Running a 9 hp saw in my garage - realistic or not?

I have my sights on an SCMI (Italian) sliding table saw with a fairly big 3 phase motor and was wondering if it is realistic to run it in my residential setting off of a rotary phase converter due to the current draw. I have 200 A service to my house and I plan to install a 100 A sub panel in my shop where the saw will be. The saw has a 9 hp main motor and a 1 hp scoring motor. I’d use the scoring motor infrequently, but when used it runs at the same time as the main motor. I was originally hoping to get by with a static converter, but after reading posts here it’s obvious that a RPC is the way to go. Also, the saw has electric controls that raise/lower the blade as well as controlling the blade angle, and I have a feeling that a static converter might cause problems with the circuit boards. I’m guessing the saw would be a “medium” start up load, and with 9 hp, I doubt it’s ever gonna’ bog down.

Assuming I can get my hands on a surplus 10 hp idler motor, I’d like to try building a converter. If I take the time to balance the phase voltages, can I get by with a 10 hp idler or do I really need a 15 hp idler? A 15 hp idler seems scary big, would draw a lot more current compared to my planned 100 A sub panel, and the motor would be a lot more expensive and hard to find. Also, I don’t want my wife or neighbors wondering why the lights are flickering on and off.

After reading posts on this forum (and they’re great!) I tracked down Fitch Williams’ plans from 1997 (I think) and when I have time I’ll track down some of the other designs mentioned. I’m pretty mechanical and have a decent understanding of electricity and motors, but translating that into practical knowledge gained from experience is another matter entirely. It gets a bit complicated with all the different types of contactors, switches, controllers, and especially, where to reference/find them etc. I’m still trying to figure out what “heaters” do! Anyway, I guess I’d like to build a project that I can be proud of; something that works well and is relatively convenient to start up and run since the saw tends to go through a lot of on/off cycles (although, it looks like just letting the idler run continuously would cost only about $0.30 or so an hour). Whatever I build doesn’t have to be the absolute “dream” RPC though. So, I was wondering, is there a particular design that I should hunt around for, and have there been certain modifications to some of these basic designs that I should be aware of? Again, I’d really like to get this saw but I just didn’t know if it’s realistically feasible in my residential setting.

I really appreciate any thoughts and advice offered, and hopefully, I can contribute something here in the end. Thanks again!

Noah

Exact same service, 200 house, 100 sub in shop. 10HP Phase Perfect would have zero problem with what you intend, and I run mine on a 70 amp breaker as suggested by maker. RPC should be similar. I would not use a 10HP idler if the 9 HP main has any sort of starting load. Once the nine is going starting the scoring will pose no problem.

John Oder

SCMI is a great saw. Hope you can work things out.

To answer your question, you can probably run the saw but you may dim the lights when you start the RPC. I do not know how hard to start the italian motor will be but here is my best guess as to a RPC that should work in this application.

You can use an idler larger than the single phase circuit just to get more starting capability. If you use a 15 hp idler on a 60 amp circuit you can expect to run a about 11 to 12 hp total load. I would suggest this based on your loads and you shop circuit. I know of one of my designs operating this way but it is not as heavily loaded as yours will be. It is a capacitor start design.
It is unlikely you will find any posted plans for such a RPC but they are around.

If you want to minimize your start up current you can use a pony motor to start the idler. I have one of these almost built right now. I am using it as a demonstrator and only have a 7-1/2 hp idler however the control panel is designed to handle a full 15 hp. The design I am close to starting up is a belted pony motor where the pony motor remains belted to the idler at all times. It is a single push button start.

I think a 60 amp single phase circuit and a 15 hp idler (maybe pony motor start) will get you going and not brown out the neighborhood. RPCs do not use much electricity while idling so that is no issue in a hobby shop. A 15 hp idler will consume about 5 to 7 amps which is about 1 to 1.25 kilowatts per hour of operation. If you pay 10 cents per kilowatt-hour then it will cost about 10 to 13 cents per hour just to let it idle.

This is buildable by a mechanical guy but you will need to learn some electrical jargon and do some studdying. Getting the parts is the hard part. Once you have the parts the rest is mostly mechanical work and following connection diagrams. This is no small undertaking and you will have to spend a lot of time gathering parts, laying out and building.

The cost of a used 15 hp idler will be about the same as for a used 10 hp. it all depends on where you can find one. I usually pay about $100 to $150 for one known to run.

Send a private message and I will send a phone number. I do not have lots of computer time.

johnoder said:

Exact same service, 200 house, 100 sub in shop. 10HP Phase Perfect would have zero problem with what you intend, and I run mine on a 70 amp breaker as suggested by maker. RPC should be similar. I would not use a 10HP idler if the 9 HP main has any sort of starting load. Once the nine is going starting the scoring will pose no problem.

John Oder

Click to expand...

The phase perfect device sounds wonderful but unfortunately, it's $3000, which = BY BY SAW. My wife would shoot me!

Same shop, not using Phase Perfect. No light dimming. This could easily be a RPC, but isn't. Disconnect just has 50 amp slow blow fuses.

http://www.practicalmachinist.com/vb/showthread.php?t=158528

John Oder

9hp in garage

Hi Noah,

Depending on how electrically resourcefull you are; you may want to look into a autotransformer static converter. Sold commercially as "ADD-A-PHASE" by Ronk Electrical (not cheap). Google Ronk for info. They can be home built successfully using readily available single phase transformers. They are essentially a static with a run cap operated at a higher voltage. For your application a big advantage is the very low starting current, as low as 2-21/2 FLA and full hp capability. Plenty of info here on this Forum "Autotransformer Converters" or search my posts.

As John said Phase Perfect is the best but also the most expensive. The RPC is probably the easiest to build youself and there is a huge amount of info available here on this Forum. Your application would probably be rather large for a VFD since you would need to get a 20hp to run your 10hp on single phase. 20 hp vfd's ain't cheap.

Note Toolnut's accurate comments about building your own.

If you have buy a RPC is probably your best bet unless you can convince your wife of the virtues of a "Phase Perfect".

Joe