Monday, 23 May 2022
Saturday, 21 May 2022
Let's power this beast up!
My first step in commissioning my current conversion (Tree CNC lathe) failed at the first step and a couple of hours in, I've barely moved forward. I used the 2 Meanwell PSUs that came with the Acorn and the ETHER1616. Here's the datasheet for the LRS-35-24 which is one of them.
With nothing else connected, closing the isolator trips the 30mA workshop RCD. This happens with either or both PSUs connected up. Lifting either L or N avoids the problem, as does running without the RCD in circuit. WTF???
I ran the machine from a 240-240 isolation transformer (output N connected to GND) with a 30mA RCD, to avoid tripping the workshop supply. Same issue (but different RCD doing the tripping).
There's no measurable path to ground from either L or N in the PSUs using a decent quality DVM, as indeed there shouldn't be. Meanwell claim 300VAC / 5 seconds as well as the normal 85-264VAC input range. There are no TVS / MOVs in the circuit and I assume the X and Y caps are approved parts. WTF??
Operating it at 120VAC input helped a bit but it still tripped out after a few minutes. I reverted to an isolation transformer (with a grounded chassis and neutral) but it's not what I'd want in the final setup. I have a couple of DIN rail Class II PSUs (ie require no protective ground connection) that I could use in place of these enclosed models but it's not something I was planning - or expecting to be required to do.
I measured the mains voltage at between 250-260VAC today. I can't safely measure it with a scope but I get similar results using 2 different DVMs. That's not unusual here these days but as I say, the PSUs are specc'd and tested (not least by UL, CSA etc) at 264VAC and even 300VAC.
RCD trips are caused by ground faults or leakage and aren't usually bothered by inrush currents. I have 3 other CNC machines full of VFDs and servos and a large single phase (70Arms) TIG welder and NEVER have an issue with nuisance tripping. Yet when I connect up either of these tiny 35W PSUs, I can't even power them up without tripping. The leakage current is supposed to be limited (and tested) by the UL/CSA/CE approvals, so without deploying more exotic test gear which I don't have at home, it's hard to think of a convincing explanation. Perhaps when I replace them with the DIN rail PSUs, I will take a closer look.
My solution (for now) is to power the machine from an isolation transformer with the neutral grounded and a good protective earth connection to the machine chassis. This won't be bothered by leakage currents, won't trip the main RCD / RCCB and gives the benefits of a dedicated circuit without requiring alterations to the workshop itself. At the time of writing, I've been running them continuously for 24h without issue. Furthermore, I've now got my machine up and running, so progress has been made!
Well that was a waste of time, falling at the first hurdle.
Let's get these servos running.
- Correct the pulley ratio for the X axis. I have both axes set to a 1:2 pulley ratio, as I couldn't recall the pulley sizes. I chose the smallest driving pulley I could fit the the LiChuan motor but that was only down from 20t to 18t ie ~10% improvement in torque.
- Set up the spindle VFD and get it running under Acorn control. I have told the Acorn wizard that the max spindle speed is 4000rpm at 10V control voltage. I need to program the VFD to run at the equivalent motor speed - this will be around 2800rpm hopefully. The driven pulley is smaller than the driving pulley but some measurement is required to get it right. I have a spindle encoder and can drive the unloaded spindle at a fairly accurate speed, so determining the actual ratio should be fairly simple.
- Set up the soft travel limits. I just pulled some safe numbers out of my ass to get me started but that's limiting the available travel.
- Connect up the lube pump.
- At some point, start to investigate the turret / hydraulic pump etc.
Monday, 16 May 2022
Spent a fair chunk of time over the last week or so making up the myriad cables, brackets, holes, mounting plates etc required to "wire up" the various components within the machine enclosure. There will doubtless be some cockups along the way but hopefully nothing too serious or embarassing. Only time will tell....
Hydraulic block plugs:
Luckily, the 9/16"-18 UNC SAE plugs I ordered several weeks ago finally came. Unluckily, I had the wrong thread so these didn't even remotely fit. What a complete twat, not least as I dropped £22 for the privilege. For a Scottish-blooded Yorkshireman, that's quite a hit. Turns out it's actually 1/4" NTP (national pipe, taper). On the upside, these are available from Amazon at £8 for 10.
So there we are. Arguably approaching 80-90% wired up. Some stuff not connected yet:
- Hydraulic pump umbilical - not required yet. Will connect into the ETHER1616 board once the main system is sorted.
- Limit and homing switches. I should connect these up before powering up the servos.
- IEC socket for the monitor. I haven't decided how to mount the monitor or finish off the front panel, so no rush there.
- Lube pump. Bit premature to power that thing up without anywhere to discharge the effluent. It simply wires up to the mains after the isolator switch, so no sweat.
Sunday, 8 May 2022
- 1 pint(!) reservoir
- 7.5 minute cycle time (at 50Hz)
- 0.2 - 1.0 ml per stroke, set by an adjustable collar.
130mH is an impedance of about 40R. In conjunction with 27R resistance, that would suggest a total impedance of around 50R. Clearly not - I guess the inductance increases significantly when the solenoid closes.
The work continues...
Wiring up the induction motors for the spindle and hydraulic pump into the VFDs. The phase connections are totally enclosed in flexible steel conduits - up until the last few inches. And as mentioned, the VFD supply sides both have EMC filters, so should be reasonably quiet, electrically speaking. Note also the red braking resistor for the spindle. There's no need for one on the hydraulic pump, obviously.
Friday, 6 May 2022
Before I get too carried away with the signal wiring, I have one critical subsystem to investigate. I'm planning to run the hydraulic pump from a 240V single phase VFD but that requires me to reconfigure it as a 240V (delta connected) machine. I'm pretty sure it's in 415V (star) at the moment.
Inevitably it's a bit of a mess down there, with oil and dust sticking to everything. On the other hand, this kind of muck usually responds well to WD40, a pot scrubber and loads of paper towels, so it shouldn't be too painful. Off we go...
First, take a note of the hose connections. It should be pretty obvious which hose goes where but you never know....
Now let's terminate the other end of the conduit next to its VFD and repeat the wiring exercise for the main spindle motor...
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