Bridgeport S2 Boss 6 CNC Transformer T2 question

[jacampb2]

Hi all, I am going on with my servo conversion and working on sorting out the power supply for my amps. I converted the originally Boss 6 controlled stepper driven mill to PC based control and new stepper drives a few years ago. When I built the power supply for the steppers, I used the original 480/220-->110 single phase step down transformer. I wired that xformers taps for 220 and supplied 110Vac to it, rectified I ended up w/ about a 75Vdc supply. This was perfect for the steppers, but I need a bigger DC supply for the servo amps.

My thoughts are this, use the old T2 transformer, wire the primary side for 220Vac, but supply 480Vac, that should give me 160Vac on the highest secondary taps. That should put my supply at ~220Vdc after the rectifier. I plan to use a big 3 phase bridge rectifier to get my DC output. I have 25,800 uFd of wet caps salvaged from a parted out VFD, I think this is going to be more than enough capacitance, especially w/ a 3 phase supply.

Does anyone see any issues with my plans? The servos are 200Vac supply, 18.6Arms continuous, max instantaneous current of 80Amps.

Thanks,
Jason

[Al_The_Man]

You may experience close to saturation or otherwise excessive high current on the transformer by feeding 480 into the 220v winding.
You could look at modifying the secondary voltage by checking to see if it is viable to remove turns on the 3 phase transformer and run the primary at the right voltage.
Al.

[jacampb2]

Originally Posted by Al_The_Man You may experience close to saturation or otherwise excessive high current on the transformer by feeding 480 into the 220v winding. You could look at modifying the secondary voltage by checking to see if it is viable to remove turns on the 3 phase transformer and run the primary at the right voltage. Al.

The primary side of T2 (3 phase xformer) has two sets of primary windings, the primary is delta wound, and the windings are series for 480 and parallel for 220. All of the windings are brought out to a barrier strip. It may be possible to rewire the transformer and only use one of each primary winding, which should be half turns, and would end with the same results if I give it 480Vac. It seems that this would reduce the current capacity of the primary windings though.

I had considered that the transformer may saturate in my original proposed scenario. Is there a way to calculate a rough saturation point? This is really a fairly good sized transformer, I think it is rated at 3.6KvA.

I just shelled my back carrying the POS to the workbench, so it could be a while before we find out whether this will work or not

Thanks,
Jason

[Al_The_Man]

What three phase supply do you have currently available?
I was thinking of decreasing the secondary, these are usually wound on last.
You would have to hook it up to see if it approaches saturation and check the current at no load.
There might be heck of an inrush also.
Al.

[jacampb2]

I built a 10HP rotary converter which supplies ~225Vac 3 phase. The legs are all well balanced, I spent a lot of time tuning it. I supply a 480-->220 45Kva entrance transformer from the rotab, I run the entrance transformer backwards, so it steps up rather than down. From the entrance transformer, I run the bridgeport CNC mill and an older Gorton 016-a manual mill. I feed the Lathe three phase 220 before the entrance xformer. They don't all run at once, but I have enough capacity to if needed.

So, I already supply the CNC w/ 480, it was a lot cheaper to buy a 480 3 phase AC drive for the spindle, than it would have been to buy a 220 one. I stripped all the old power supply out of the mill when I converted it to new controls. I built the stepper DC supply from one of the old step down transformers in the machine that provided 120 single phase from 220 or 480.

The T2 3 phase transformer has secondary taps for 60, 70, and 80V. If I am not mistaken, removing windings on the secondary side is going to lower the secondary voltage, not raise it.

On another note, I may have a set of three 1:1 isolation transformers that came out of the above mentioned parted out VFD. This one was a 30HP drive, so the transformers should be able to happily handle the current. I could just use them to isolate the line and then rectify the 220Vac...

I'm going to go hobble out to the shop and see what exactly I do have for xformers.

Thanks,
Jason

[jacampb2]

Okay, checked on the Isolation transformers, and I only have two. I know I saved everything from the drive, so that leads me to ask, why the heck there were only two of them? The drive was 3 phase input only-- I didn't take a close look at how they were wired, but I guess I should have. I can't see two individual single phase transformers working...

They appear to be single phase transformers. They are not very large, about 4" cube, but the windings are probably 8Awg enamel wire. I will hook one up to a LV supply to be sure they are 1:1.

And now the next question, do I really need to have an isolation transformer in place? I have always heard that that was the rule to live by in PS design, however, I have had other drives apart that simply rectify the AC line and have no isolation transformer at all. If I don't need a transformer, I will just rectify the 3 phase 220 directly and use that for my supply.

Thanks,
Jason

[Al_The_Man]

Originally Posted by jacampb2 And now the next question, do I really need to have an isolation transformer in place? I have always heard that that was the rule to live by in PS design, however, I have had other drives apart that simply rectify the AC line and have no isolation transformer at all. If I don't need a transformer, I will just rectify the 3 phase 220 directly and use that for my supply. Thanks, Jason

No not absolutely required, this is done on VFD's etc, also AMC use this system for their AC powered drives, this allows supplying 120/240ac direct, or any other AC voltage within the range of the drive.
The only time you could run into a problem is if the AC is referenced to ground and the DC supply has a ground reference also, only one can have a ground reference, not both.
Al.

[jacampb2]

That might just be the route I take then. I will have to pull more wire to get the 220 3 phase over there, but even then it will be cheaper then sourcing the appropriate transformer. On top of that, it makes the power supply a lot lighter, and my jacked up back will be happy about that!

Thanks,
Jason