wilki with info on starters.

Mike

Thanks for the responses. I did some looking and I think I can get a new 5HP 1PH Leason delivered for between $300-400. I am sure I would have to do something about the mag starter. It might be possible to change some elements in the starter, not sure. The rest should be simple. The compressor is $500...joe

Update, I just found this posting online claiming that most mag starters for 3PH work for 1Ph too. I need to find an electrician.
http://www.hotrodders.com/forum/new-...elp-75695.html

[Rob Russell]

Rob,

Your post illustrates why, if the OP only plans to run one piece of 3PH gear, that it is probably less expensive and simpler to just switch to a single phase motor. Quincy sells the QT-5 compressor with 3PH or 1PH 5HP motors so it's not like it will be a difficult switch.

Greg

Greg,

I should have explained my post/picture more ... call it me being a bit cranky last night.

I was building an complicated rotary phase convertor (RPC). Most RPCs have a single idler motor (the extra 3-phase motor you use to generate the 3rd leg). I was using 2 idlers in my design. Why? You want to balance the voltage and current across the legs in a RPC and you use oil-filled run capacitors to do that. Problem is, if you have loads of significantly different size to run, the balancing becomes problematic.

I chose to build a RPC that used a small 5HP idler that could be running when I wanted to use a lower HP machine, like the 3-phase Rockwell radial drill press which only has a 1HP motor or something like that. The main load for the RPC was the Felder equipment which, when running with the dust collector would be something like 13 HP. Designing and building an RPC control panel for that is WAY more complicated than most people need. For something as simple as what Joe wants, a simple single idler RPC would be fast to build and cheap.

In fact - Joe I have the 5 HP motor to get rid of - if you want it for free (just pay shipping), send me a PM.

Building a single-idler RPC isn't difficult and could be considered fun. You definitely learn about electricity, although I'd never call my self an EE like Rick is.

[Rob Russell]

A motor swap will be the most cost effective unless you intend to get into more 3ph machines. For example, a VFD would run you in the $800 neighborhood minimum. The starter on the machine now will probably work with single phase, though you'll need to do a bit of rewiring. You'll also have to replace the heaters in all likelihood. It's not difficult stuff, but you'll need to educate yourself on 3ph and starter technology. OWWM.com has a decent wilki with info on starters.

Mike

You can get a 5HP VFD that will run from 240v single phase current for $600 or less (sample VFD at driveswarehouse.com).

[Rick Christopherson]

You can get a 5HP VFD that will run from 240v single phase current for $600 or less (sample VFD at driveswarehouse.com).

Whoa there little buckaroo!!! You found a single-phase 5 hp VFD at this price?!?

That changes everything!

Leave this 3-phase motor in-place, and get this VFD. When starting under load, a 3-phase motor will start Far smoother versus its single-phase counterpart. Even if you could buy a new single-phase motor for $100 less than this VFD, I would still pay the extra $100 to get the VFD. You will get more performance and longer life with this combination, as compared to the single-phase change-out.

Nice going Rob!. I haven't gone looking for one of these before, but this is the first time someone has shown me a 5 hp single-phase VFD.
=================
By the way, if you go this route, the VFD completely replaces your motor contactor and its overload heaters, so you could sell the contactor for a decent price and completely make up any difference. The compressors pneumatic control switch would feed directly into the VFD control. For a compressor, make sure you disable the soft-start and soft-stop of the VFD.