Thread: Siemens 6RA26 DC spindle drive always at (beyond) max speed

Hi everyone,

It's been a while since I've been on the forum, sorry about that.

I'm having a problem with the Siemens spindle drive of a machine I'm trying to repair, and I'm really running out of ideas, thank you for taking the time to have a look at it.

The machine is an Hermle UWF851 (Deckel type of the 80's), with a TNC155 controller, it wasn't working when I bought it last year, the seller told me that the spindle motor "probably didn't get a signal".
I started to repair it 2 weeks ago, the Heidenhain controller works fine, axis move, but when I've tried to have the spindle runing, the spindle drive made a terrible noise and the motor turned very very slowly, I guess there's something wrong with the thyristors or their firing logic.
I didn't look very deep into it because I had a spare drive on a shelf (the exact same model, I got it from a scraped machine some times ago), I set the potentiometers with the same values as the drive I was removing, wired everything. Then entered an M03 S200 in the controller, the spindle started to move, everything was normal but when I asked for S1000, nothing would happen, I guessed the drive needed some tuning.

Here are the data of the drive and spindle motor:

Drive:
Siemens 6RA2620-6DV55-0
Input: 3x380 volts
Output: +- 380 volts 35A (that's from the plate on the drive, Siemens's manual says +-380 volts or +- 500 volts)


Motor:
Siemens 1GL5108-0EW40-6HU7-Z
Armature: 40-380V ; 50-2340 rev/min ; 18,5-19A ; 0,13-6,1kW
Field: 310V ; 1,1A (there's also 47V 0,18A on the next line)


Tacho:
Huebner 20mV/ rev/min

I measured 2,1 ohms for the armature, 214 ohms for the field, around 300 ohms for the tacho. Isolation seems good (I don't have a megger), collector and brushes are in nice shape on the motor and the tacho.

I wired the drive separately (the 3 phase power supply) and removed the belt that link the motor to the gearbox (the machine's gearbox is empty, I'm waiting for oil to fill it). I followed the instructions of the drive's manual, removed a solder link on one of the board, checked the wring, phase order,.. Set the max and minimum field voltage.
I have a first question here, the manual says to set the minimum field voltage at 70% of the rating plate, so I set it to 310x0,7 = 217V, is it good? The "47V 0,18A" on the rating plate keeps me confused, especially as I'm far from the minimum field potentiometer setting of the defective drive that was on the machine (on a 10 scale, it was at 2, I'm now at 9) and it was protected by a small piece of tape.

So far, the tuning procedure went smoothly, I wired a 1,5 volts battery on the +-0-10 volts input the motor rotates and stabilize, I checked the tacho polarity as asked in the tuning procedure, the polarity is fine but I get 72 volts, so the motor is rotating at 3600 rpm! I check the armature voltage , there is 520 Vdc, the field is at 220 Vdc, so it seems that the drive tries to accelerate the motor by all means it has to do so, if I lower the minimum field potentiometer, the speed increase. If I reverse the tacho, I have the same problem, if I removed the tacho leads and put 1,5 volts on the speed input, the motor rotates a few turns then a led lights up "Verz.stop", I guess it's a fault condition.
I checked the wiring of the field and armature, according to the mill wiring diagram (I noticed that the motor was rotating in what I thought was the wrong way, a positive speed input would have the tool rotate like an M04, but I'm not very familiar with the way cnc work), the armature leads had been permutated, after putting them back on their respective terminals the motor rotates in the "right" direction, but still at 3600 rpm.
If I try to tweak the R4 potentiometer, which is supposed to set the maximum armature voltage, or R28 which is supposed to set the speed aligning, it doesn't change anything, I still have 72 volts on the tacho (or -72 volts if I change the speed input polarity). The only parameter that have an effect on the speed is the minimum field setting.

Would someone have an idea of where I should be looking?
I really have no idea of what's going wrong, it seems like almost everything is working fine, just like if the drive was not "seeing" the tachometer input (I measured directly on the board, so the voltage arrives there).
I attached the manuals, the V50 is in english but the copy is of very poor quality, the V56 is in german but drawings and values are easier to read, by far. There's also the wiring diagram of the drive (from the machine's manual), and a picture of the spindle motor's rating plate.

Thank you for your help, sorry for the mistakes, my english is getting very rusty.

François
Belgium

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