HELP! VFD and phase converter ???'s

HELP! VFD and phase converter ???'s · 07-13-2009, 10:53 PM

Ok, I know there are some electric guys out there that can help me out with this one. I am looking into some lathe stuff and can't seem to wrap my head around something.

I think I have the first part: If I get a 3 phase 3hp motor, I will need to get a rotary phase converter, and run a 220 line, right?

Now here is the part I stuck on: I was looking at some VFD's and they also appear to be phase converters, and are also used to control the speed of the motor???

In the end I would like to be able to set up an AC electric motor, have enough HP & torque to turn large items, but also be able to turn from 0-1500 Rpm.

Man I barely do the woodworking thin well, this electricty thing is making my brain hurt.

scsmith42 · 07-13-2009, 11:11 PM

All that you need is a 3hp VFD with a 240V single phase input, and a 240V three phase output.

Jeremy (Woodwrangler) has this same setup on his bandsaw (or bandsaw power feeder, I forget which).

With the above referenced VFD, no rotary phase converter is required.

Around $260.00, plus shipping.

Here is a good company (and reference) to consider:

http://www.driveswarehouse.com/Drive...0-022NFU1.html

Get your motor plate information and ask them for their recommendation. You'll probably be ok with a VFD that outputs from 0 - 60 hz. If you need to spin your lathe faster, then get a unit that outputs above 60 hz.

Also, make sure that the voltage on the motor matches up to the output voltage on the VFD - some motors are 240V, others 480, etc (also known as 220 and 440)

Scott

DaveD · 07-13-2009, 11:30 PM

Can't help on the VFDs since that isn't the route I took (didn't want to mess with the contactors in the lathe). On the other hand I have a 10HP rotary phase converter (RPC) setup I installed myself.

Machinery is typically going to require a RPC over a static phase converter because of the starting load. The static converter poorly fakes up the 3rd phase. the rotary gives you actual 3 phase.
In my case its a real 5HP motor on a gear head metal lathe. It was debatable if a 7.5 HP RPC would consistently spin up the lathes 5.0 HP motor. That's why I went with a 10HP RPC.

Bought the phase converter control panel from a place on EBAY (about $300), and the 10HP 'idler' motor (that's what you call it) I got from grizzly ($200).

A 1800 RPM idler is better than a 3600 RPM idler.
You should get a 5HP idler for that 3 HP lathe. Idlers can be bigger.IE a 10HP (or even bigger) idler in your case will work just fine. It jsut takes bigger wire/breakers/etc.

There are various ways to build a RPC. It can be as simple as slap the 220v single phase to the idler and spin the shaft up with a piece of rope (or a small single phase motor). The 'more correct way' is to do it with start capacitors and a contactor or two, then use 'run' capacitors on the RPC to even the 3 phase voltages out.

What I didn't like about most RPCs being sold (particularly on EBAY) is they don't tell you what is in them or give a schematic.

Go here to learn more than you ever want to know about VFd and phase converters. All kinds of designs and schematics. Pictures too.
http://www.practicalmachinist.com/vb...ns-101231.html
That site (practicalmachinist.com) is more for 'professionals' so be aware they 'may eat you alive' because they sure can be a contrary bunch over there (a wealth of machining info though).

If the only 3 phase machine you will ever have is the lathe then I'd seriously look into the VFD. Yes, it makes the machines motor a variable speed motor.

If you want to come see mine of talk about RPCs just let me know. I'll try to help.

Alan in Little Washington · 07-14-2009, 12:36 AM

Originally Posted by NCTurner

In the end I would like to be able to set up an AC electric motor, have enough HP & torque to turn large items, but also be able to turn from 0-1500 Rpm.

Sorry, that is not going to happen with any system.

First a rotary phase converter will make three phase from single phase input. I may be wrong but it can not be used to control the speed.

A VFD will convert single phase to three phase also, but allow you to control the speed if you want, but it will not allow you to turn a three phase motor from 0-1500 RPM without losing horsepower at low RPM.

A VFD works by varying the frequency of the three phase output typically from 0 Hz to as high as 400 Hz. Most induction motors are designed to run at rated speed on standard frequency of 60 Hz. When the frequency of the power supplied to a three phase motor is changed the motor responds by running faster or slower depending on the frequency. The problem comes with the design of the motor and the magnetic properties of the metals used. The magnetic field that turns the motor is weaker at low frequencies. As a result, the low end hp is greatly reduced and the motor can overheat as well if run at low RPM due to the magnetic properties and reduced air flow. Some VFDs are designed to counteract the loss of hp with variable torque, but that can result in even faster overheating.

What this means to a lathe is that hp required to spin a large blank at low speed during initial roughing may be insufficient and could result in an overheated or damaged motor if operated that way too long. You can compensate for the loss of torque to some degree with a larger hp motor, but you will still have the overheating problem and the prices of VFD's climb quickly as they get over 3hp. Another partial solution is to mechanically gear the motor so it runs very very slowly at 60 Hz. The problem with this solution is that you will need to overspeed the motor (premature bearing wear and a lot of noise) in order to get the spindle to turn 1500 RPM.

