Fixing the electrics in my old German mill Ruhla 250/710

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Bill Shields
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Re: Fixing the electrics in my old German mill Ruhla 250/710

Post by Bill Shields »

many places use contactors as relays -> to avoid the problems of stocking many different devices...

as long as you are not putting any motors across any of those relays, you will probably be OK for contact rating...HOWEVER.

you need to be aware of the potential problem of 'transients' pulling small relays in to 'make them'...and possibly cause a dangerous fault...

you mentioned that the new relays take a lot less current to pull in the coil...which is exactly the point I am making..

This problem is generally avoided by big heavy contactors that need a good solid signal across the coil to cause them to close.

been there - done that - nobody got hurt but scared the c**p out of a bunch of people -> and never repeated that mistake in any of my designs or repair jobs.
Too many things going on to bother listing them.
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liveaboard
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Re: Fixing the electrics in my old German mill Ruhla 250/710

Post by liveaboard »

I appreciate the warning; I'm not too worried though.
For one thing, the motors are only 1.5 or 2 HP, and 380V 3-phase. Current is just 3.5A.
And the coils are 220V, so even if their current is low, the pull in voltage is pretty high.

If you turn out to be right... well then, you'll be right.

The low current contactors are the same units as the ones that carry motor current, so yes the builders were limiting the parts list when possible.

One of the bad contactors I pulled today is a newer western German one that was swapped in at some point.
"Newer", only 30 or 40 years old.
meuller relay failed.jpg
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Bill Shields
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Re: Fixing the electrics in my old German mill Ruhla 250/710

Post by Bill Shields »

You would be surprised at the transients that ate generated with stopping and starting motors.

Takes a recording scope to see them....cannot measure them with a meter.

all they gave to do is pull in a contact for an instant and cause a control circuit to engage and latch when you do not want it.

He careful and aware...
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armscor 1
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Re: Fixing the electrics in my old German mill Ruhla 250/710

Post by armscor 1 »

I had to maintain a relay logic control system using about 200 relays controlling a CO burner generating steam from the flue gas from a catalytic cracker.
The relays had economy resistors in parallel with a NC contact, on energising the NC contact would open and the relay would hold in with the much reduced current flow feeding the relay coil, definately extended the life of the coil.
What we did before PLC's came along!
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liveaboard
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Re: Fixing the electrics in my old German mill Ruhla 250/710

Post by liveaboard »

thankfully, my system isn't quite that complex...
I bought the 3rd contactor today, $35.
Modular types with extra contacts (NC as required) cost $60-$120 each, and I need many.

Anyway, today is car day (inspection fail!), tomorrow I have to spray the orchard, then mow the weeds.
It seems like there's always something in the way of doing what I want to do.
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Re: Fixing the electrics in my old German mill Ruhla 250/710

Post by John Hasler »

Bill Shields wrote: Mon May 24, 2021 8:14 pm You would be surprised at the transients that ate generated with stopping and starting motors.

Takes a recording scope to see them....cannot measure them with a meter.

all they gave to do is pull in a contact for an instant and cause a control circuit to engage and latch when you do not want it.

He careful and aware...
That's one of the reasons for a control transformer. With proper design those transients never come near the relays.

Snubber networks can help protect your relays from false tripping and also extend their useful lives.
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Bill Shields
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Re: Fixing the electrics in my old German mill Ruhla 250/710

Post by Bill Shields »

Agreed.. but I know nothing of the remainder of the system -> only what I see -> and I am a cautious critter.
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tornitore45
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Re: Fixing the electrics in my old German mill Ruhla 250/710

Post by tornitore45 »

What can be toxic is the fumes from a selenium rectifier that is in the process of burning out.
Top
Ahhh the intoxicating aroma of burning Selenium Rectifiers, what memories!
Mauro Gaetano
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Harold_V
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Re: Fixing the electrics in my old German mill Ruhla 250/710

Post by Harold_V »

tornitore45 wrote: Wed May 26, 2021 3:00 pm Ahhh the intoxicating aroma of burning Selenium Rectifiers, what memories!
One I remember well after walking in to my shop one morning, many, many years ago, and smelling "garlic". My Graziano had such a rectifier, and I didn't have a habit (then) of turning off the main power to the machine at the end of the day.

H
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liveaboard
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Re: Fixing the electrics in my old German mill Ruhla 250/710

Post by liveaboard »

I'm amazed it's still in there.
The drop across it is huge, and increases with time.
The transformer that supplies it has multiple taps, and has been set on the highest to compensate for the ancient rectifier.
Yet no one just put in a new one that costs $1.

I took out another 2 relays; each one had a bad contact. It's possible the machine hasn't operated in many years.
I was blaming the east block components, but even the old west German ones are bad.

Of those tested so far, only the newest 2 contactors, the new type Klockner Mueller, are good on every contact.

There's another tiny little transformer in there that's failed, it's 18vac only to supply a few indicator lights. I don't know why the builders didn't simply use 2w out of the 24VDC supply for the clutches and brakes. The bulbs are 24v standard automotive.
Anyway, I found a 20V transformer in my electronic bit junk box.
It came in a broken home plastic bag sealer I found on the trash last year.
I'm so bad...
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Bill Shields
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Re: Fixing the electrics in my old German mill Ruhla 250/710

Post by Bill Shields »

if the old contactors did not hold up...consider how your relays may suffer...

consider inrush current, and the frequency of 'jogging' things which by definition are typically during inrush cycles.. and how that is going to affect the contacts... :shock:

or are the axis feeds all clutch with the motor running constantly? :?:
Too many things going on to bother listing them.
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liveaboard
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Re: Fixing the electrics in my old German mill Ruhla 250/710

Post by liveaboard »

the little relays only switch other relays.
As far as I know.

Anyway, can we ease off on the doom-saying?
You said it, I acknowledged you said it, I've bought the relays they're on the bench and they're going in.
If they burn, then they burn.
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