Ok all you old DC drive guys... I need help on an "S21" regulator
Ok all you old DC drive guys... I need help on an "S21" regulator
(OP)
Today's challenge for the graybeards:
I must come up with a way to interface a new AC vector drive system (for a hoist) to an existing control cab for a GE DC M-G set drive system that was 50+ years old. The customer is insisting on retaining the existing operator control system that he says is using something called an "S21 regulator" on the Master Switch for speed control. It apparently takes a 120VAC source and splits it with a special transformer that has a moving coil with a variable air gap(?) so that if you push the handle forward from center, it drops the 120V as the air gap increases by 0-30VAC in-phase with the source, then if you pull the handle back past the center point, it drops the AC voltage by 0-30V, but at 180 degrees OUT of phase with the source. The phase difference becomes the sign of the signal, the 0-30V is the amplitude of the signal. This was then somehow fed into the M-G set power control to provide DC voltage control to the hoist motor.
I had, in my misspent youth, yanked out several Ward Leonard systems and replaced them with Solid State DC drives, but we NEVER retained any of the old operator interface systems so I never bothered to see what they were. I therefore have zero experience with this, but the guy I'm doing the project for insists these are (were) very common on old Ward Leonard and GE DC drive systems. Google is ignorant of the term "S21" with regard to anything speed control related, other than to confirm that is was indeed used on old crane controls. All I'm really looking for is a more accurate description of what it is/was and does so that I can come up with a way to transform it to something a modern drive can deal with, such as +-10VDC, but with a degree of accuracy one would expect for a hoist drive. A drawing would be fantastic.
I have by the way already offered to just replace the Master Switch with a more modern version, but he is insisting that anything that changes the operator's perception of how it works, real or not, is unacceptable. I MUST use the exiting system, although a transducer is acceptable once it leaves the control cab. Needless to say, I cannot find any sort of 0-30V In-Out of Phase to +-10VDC transducer and before I start cobbling together components to make one, I'd like a more accurate picture of what it is.
I must come up with a way to interface a new AC vector drive system (for a hoist) to an existing control cab for a GE DC M-G set drive system that was 50+ years old. The customer is insisting on retaining the existing operator control system that he says is using something called an "S21 regulator" on the Master Switch for speed control. It apparently takes a 120VAC source and splits it with a special transformer that has a moving coil with a variable air gap(?) so that if you push the handle forward from center, it drops the 120V as the air gap increases by 0-30VAC in-phase with the source, then if you pull the handle back past the center point, it drops the AC voltage by 0-30V, but at 180 degrees OUT of phase with the source. The phase difference becomes the sign of the signal, the 0-30V is the amplitude of the signal. This was then somehow fed into the M-G set power control to provide DC voltage control to the hoist motor.
I had, in my misspent youth, yanked out several Ward Leonard systems and replaced them with Solid State DC drives, but we NEVER retained any of the old operator interface systems so I never bothered to see what they were. I therefore have zero experience with this, but the guy I'm doing the project for insists these are (were) very common on old Ward Leonard and GE DC drive systems. Google is ignorant of the term "S21" with regard to anything speed control related, other than to confirm that is was indeed used on old crane controls. All I'm really looking for is a more accurate description of what it is/was and does so that I can come up with a way to transform it to something a modern drive can deal with, such as +-10VDC, but with a degree of accuracy one would expect for a hoist drive. A drawing would be fantastic.
I have by the way already offered to just replace the Master Switch with a more modern version, but he is insisting that anything that changes the operator's perception of how it works, real or not, is unacceptable. I MUST use the exiting system, although a transducer is acceptable once it leaves the control cab. Needless to say, I cannot find any sort of 0-30V In-Out of Phase to +-10VDC transducer and before I start cobbling together components to make one, I'd like a more accurate picture of what it is.
