View Full Version : Motor Question

12-19-2008, 10:57 AM
The lathe I'm rebuilding came with a 1 HP 3 phase 220 volt motor. I only have single phase power at my shop. My question is - Do I replace the motor with a single phase one or get a phase converter to run the 3 phase motor?

As to price, the new single phase 1 HP motor is the same price as a solid stage phase converter. On the up side, the 3 phase motor fits in the machine but on the down side, I have no way to test run it. The shaft turns freely by hand so at least it turns. Are there any electrical checks I can do to see if the motor is good? I'm electrically challenged and the only electrical measuring device I have is a VOM.

Is there any "advantage" to the three phase motor over the single phase? The current draw for the three phase motor is rated at 3.6 amps while the single phase motor I'm looking at draws 14 amps. To me the single phase motor would use more electricity than the three phase but then again, how much is lost in the phase converter?

Would appreciate any help from those who are friendly to and conversate with flowing electrons.


12-19-2008, 12:08 PM
3 ph is a better power source for turning machines, I take it you didn't get a chance to hear it run if your asking about the motor?

I've never used a static converter so can't comment on their operation, this is a smaller hp requirement so a VFD maybe ab option for you.

You can test the unhooked windings by either setting your meter to ohms and comapring the windings resistance, you will have to test L1-L3, L1-L2, L2-L3. After that once it's running you can take amp readings, if it turns in reverse switching any 2 connections will make it turn the other way.

Theres really no savings on single phase power of different voltages specifically 115/230 as though the nameplate info for 230 is half you have 2 115 leads suppling it so you are still consuming the same amount.

A VFD maybe the cheaper route as you can probably setup all your parameters in it's menu, using a converter you should have a starter and heaters to protect the motor which is all additional equipment.

Forrest Addy
12-19-2008, 04:14 PM
There's choices and there's choices. Everything depends on your budget but don't change out that three phase motor with a single phase until you;ve explored every alternative.

A VFD will run your lathe motor to full HP with frully balanced three phase juice from a single phase service. This device will also give you variable speed at constant torque over a wide range. You will still need the mechanical speed changes on your machine for heavy stock remaval but the variable speed is very convenient for "filling in" between speeds and of course wide ranging for light cuts. Their drawbackss are only one motor at a time can be supplied from a VFD and they cannot be connected to electrical apparatus other than motors. A 1 HP 115 single phase input VFD from Dealer's Electric cost's $133 and up.

You said "solid state" phase converter. I wonder if you don't mean "static" phase converter. This is a box with capacitors and a start relay. They are every light and not particularly desirable for machine tool service. They will run a three phase motor from a single phase service but the motor will only develop about 1/2 the motor's nameplate rating.

Then there is the real solid state phase converter where the third leg is electronically converted from the single phase service voltage via sophisticated power electronics. These work very well, drive the motor to its full HP, serve any associated electrical equipment but are quite expensive.

The rotary phase converter is a very satisfactory solution to runnng three phase equipment from single phase power. They can be made to most any degree of sophistication in the home shop often using salvaged components. Their drawbacks are added noise, some additional electrical cost of operation, and the motors they drive develop 70 to 80% of their name plate power.

For a single motor as in a lathe drive, VFD is probably your best choice. If budget is a problam I suggest a home made rotary phase converter. Frankly I wouldn't consider a static phase converter. An electronic phase converter as from Phase Perfect is overkill for a small home shop machine.

If I may tout myself as an axample, I have five machine tools on VFD's. I used to use an RPC that while satifactory to a point didin't permit full HP cuts. It was noisy and added about $6 to the power bill (1994 prices.)

12-19-2008, 04:18 PM
The lathe was bought in boxes. It was to be a restoration project for a man who passed and I bought it from the estate. The motor was in one of the boxes so I never had a chance to see the machine running.

Thanks on the tips for checking the resistance of the leads.

The phase converter I was calling a "solid state converter" was just an electrical box, no motor (in the $180 range). The rotary phase converters I looked at started in the $700 range and went up from there. I would be interested in getting more information on the "build your own" converter. As a said, I'm not friendly nor conversate with electrons!


12-19-2008, 04:38 PM
To run a single 3ph machine in your shop, get a small VFD. You can get 1 to 1.5hp VFDs cheap on ebay, sometimes on Craigs List, or from Classifieds on sites like PM or Woodworking. Very easy, and can be very convenient particularly with older machines using the variable speed option.

If you want to supply a shop with multiple 3ph machines in the future, RPCs are easily built. Idler motors in the 2 to 3hp range are easily found, often free or nearly so (check with AC service/install companies or scrappers). Plans are on the Internet and can be as simple as a cheap start cap with a relay and momentary switch to engage the start cap. This simplest RPC works well enough, but you need an idler at least twice (or more) the size of the load motor if you plan to put much strain on the load motor. Step up to timers or potential relays, add in balance caps, go as far as you want or need to. I recently finished a new dual idler RPC with full set of controls and independently selectable levels of 5hp, 10hp, or 15hp. Not hard at all really, but I had to educate myself a bit as well as relying on the help of generous folks helping out on forums...

Ed Tipton
12-19-2008, 04:56 PM
Big Boy 1...I am admittedly not up to speed on the vfd's. I've read a lot about them on this forum, and I know they are favored. Having said that, they are relatively expensive. I have two three phase lathes and one mill in my shop. I run all three from the same rotary convertor. I built this convertor from surplus parts for about thirty dollars. the largest purchase will be the motor, and for your needs, I would suggest something in the 2-3 HP range. I admit that I had some of the required parts lying about...but if you purchased the entire thing I think $100.00 should get you all the way home (buying surplus). There is plenty of information available on different designs, but they are all pretty easy, and should be do-able for someone with a limited electronics background. My unit has performed admirably for about 15 years now without so much as a hiccup. I've read all about balencing the leads etc. and I did some of that... but the main thing is to use plenty of capacitors since any balancing will only be balanced for a specific load. For all of my purposes, I don't feel that the balancing is all that critical. My single convertor will run all three machines either singly or all at the same time, and given my success and the cost factor, I'd be hard pressed to recommend anything else. Dont be intimidated by the "electronics, it's really not much more than a single afternoon project. Of course, you can spend additional time and money "prettying it up", but you can get up and running quickly and cheaply.
For both your lathe motor and the convertor motor, there's really not very much to go wrong with them. A quick check of the bearings should probably tell the tale. If it rotates smoothly and quietly, your probably good to go. As for additional noise, mine is negligible. In my shop, with everthing turned off and nothing making noise, you have to "listen" to make sure you aren't walking away and leaving it turned on. Go for it!

12-20-2008, 02:37 AM
Most will say the static converters are not any good but I have been running a 2 HP BP on one since 1987 without a hiccup. I also use one to run a 5 HP
24" swing, turn of the century (1900) lathe with no problems at all.
I now use the 5 HP motor, on the 24" as a rotary phase converter to run my SB 16" lathe. I start the 5 horse with the static converter and let it idle as the RPC. I have seen reference to balancing the output of each leg but have never done that.
All of the VFD users I've seen need some sort of help to get up and running where as the static converters are plug and play.

12-20-2008, 09:20 AM
My VFD was pretty much plug and play. Read the manual to find out where the wires go. Power on and then follow simple instructions to enter data from the load motor name plate. Done...

Bill Pace
12-20-2008, 09:25 AM
Another vote for VFD --- comparable cost and while requires a bit of figuring out, probably not any more than the other routes, and with the added advantage of the variable speed..... Well, IMO theres just no other way to go!