Question about VFD's and Speed Control.

[Todd Glover]

I recently purchased a 1971 PM90 that the previous owner had taken the 3phase motor off of and replaced with a single phase 1hp motor. I'm quite happy with the power of the machine so far, but as I start turning larger things I'm thinking a min speed of 500 might be too much.

So my question is, to get the lower speeds that I see some of you folks talking about in some of your posts, is my option to put the 3 phase motor back on with a VFD/Speed controller on it?

I'm not interested in doing HUGE bowls, but I am quite interested in safety. So while I understand the PM90 can probably handle some fairly good sized stock that isn't perfectly round without walking across the shop, I'm not so sure a large out of round piece wouldn't shear the connectors I use to attach to the faceplate and put a rather large dent in my rather large noggin.

Thanks again for any input.

 

[john lucas]

If you still have the 3 phase motor that is definetly what I would do although you might need some help buying the correct VFD.

 

[Bill Turpin]

Be careful with VFD

Do not purchase a VFD unless the seller will help you program it! VFD manuals are some 150 pages. There are ± 250 programmable parameters, with about 15 that need to be changed. The horsepowers/voltage/phase MUST match. A 230v/1phase VFD can create the third phase. Do not purchase a used VFD without the manual. Downloading and printing from the internet takes forever. It is difficult to read internet instructions on your puter while kneeling in the sawdust correcting programming errors. I have twenty years of experience with VFDs, some of the instructions still take a EE to understand. VFD controlled lathe motors are the only way to go.

 

[Bill Boehme]

The old motor probably is very not suitable for inverter duty. If the nameplate shows the insulation class to be anything less than Class E, the motor will probably die a slow death caused by insulation failure in the windings. Many woodturners use motors that are not rated for inverter duty and say that they have not encountered any problems, but the failure is insidious so most people do not recognize it. The cause of the failure come from the method that the three-phase power is synthesized in the inverter. Inverters use pulse width modulation (PWM) with a carrier frequency that is nominally around 5 kHz. The amplitude of the PWM signal is a few hundred volts, but because the motor represents an electronically inductive load, the leading edges of the square wave pulses can have an amplitude well over a thousand volts. The winding insulation in older motors is thin compared to that of inverter duty rated motors, so insulation breakdown between adjacent wires is likely, especially where the wires wrap around the ends of the winding slots on the stator laminations. This type of failure is not immediately noticeable, but each time that there is an insulation failure between wires, the output power of the motor decreases a tiny amount until it reaches the point of avalanche failure. So, five thousand times a second the motor stator windings are being hit with a high voltage transient that is between one and two thousand volts. Class A and B insulation can't take that for very long.

There are a couple other considerations in choosing a motor. The normal general purpose motor is designed to operate at one speed and its internal cooling is optimized for peak efficiency at that speed. Running a motor much slower than its base speed can result in the internal temperature going extremely high which can exacerbate the insulation failure mode that I described above. The other consideration is that the bearings used on general purpose motors are rated for the base speed. If the motor is operated at much greater than about 125% of base speed, the bearing life will be greatly reduced.

In addition to all of he caveats about the motor, something very important to realize is that the lathe spindle power output will not be comparable to using the same motor with a stepped pulley drive arrangement. With the mechanical drive, the available output power will always be equal to the motor power minus small transmission losses while the electronic drive system will only be able to produce the motor rated power output at base speed. Below base speed, power decreases linearly with speed.

Having said all of that, a properly designed VFD system is the way to go, but the key words are "properly designed" which is much more complex than just slapping any motor onto the machine. It also includes motor sizing and additionally using the pulley drive ratios for optimum performance. Otherwise, it is easy to wind up with a lathe that is a high tech poor performer.

BTW, I've never had a problem with 500 RPM as a minimum speed.

 

[KurtB]

Speed Control

Todd,
If you are a relatively new to turning, your lathe will be fine for spindles, peppermills and smaller pieces that do not weigh much. The concern starts when you mount an out of round, unbalanced block on the lathe, particularly if it starts to weigh in excess of, say, 20#. Even then, if you take the time to balance the load of the piece, and prepare it properly with a bandsaw so that it is nearly round, many of the problems will simply go away.

In the early stages, many turners would be well advised to spend that VFD money on a bandsaw, or upgrade their bandsaw, so that blank prep would be improved, and the 500 rpm minimum would not be a problem.

Don't get me wrong, VS is wonderful, and you should go for it when you can afford it; but you might want to wait until you're more sure about what it is you most want to turn, and then buy the lathe most suited for that.

 

[KEW]

I agree with KurtB. Take your time and figure out what you want in a lathe before making this one into something that it is not.
IMHO, unless you are in it because you enjoy the project of adding VFD, it is a mistake to add it to a PM90.
It is, as it sits in your shop (with Reeves drive), an excellent spindle machine. With the maximum swing of 12" it is inconvenient to turn anything over around 9-10" (blank prep, positioning of banjo to avoid interference, etc.).
If you can figure out how to take advantage of the gap bed, let me know. By the time you use a chuck or other mounting system you can't do much but the flattest of platters.
Outboard turning is a possibility, but it is a whole 'nuther project to get both your lathe and outboard tool stand where they will be rigid (or at least move in unison) - the base of the PM90 is its weakest aspect.

