The Craftmans Shop > New from Old
The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
awemawson:
The hunt for the earth leakage goes on!
Consistent approx 300 ohms 'phase to chassis' with the huge breaker giving the KTK Mentor it's 415v 3 phase closed , but clears with breaker open. First job, open up the KTK Mentor DC Drive.
Well I really thought that I'd found the fault - several 0.1 uF capacitors in a very sorry state. So I disconnected the terminals putting power on the device, but noticed that there were much thinner cables going elsewhere.
Measuring to the cable ends the 300 ohms problem was coming NOT from this bit of the Mentor, but the small cables - but where do they go? A major bit of trunking dismantling and cable tracing found that they went down to the bottom of the KTK Mentor via some smaller fuses.
Isolating the fuses the fault persists (so not this bit of the Mentor drive), but hang on, there again is another cable coming off these terminals back into the trunking - but to where?
Answer - it goes to a small motor starter unit, who's output cables plummet down to the main termination rails at the cabinet base FOR THE FAN FOR THE MAIN MOTOR. Faulty fan perhaps, yet I know it turns as that was one thing I got going on site before I bought the machine.
But hang on what is that white domestic looking three core flex doing connected there and why is it's blue neutral wire connected to earth.
Answer: some plonker has decided to add a Ventaxia Window Fan to the roof of the cabinet that houses the turret electronics and a pair of 24 v power supplies. This machine has no neutral feed, so Mr Plonker has used Earth instead :bang: I've temporarily isolated the fan.
So the elusive 'earth leakage fault' has proved to be someone in the past doing silly things. Now the machine happily starts up on my 100 mA RCD protected supply without tripping, and I can consider returning the Sinumerik 820T controller to it's proper place, but before then I must decide what I am doing about relocating the batteries that caused the problem in the first place - replacements arrived today.
. . . mind you those 0.1 uF capacitors will need changing before long
charadam:
Andrew, I am lost in admiration at your fault-finding ability and sheer breadth and depth of know-how.
I must however sound a note of censure - your boots man, they are a disgrace!
I offer in evidence post #106.
awemawson:
You are NOT the first MadModder to comment on my footwear :clap:
They are 'Crocs Bistro' of the unvented side variety in white (yes honestly!) intended for the catering industry and I get through three pairs a year. This particular pair were actually thrown in the bin a few weeks ago, but I rescued them when I was Creosoting those chicken sheds, as the stuff goes everywhere. I do like to properly wear them out and get my monies worth.
I wear Crocs all day every day about the farm, or if I'm walking the dogs in the woods or countryside, and as I take UK size 13's which are difficult to find I buy them well in advance if any turn up at a sensible price - I do have four brand new pairs in stock for future use :ddb:
awemawson:
Now of course the mistake Mr Plonker made fitting this fan was to draw 240 volts from one phase referencing it to earth. All well and good if the earth current isn't being monitored by a safety device (The RCD)
What he SHOULD have done in this situation where no Neutral is available, is to use a small transformer primary between two phases (415 volts) and secondary giving a floating 240 volts. The fan can still be earthed (though it wasn't !)
But what size transformer? Well I can't find a model number on the Vent Axia so I needed to measure the current draw rather then look it up. So I temporarily wired it to 240 volts leaving a long enough neutral wire for my Fluke clip on meter to measure the current.
Answer : 200 mA . Now I'd expected a start up surge, but actually it starts at 100 mA then increases to 200 mA. so a 50 VA enclosed 'panel transformer should do the job nicely - off to eBay to find one :clap:
Pete.:
I think I've seen one of those in a disused panel at work Andrew. I'll have a look tomorrow.
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