Don't Forget to Change Your Backup Batteries !

[awemawson]

My Beaver Partmaster CNC Mill is controlled by a TNC355 Heidenhain controller. The internal parameters and setting are held in memory made 'non volatile' by three AA batteries deep in the bowels of the machine.

The book advice is to change them every year, so to remind me I have a label on the machine "Batteries Changed dd/mm/yyyy" and it works out that around Christmas the batteries indeed get changed, and the removed (but probably still OK for non essential applications) batteries get distributed for grand children's toys.

To my absolute horror I just noticed that the label tells me they were changed in December 2018 

I must have been too involved in other things!

So this evening I powered the machine up (batteries get changed with power ON) and was presented with the error "EMERGENCY STOP DEFECTIVE" and nothing would clear it - argh !

Have I lost the parameters? Well no because if that happens it comes up and speaks in German - do the sensible thing - power off - say a prayer to the machine gods, power it back on. Phew - fault cleared. Quick, change those batteries, re-print another label AND DON'T FORGET NEXT TIME !

A close shave but I got away with it - phew.

(I do have all the parameters both as hard copy, paper tape, floppy disk, DVD and on a PC so I'm covered but it's a pain reloading them)

[Sea.dog]

I always set reminders in my Outlook and Google calendars.

[AdeV]

My old Interact II had the same batteries in it for at least 5 years! It also spent the better part of 3 years not powered up, after I moved to this workshop....

When I did finally get around to trying to recommission it, I got the "batteries low" warning. So I did the right thing & changed them immediately - only to be presented with German next time I powered it on! Turns out, some dirt had got between the battery terminals somehow & thus no power = no parameters. Luckily, having been pre-warned about these things, I'd taken a copy when I first got the machine. Manually re-typed all 200-ish of them (serial link wasn't up); yay, English! Re-boot. Boo, German! I must have gone through this process 3 times before I scuffed up the battery terminals with a file; no problems since then...   

At least, no problems relating to the batteries.... I'm fairly sure I've killed my thyristor drive board (new one all the way from Canada is sitting on the bench waiting to be fitted); and last time I tried to power it up it's burnt out another RF suppression cap elsewhere in the machine, so now I need some new ones - and typically, they're not the same spec as the ones I've already got.  I am starting to think it might be time to bin the old electronics and drive it with something more up-to-date; the only pain is, YOU try to find a driver board for a 22kw 8KW 400 volt DC motor... I have yet to find anything! Then there's the servomotors on each axis. Steppers I can understand.... Servo drivers, not so much. Plus whenever you search for servo driver, you end up with a billion stepper driver hits...

[awemawson]

KTK Mentor DC drives turn up occasionally on eBay - I keep a watch out as I have one driving the spindle of my Beaver TC20 lathe.

[AdeV]

I think mine's a Contraves - is that the same thing?

[awemawson]

I've heard of Contraves but never seen one, but I seem to remember that they have a reputation for being expensive to repair - but that's only hearsay.

[nrml]

Google calendar is a godsend for these things along with birthdays and wedding anniversaries . You get emails and text messages to remind you as close to the event as you choose to set it.

[AdeV]

...I seem to remember that they have a reputation for being expensive to repair - but that's only hearsay.

Sounds about right....

I have a spare motor (picked up for relatively cheap on fleaBay), but no spare controller...

Maybe I should rig the spare up on the bench & start experimenting with mosfets! 

[awemawson]

IIRC the Mentor is basically a massive H bridge driven by some cunning microprocessor trickery. The field winding is excited separately by another unit.

[AdeV]

IIRC the Mentor is basically a massive H bridge driven by some cunning microprocessor trickery. The field winding is excited separately by another unit.

I think that's basically true of the Contraves as well. It uses thyristors (must google them again, but IIRC they're like old-school high power mosfets) to provide the drive. I'm fairly sure it's the field exciter circuit that's let the smoke out this time.

[awemawson]

Unlike mosfets thyristors once triggered continue to conduct until either the power source is removed or they are capactively forced into non conduction by a negative pulse. (Comutated)

[AdeV]

Unlike mosfets thyristors once triggered continue to conduct until either the power source is removed or they are capactively forced into non conduction by a negative pulse. (Comutated)

I've been googling In fact, Thyristors are hardly like MOSFET at all..... but hey ho.

Having search, in vain, for any kind of documentation on my Contraves motor, the best I can do is a bad picture of a ratings plate on one on eBay, which doesn't tell me (for sure) that this is a DC motor, although I'm pretty damn sure it is, there are only 2 high current wires leaving the thyristor board (3 phase wires coming in). The motor is rated 400V and 20amps, which seems to be well within the capabilities of a MOSFET.

I'm going to have a chat with my friend the electronics genius ( I don't use that word lightly), if I'm lucky he'll offer to design a board for me 

[awemawson]

I seem to remember that the only complication is profiling the field current value against rotor speed - iirc the current has to decrease as the speed increases which is counter intuitive.

Attached is the original test sheet for my Mawdsley DC motor when it left the factory. It's much the same order of magnitude as yours, and the test results show rotor and field current versus speed - might be useful as a starting point