Bridgeport Series II Retrofit

[Cold Fusion]

Here we go again

A month ago I posted on another cnc forum asking for some advice on buying a knee mill. A gentlemen responded and told me that he had a Bridgeport Series II NC mill for sale locally. Originally it had a tape drive, but that fried a long time ago. The ways and screws look to be in great condition because of the lack of use. It has a 4hp head, ground ballscrews, box ways, an air assisted knee, 40 taper spindle, 30x15 travel, and weighs 6000 pounds. My plan is to use the original 1400oz Stepper motors in conjunction with some really nice Slo-Syn drivers with built in power supplies. The head needs some work, but there may be a more elegant solution. Originally it was a Vari-Speed (4J) head but all of the belts and some other pieces need to be replaced. I have been wondering if it might be better to lock it into 1 or 2 ratios and do all the speed changes via the VFD. This would save time, money, and be much more convenient. Since my cnc router just sold on ebay, all I'm waiting on is the cash in hand. It should be 1-2 weeks before I can get it delivered to my house.

Let the fun begin!

[Al_The_Man]

There is a few post regarding this in these forums, this one for a start
http://cnczone.com/forums/showthread.php?t=7172
Al.

[ChrisJ]

I will give you my two cents on VFDs. Others, please chime in on your knowledge.

You may not find a VFD in the 4hp range that has 220V single phase input. Automation direct stops at 3hp. The Yaskawa reps tell me that you can take their drive (3 phase input) and derate it by a factor of two and run it with single phase. So, in your case 4hp x 2. So the next available size which would be 10hp. I do not know if you will find other manufacturers that will proclaim this functionality. Or of course pick up a phase converter. Don't know which way would be less expensive.
Don't know what your motor is on that machine. But you may want to replace it with something like a Marathon Blackmax which has a TENV (non-vent) enclosure. This motors are rated for constant torque down to 0 RPM.
An 1800 RPM motor can be clocked at 2x nameplate with no problems. So you can achieve 3600 RPM without a problem, possibly more.
Look for a drive which has "sensorless vector" capability. Automationdirect sells a Hitachi SJ100 that has this capability. Don't know if they will share information that you can double the size of the drive and run it with single phase or not. Worth a try. A vector drive is even better but you will need an encoder on the motor. Big bucks!

Or replace the motor with a 3hp and then there are no issues since those drives are readily available in single phase input. If you can live with a little less power.

That is my two cents.
Good luck
Chris

[Karl_T]

The Yaskawa reps tell me that you can take their drive (3 phase input) and derate it by a factor of two and run it with single phase. So, in your case 4hp x 2.
Chris

The derating factor I've heard is 2/3. Makes sence cause you got two hot lines feeding the VFD instead of three. The diodes on the input are most liekly the limiting factor.

FWIW, I've run a 5 hp motor in my 10EE with a 5 hp. VFD (no derating) single phase input with no trouble at all. VFDs are programmable for ramp up/down, if you get OCR kickouts (overcurrent rating) adjust the ramping.

Karl

[ChrisJ]

It wouldn't surprise me that the Yaskawa rep is just being conservative using the 2x.

Chris

[DSL PWR]

In theory it is 2/3, but you need to remember 3 ph power has a different pase shift. There for when you use 2 legs out of the three the pase shift accounts for more loss. With single phase pwr the 2 legs are 180 deg out of phase, with 3 ph they are 120 deg.

Typically vfd's are rated if you are feeding 3ph in and getting 3ph out. When you put 1ph in and get 3ph out, you need to derate the drive.

If I were you CF I would put a 3ph motor on the mill (I'm assuming you have a 1ph now), and get a vfd so you can run on the house power.

I'm not even sure a 3ph vfd would run a 1ph motor, as the windings are totally different. A 3ph has 3 different windings, and a 1ph only has one. So I think if you would hook 3ph pwr (even if it is only 2 legs) into a 1ph motor you would 1) blow the vfd, 2) burn out the winding in the motor.

[Al_The_Man]

You cannot run a single phase motor on a 3ph VFD, There are 1ph VFD's out there for 1ph motors that are cap-start-cap-run (no centrifugal switch) but they are very unreliable and tend to drop out of control as soon as rpm drops too low.
The reason for de-rating a VFD when hooked into 1ph is that the DC supply has less capacity on 1ph than 3ph, the AC input is rectified filtered and then converted to variable 3ph out. If you dont intend running your 3ph motor up to maximum current then you can get away without derating.
Also usually the reason for faulting out when using fast accel. decel rates is overvoltage not overcurrent due to the regenerative action caused by rapid braking.
Al.

[Cold Fusion]

I need to clarify a little bit. The 4hp motor is already a 3 phase. The vfd I bought is a 7.5HP model. I believe that this won't be a problem, because 99% of the time I won't be using anywhere near the full amp draw of the motor.

[lerman]

My understanding is that the issue of running VFDs on single phase versus three phase is the capacitors. Three phase power can give you three times the ripple frequency (360 Hz versus 120Hz for full wave rectification). That means you can use a capacitor of one third the size for the same amount of output ripple.

I believe this is also the ripple current in the capacitor (although I'm not certain). The ripple current of the capacitor causes heating of the capacitor and is a direct cause of failure of the capacitor.

So, you might find that the VFD works just fine without any derating. Until a capacitor fails and it dies. :frown:

Ken

[ChrisJ]

CF,

7.5Hp drive is close to 2x the 4hp motor. So, nothing is 100% but you should be o.k. there.
Don't be so sure that your existing motor is going to like being driven by a VFD if it is the original motor. But then again, run it till it burns then replace it with a inverter duty motor, provided no damage occurs to the VFD in the process.

Chris

[Cold Fusion]

True, but for now I'll just run the motor within a 25% window of the original 1750 rpm's so that nothing is damaged. After I earn a little cash off of the machine, I'll upgrade to a 3hp Inverter grade motor.

[ChrisJ]

CF,

BTW: I found some of the information in this article informative. It is on VFD/motors:
http://www.reliance.com/pdf/misc/225412.pdf#search='torque%20decrease%20with%20speed%20decrease%20inverter'

Chris