Let's commission this mother - OOOF! (trouble with the RCDs and Meanwell PSUs) - WTF????

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.

As a former SMPS power supply developer, I have to say I'm puzzled. MW are a reputable company and I'm sure these products generally work well in hundreds of applications.

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. 

There are plenty of LEDs on these boards. with the workshop lights turned off, it looks like a disco.

I left it running over night and it was fine next day. I then spent several hours rewiring the Home and Limit switch circuits. This is due to the fact that you have to choose between "LIMIT ALL" (all NC limit switches in series) or individual inputs for each LIMIT switch. The Stupid Fat Bloke had naively brought the switch signals to the Acorn with a common 0V. Ho Hum. Now I have one input for all 4 limit switches and one input each for the X and Z HOME switches.

Once I'd finally got the limit and home switch wiring sorted, I was able to get the servos jogging without needing any config changes. Furthermore, the control wiring actually worked. One minor change was to use DO1 (servo OK) rather than DO2 (servo alarm), so that the Acorn will recognise an unpowered servo drive as a FAULT condition. Otherwise, if I forget to turn on the power to the servos, the Acorn won't know any different. The FAULT output from the LiChuan servo drivers is active low - which means that an unpowered servo will appear to be reporting DRIVE OK.

Before I start jogging and homing the machine, I need to lift the drag chain away from the moving axes. Of course, when I refit the enclosure, this will be taken care of. But for now, I've fastened a deal of wood to the remnant of the support frame...

...and secured the drag chain to it.

A few cable ties and it's safe enough to allow some proper commissioning.

So now I've got the machine axes 90% configured. Homing and limit switches are working. I'm getting there now....

What next?

• 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.