I don't mean to berate this lathe, it is truly a great machine; however at the end of the day it was designed for turning spindles. There are people who have successfully made a project of converting them into a decent bowl machine.

I had one for my second lathe (after a mini) and learned a lot on it. I thought and thought about upgrading it with VFD and getting the outboard setup tight (I had the freestanding toolrest). However, once I considered the time and money required, I decided to sell it and used the money to buy a used Nova3000 (manual belt change). The Nova possibly may wear out while there was no way I could have worn out the PM90. The PM90 is definitely more machine, but the Nova better targets the type of projects I make.

Most of us like to reaffirm our decisions and I suppose I'm no different, but I'd recommend you do as I did. Use the PM90 to learn spindle turning and it is a fine lathe for small (say 8") bowls. During this time, go ahead and accrue more tools and accessories, refine your turning and sharpening skills and figure out what you are looking for in a lathe. If you decide spindles are your thing, stick with what you have. It is also a good machine for ~6" diameter hollow forms.
If you want to turn salad bowls, find a lathe with lower speeds and enough swing over the ways so you can use the tailstock with them (not an option for outboard turning).
I suspect if you do decide to sell the PM, you won't loose a penny. The difference between a 38 year old PM90 and a 40 year old PM90 associated with use by a hobbiest doesn't impact its value.
Good luck and enjoy yourself!

 

[Dave Peebles]

Hi Todd,

Years ago I had a reeves drive Boice-Crane lathe. It also came with a 3 phase motor that I changed out to a single phase. The minimum speed was around 500 rpm. That was great at the time because all I was turning were spindles for Shaker chairs.

But as I got hooked on bowls and such I felt like I needed a little slower speed. What I did was changed out the motor pully... or sheave for the purist to a smaller size. I believe it came with a 4" and I changed it to a 2".
I used a link belt to get the proper tension. I was told later that you cannot use a link belt on a reeves drive, but it worked for years that way with no problems. I am happy I didn't hear that warning before I tried it.

One other thing you might try. If you can get a copy of the Del Stubbs woodturning DVD, he has a very nice section on his method of making a lathe variable speed. It is low tech and works great.

Good luck,

Dave

 

[Todd Glover]

Alot of great information here so far, thanks a ton.

I guess I'm going to have to figure out an outboard toolrest system as I will definitely want to turn some bowls and I don't think I'll be upgrading anytime soon. (Made the mistake of telling the wife this will be the last lathe I'll ever need )

I have all the tools I currently need (until I start hollow forms), and I have the sharpening gear, now it's time to start turning some practice bowls.

I'll get some rough dimensioned on the bandsaw and see what I can do from there I reckon.

Again, thanks for the advice.

Here she is next to the one she's replacing:

 

[KEW]

Looks nice and brings back lots of fond memories.
What is the replacement cover on the outboard side of the headstock about?
One thing you'll want to work out for outboard turning is go ahead and find/buy the LH insert to adapt your chuck to the outboard spindle. I had trouble fitting mine - apparently they didn't always use the same thread.
Mine was a '78 (made in USA) model.

 

[MichaelMouse]

I had the Delta 4 speed 12" with its computed minimum of ~600. With a decent bandsaw, and with an occasional lag opposite the heavy spot on weird stuff, design permitting, it didn't move much. If you can't do a lag, perhaps a couple of Forstner bores in the heavy area can do it. I still like to be nice to my lathe rather than brag/complain in forums about how heavy and out-of-balance it can/can't turn without wobble, so I trim up with the bandsaw.

If you want to turn outboard, tie the rest to the lathe so it moves together if it moves. Opposite movement of rest and work can spoil your day. If you need to slow it, you might consider a DC motor if buying new, or a rotary converter for the motor you have. Keeps things more sinusoidal.

 

[KEW]

Todd,
What type of chucks do you have?
A 1-1/2" by 8 tpi insert cannot be had for any of the current Nova chucks and I believe only the Stronghold of the Oneway chucks can be fitted.
However, if you have Nova chucks and want to keep on using your jaws, this guy has some of the Original Nova Chucks. He may not have a 1-1/2" X 8 in stock, but he also runs a machine shop and this chuck has enough meat that he can drill and tap it to fit.

http://www.timstoolcrib.com/

Tim is the Teknatool service rep for the USA. I got my Nova chuck for the PM90 from him. He is a good guy to deal with.

If you don't have Nova. Grizzly sells a knock off of the VicMarc chuck with a 1-1/2" X 8 insert.

 

[Todd Glover]

I have the Oneway Stronghold.

I talked to Peter at Oneway this past Monday and ordered the correct insert. He told me it would be here by Thursday, but I haven't seen it yet. Was hoping to do some turning this weekend but haven't been able to.

Probably a good thing it didn't show up yesterday, the wife probably wouldn't appreciate me spending all day in the shop on mothers day!!