PM66 3phase motor

[Andy Livingston]

I bought a used PM66 with a 3 hp 3 phase motor from an estate sale. I think I got a great deal. It came with a 7.5 hp rotary phase converter that is in excellent shape. I am thinking of selling the phase converter and replacing the motor with a single phase model.
Two questions:

1) Where would I go to find a new/good rebuilt single phase motor for this saw?
2) If it was your saw, would you just keep the phase converter and run the saw as is? I would probably need an electrician to hook it up for me. I am comfortable working with single phase power, but have absolutely no experience with 3 phase.

It is necessary to turn on the converter seperately from the saw, and it will use a lot of juice. I am thinking it will be better in the long run just to replace the motor.
Looking forward to replies from those who know or have been there!

Andy

[Rob Russell]

I'm going to assume that the saw has a motor starter on the back and the actual start/stop button is controlling the contactor.

If that's true, you can't just replace the motor. You'd also have to replace the motor-starter and ensure that the "heaters" or overload protection were sized to the single-phase motor. There is also a good possibility that you'd need to replace all the wiring (except for the stop/start switch) because of the increased ampacity requirements. The cord and plug would need to be replaced with at least #8.

Going to a single-phase motor when it's 7.5HP isn't as simple as a smaller motor.

[Andy Livingston]

Rob,

The phase converter is 7.5hp. The saw itself is 3 hp. I was planning on replacing the motor, motor housing and the start/stop switch. I didn't realize there would be more wiring to be concerned about. I may just end up hooking up the converter and going with what I have.

[Charles Shenk]

Instead of using more electricity your saw will draw less and will run better, especially under extended heavy uses. I am assuming all things are working properly. If it ain't broke don't fix it in my opinion.

What vintage is your saw? Got any pix?

[Karl Brogger]

How old is it to be a 3hp 3ph? I don't think I've ever seen one. (Which doesn't really mean much)

[Mike Heidrick]

Take some pictures of the motor, its nameplate, the ROC, and its plate and terminals and we can help you get it wired up possibly.

[Rob Russell]

Rob,

The phase converter is 7.5hp. The saw itself is 3 hp. I was planning on replacing the motor, motor housing and the start/stop switch. I didn't realize there would be more wiring to be concerned about. I may just end up hooking up the converter and going with what I have.

Yeah - it would help if I read your original post a little more carefully.

The difference in FLA current draw for a 3HP 3-phase vs. single-phase motor is roughly 10 amps vs. 17 amps.

If you don't mind running the RPC, it gives you the ability to get other 3-phase machines which frequently run far cheaper than single-phase because of the power requirements.

[Rick Christopherson]

Instead of using more electricity your saw will draw less and will run better, especially under extended heavy uses.

If you don't mind running the RPC, it gives you the ability to get other 3-phase machines which frequently run far cheaper than single-phase because of the power requirements.

Nothing comes for free. It doesn't matter if the motor is single-phase or 3-phase, the power consumption from your single-phase supply will be the same, not including the consumption of the converter.

If the 3-phase motor draws 8 amps @ 3-phase, the incoming converter power will be 14 amps @ single-phase. The difference between these figures is the square root of 3. You need to make sure you take this into account when you size the incoming circuit. The same is true for the converter. Multiply the nameplate rating by sqrt 3, except the converter will only draw 50% nameplate amps, plus the tool motor amps.

When your converter is running, it will draw about 15 amps, however, this is at a very low powerfactor, so the cost to run an idling converter is nearly zero. It would be equivalent to about 2 amps of real power consumption.

Oh, what Charles said is correct in that the 3-phase motor will perform better than its single-phase counterpart, but there is no savings of electricity in the process.

[Rob Russell]

Rick,

My point was that used 3-phase machines frequently cost less to purchase than used single-phase machines, not that there is any savings in running the 3-phase machines.

Rob

[David Freed]

I would be more likely to change one of my 1 ph machines over to 3 ph rather than the other way around. I will use 3 ph wherever I can.

[Rick Christopherson]

If you don't mind running the RPC, it gives you the ability to get other 3-phase machines which frequently run far cheaper than single-phase because of the power requirements.

OK, I understand what you were trying to say. Instead of "run far cheaper" you meant "cost less to buy", which is typically true.

[Andy Livingston]

Thanks for all the responses guys. I have gained a lot thru this discussion! I'm not certain what I'll do yet, but when I do get it set up I'll post pics and details. I also received a great private message giving advice on auction site postings for single phase motors and shaft size info.
Thanks again for all the responses. If anything else comes to mind, feel free to post!
It is an older saw, and it is in great shape. It came with a relatively new accufence and a Forrest WWII blade in also in great shape. I've got a couple Forrests, so I may sell the blade. I got the whole thing for 1000, including an almost new 7.5 hp phase converter (they go for over 500 new). I could part this thing out and make money, but I really want the saw.