Hi, I'm a regular on rec.crafts.metalworking. First post over here. So go easy on the FNG.
I'm refitting the control on a very large milling machine with brushed DC servo motors and spindle. The original power supply can't be re-used. It was rated at 400 volts 100 amps using three phase input.
My three phase is "made" from single phase with a couple large three phase motors totalling 35 hp. and a whole bunch of run caps. Its balanced as much as possible but still has a "wild" leg that tends to run higher voltage.
The DC supply is almost too simple, 220 three phase line connected to three 100 amp fuses, then a large motor contactor, then six very large diodes to rectify to DC. Then to a bus for fusing each of the five AMC servo amps.
At this point, no isolation transformer so DC- is not grounded. AFAIK, this is not a problem.
Should I add electrolytic caps for DC smoothing? and how much? if using these how should high startup surge current be managed?
I have a three phase rectifier or choke if this is needed for AC noise suppression.
We are all expert in some areas and clueless in others. This is my clueless area. what am I missing here?
Wait a minute -- 400 volts and 100 amps is over 50 horsepower. Are you just putting the cutting tool right on the servo motors?
I was going to suggest that if you had a sufficiently big DC machine that you stick it into the mechanical arrangement in your rotary phase converter -- but that's when I really _looked_ at your numbers.
At any rate -- if you have a 50 horsepower, 400V DC motor/generator, you could hook that up to some serious AC iron. Or sell it for scrap and get enough $$ to buy all new parts for a supply.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html
So you have read a few typical posts on this NG :-).
Why is that? Are some of the parts reusable?
Bridge rectified 3 phase power has only around 13% ripple:
and your servo drives are presumably designed to handle the input power ripple - the servo controller provides the regulation. With only 220 VAC input you will be limited to around 300 volts not 400, which may or may not matter to your servo electronics - they could have an undervoltage lockout and not run at all, could run but not at full power, or if they were designed for a wide enough range of input input voltage, they may work perfectly. Is there a nameplate on the servo controllers with input power requirements?
The spindle is 20 hp DC servo and each of the three main axis are about 2 hp DC servo. The 4rth axis will be dinky. So, in actual use I doubt there will ever be more than 30 amp for a second and running maybe 20 amp.
But the original machine was wired and fused for 100 amp power supply,
80 amp (now DC) spindle, 30 amp (DC) on each servo.
Plans are to move this machine to my son's place with real 3 phase after control is up and running. This may be more than one year on my generated 3 phase. programming and debugging the control for this machine is NOT a small task.
Original used SCRs and a PCB to control them through the fanuc computer - had to junk this. I'm just saving stuff like fuses and contactors. The drives are totally obsolete.
....
So you're saying no need for smoothing caps??? Remember my 3 phase made from an RPC is not as good as the real thing.
The DC voltage will be around 340 on rectified 3 phase and may go a bit higher with no load. These voltages are not a problem for my AMC servo amps. I'm using a model 100A40 on the spindle, smaller units on the axis. here's AMC page for the docs:
So the original supply was regulated but unfiltered - which is completely irrelevant now since you are replacing the drives. It also means that the original servo drives not needing filtering is also irrelevant.
But not that much worse either, the "wild phase" (derived phase) should still be well within the drive input voltage rating. If the drives can handle the ripple from rectified utility 3 phase power, they can probably handle the bit of extra ripple from your RPC power.
The AMC installation manual does mention the need for "enough capacitance" on the DC power supply - in order to meet the current inrush requirements, not because ripple is a problem (as I read it). Your rectified 3-phase RPC DC power should have pretty good current inrush capability; it does not have the voltage regulation response time issues of regulated supplies, and you might not need much if any capacitance in practice, especially if you run well below the drive capabilities (as appears to be the case). But I think you should call AMC for the definitive answer to the question of how much if any capacitance is required, because these caps will be expensive and you don't want to use more than you really need. (Low ESR (Equivalent Series Resistance) capacitors will be required to prevent overheating from the ripple current.)
With a regulated power supply to a servo controller there can also be issues with interaction between the power supply regulator and the servo controller causing instability, which can sometimes be fixed by "stiffening" the power supply with more capacitors; this could be the real reason for requiring "enough" capacitance.
In the unlikely event that you only reach a clueless salesman at AMC, I would try it with no caps, as these drives are well protected and it is unlikely (but not impossible) that you will damage anything IMO, although I am not offering to pay for any repairs required if I am wrong. Some of the input voltage ripple will pass through the drive controller to the motor (too fast for the drive to respond to), and this may or may not cause a problem with drive tuning, but probably not, and will cause a bit of extra heating in the motor windings, but probably not enough to cause problems unless you are really pushing the motor limits.
Ignore everything I said if you can get the advice of an AMC factory engineer :-).
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