"Will work for (the memory of) salami"





RE: Ok all you old DC drive guys... I need help on an "S21" regulator
Pity the next service tech. grin
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Ok all you old DC drive guys... I need help on an "S21" regulator
To that end, 50 years ago electrical controls were pretty well documented with theory of operation, schematics, parts lists, troubleshooting, even an alphabetical list in the back called an 'index'.
Somewhere in that plant, there's a manual for these controls because back then capital jobs always required at least 4 copies of the operations and service manuals and at least one has survived, somewhere.
It's probably a non-descript 'report cover' titled with a typed adhesive label, the kind with the metal strips that go through the punch holes and then fold over. It's somewhere, he just hasn't put any effort into finding it.
You might encourage him to participate by painting him a picture of the alternative: dealing with a series of 26 year old kid salesmen with 4 months of 'drives' experience under their belt promising the world because everybody's line is 'world class' and their service guys 'really know their stuff'. Give you the shivers? It will for him, too.
RE: Ok all you old DC drive guys... I need help on an "S21" regulator
Mike Halloran
Pembroke Pines, FL, USA
RE: Ok all you old DC drive guys... I need help on an "S21" regulator
Agree that anything else would be better than that original HMI.
Agree that there are documents describing it, somwhere.
The thing you describe is nothing but an ordinary differential transformer and all you need to make it useful with a modern drive is to put a synchronous rectifier and some smoothing between that device and your drive's analogue +/-10 V input.
If you dig into it, there is probably already a synchronous rectifier in place and that rectifier was used to feed the excitation winding of the generator. Tell the guy that wants to keep that HMI that the synch rectifier is part of the HMI and that he can't have one without the other. Adapting it is easy, just put an RC filter (no electrolytics) and a voltage divider so your +/- ca 40 V are reduced to +/- 10 V.
Of course, I could also build you one, using modern stuff like an Arduino and communicate with your drive over USB or WiFi - perhaps even via a 5 V analogue signal plus FWD/BWD logic signals. There may even be a +/-10 V signal - but that will cost extra.
But do look for that synch rectifier first. It is there. Promis.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Ok all you old DC drive guys... I need help on an "S21" regulator
RE: Ok all you old DC drive guys... I need help on an "S21" regulator
but I don't remember any phase shifting being involved.
RE: Ok all you old DC drive guys... I need help on an "S21" regulator
But I did manage to find info on old GE Crane Control Master Switch catalog for variable speed DC crane drives, and I think I see why he wants to keep it. The reference output is non-linear, the signal voltage changes in something like an S curve in either direction. So when you first come off of center, the change is very small with relation to handle movement, but the further away you get from center, the higher the rate of change. So operators will be 'used to" the non-linearity of it and if it is replaced without that consideration, there may be training issues and risks of error. There cranes are on a hydro dam for servicing the generators and turbines, so making a slight mistake as you lower a rotor into a generator can be very costly, if not dangerous.
His description seems to indicate a different version where instead of using contacts to select Fwd or Rev contactors, it is using the signal change from this "in-phase / out-of-phase" condition. He had mentioned that there were no other contacts involved.
I think now that I see this and understand his concerns, it appears that he is most likely unaware how easy it is now to pop in a simple algorithm to create that non-linear response to a signal change on analog inputs. The only tricky part may be in determining, most likely empirically, what the shape of that curve looks like.
"Will work for (the memory of) salami"
RE: Ok all you old DC drive guys... I need help on an "S21" regulator
RE: Ok all you old DC drive guys... I need help on an "S21" regulator
I have worked with crane operators (container cranes mostly) and they are very sensitive to how the control's feel. They get nervous if it feels too delicate. One can understand that. Loading and unloading containers is more art than craft - at least if you are going to get it done quickly and safely.
As I said, look for the old synchronous rectifier and use it. All you need is to smooth it and scale down the voltage. Put Si diodes there instead of the selenium rectifiers that probably were used. Synch rectifiers were usually built with switching rectifier bridges without any voltage drop, so there will not be much difference. I think that your customer can like the idea.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.