For this reason, most if not all WW lathes equipped with VFD's also retain mechanical variable speed gearing. The VFD is used to control the speed only within a range determined by the mechanical gearing.

Here is an account of one guys (metal) lathe conversion to a VFD. Note what is says near the bottom about low end torque (he means hp). I would guess hp demands on a WW lathe exceed those of a metal lathe.

mkepke · 07-14-2009, 12:52 PM

A VFD is *highly* recommended for powering a lathe since it adds speed control, braking and reversing. In fact, the big manuf. all produce VFD-driven wood lathes now, e.g. the Powermatic 3520.

What Alan says is correct about losing torque when you try to use a VFD to provide speeds outside the normal range of the lathe.

I have my Rockwell Heavy Duty lathe powered with a Teco FM100 VFD. I still use the lathe's mechanical Reeves drive to ensure best torque.

There is also a concern about overheating the 3ph motor if it wasn't designed for VFD control, but I haven't seen this when keeping within a 1/5 or greater range of the original RPMs. I think the danger zone is extended operation at 1/10th the normal RPMs (motor windings start to overheat).

-Mark

Bryan S · 07-14-2009, 01:04 PM

The only thing I could possibly add here is to be sure and install an overload relay to protect your motor

Alan in Little Washington · 07-14-2009, 04:22 PM

Most VFD's have (selectable/programmable) overload protection that protects the VFD and the motor, but you might want to get a motor with a thermal cutout. Also, don't put any switches between the VFD and the motor. Control the motor with the VFD controls only. If a spinning or decelerating motor is disconnected from the VFD then reconnected while it is still spinning the reverse current generated by the motor will fry the VFD's electronics. Most if not all VFD manuals have a big caution about this.

You might want to get the optional breaking resisitor too if you buy a VFD, plan to turn big bowl blanks, and want to use the VFD's ability to provide breaking. The internal circuits will provide some breaking, but the relatively cheap, add-on breaking resistor will improve breaking considerably.

Dusty Sawyer · 07-16-2009, 01:15 PM

I am facing the same issue. The lowest speed I can currently manage on my PM45 is 400 rpm... way too fast for those 100 lb+ out of balance stumps and burls that I have been trying to work with. I have visions of the dang thing getting loose and chasing me and the dog around the shop at that 400 rpm, not to mention warping a shaft or burning through a bearing.

I've looked at every possible solution with the pulleys, but there just isn't room to gear it down that way or to try and install a secondary arbor with two varied size pullies on it either. I'm thinking of setting up a temporary drive system on the inboard spindle for when I turn outboard blanks into round, a gear reduction motor, adding another mounting plate to accomodate that extra arbor and pullies, or just buying a Stubby, new PM or Oneway. No solution is optimal (or cheap)

I'd be extatic to get it down to around 100 rpm though from the 400, so maybe the variable unit would work for me - that's only 1/4 and not the 1/10th level where overheating and damage can occur, right?

David

Alan in Little Washington · 07-16-2009, 01:47 PM

That should be ok, but remember you need to replace the single phase motor with a 3 phase, one.

Another thing that you can do in the meantime. This is done by machinists and WW turners. It is also similar to how turned legs, spindles, etc. are made in furniture factories. One guy even used the technique to turn a huge 5'+ (10'?) diameter bowl- not a misprint, that is 5 feet! You mount the blank on the lathe, leave the motor off, then use a router or other powered shaping tool mounted in a jig to rough it and take off the high spots. Turn the blank by hand a little at a time as you go. Once it is round you can turn on the lathe motor and use gouges.

Dusty Sawyer · 07-16-2009, 03:53 PM

Hey, I just looked up a post I had put out there a while back and there was a reply that is quite uselful.

http://www.ncwoodworker.net/forums/f...59-post247863/

hope this helps you too

David

Dusty Sawyer · 07-26-2009, 11:25 AM

Thanks everyone for your excellent advice.

Looking at the cost and time invested to do this properly, I decided to start looking for a lathe better suited to what I am trying to accomplish. I heard the PM3520 mentioned a couple of times and coincidently found one for sale localy for under 2k including the outboard stand and new Baldor inverter.

After moving it yesterday I can sure feel it today

David

Alan in Little Washington · 07-26-2009, 12:30 PM

Originally Posted by Dusty Sawyer

Hey, I just looked up a post I had put out there a while back and there was a reply that is quite uselful.

http://www.ncwoodworker.net/forums/f...59-post247863/

hope this helps you too

David

Thanks. I just read my earlier post and realized I swapped terms so went back and replaced torque with hp. Congrats on the score!!! Pics!, we need to see pics!!

mkepke · 07-27-2009, 03:01 PM

Originally Posted by Dusty Sawyer

Thanks everyone for your excellent advice.

Looking at the cost and time invested to do this properly, I decided to start looking for a lathe better suited to what I am trying to accomplish. I heard the PM3520 mentioned a couple of times and coincidently found one for sale localy for under 2k including the outboard stand and new Baldor inverter.
<snip>

That's one way to solve the problem !

-Mark