I promise there is an aviation link here, albeit tenuous.
I lost a static plug for my TB20. So I took Peter’s advice and decided to make me a new one, from nylon rod. That meant digging out my 40-year old Unimat 3 lathe, from my model train days. I last used it maybe 15 years ago. To my amazement I found all the bits and pieces and got it working.
But… after about 10 minutes it trips the ELCB. If I leave it for a while I can use it for another 10 minutes. Not ideal.
New motors are hard to find and ruinously expensive. So my thinking was to put a galvanic isolation transformer, available for €100 or so, between the lathe and the power. I THINK this will isolate the earth leakage and stop the breaker tripping. But I’m not 100% sure. Could someone with more electrical knowledge than me confirm or deny?
Yes that will fix it.
A common problem. This week a window cleaner’s pump was tripping our ELCB (nowadays called RCD). I used an isolating transformer to fix that
You get this with fluorescent lights too; same solution. And fridges/freezers, and the normal way to solve that one (since you don’t want your frozen 1/4 of a cow to thaw while you are on holiday to Ibetha) is to connect these before the RCD, using a specially wired power socket 
It means your motor winding (the stator) is buggered, possibly due to past overheating…
It means your motor winding (the stator) is buggered, possibly due to past overheating…
Entirely possible, but what’s surprising is that the breaker trips even if I don’t turn the motor on, though it takes a bit longer.
I COULD take the motor to pieces, it’s probably filthy inside. But could I persuade it to go back together again afterwards…? Easier to buy the transformer.
the breaker trips even if I don’t turn the motor on
That suggests a short between Neutral and Earth (lathe chassis). Very possible with damaged stator insulation, which is only going to be wound with an enamelled copper wire…
Normally there is a small voltage there, because N is connected to E at the substation (the nearest 11kV → 230V transformer), say 1-3V, and this voltage produces enough current in the short to unbalance the RCD enough to trip.
Wiring practices are country-dependent, however.
johnh wrote:
Entirely possible, but what’s surprising is that the breaker trips even if I don’t turn the motor on, though it takes a bit longer.
This is rather odd as an RCD always trips instantaneously – it’s not like a fuse or circuit breaker which trips after a delay with moderate overload.
That means that your earth fault develops over a period of time. That would not be surprising when the motor is running as the windings will heat up which will cause mechanical movement, but with the motor turned off…?
It is also possible for an RCD to be faulty – over sensitive. I’ve seen a surprising number of those!
The N-E case can be elusive because the N-E current (the “unbalance current”) will vary with the N-E voltage, which depends on the current flowing around the L-N loop (i.e. the applicance load), and that will include the activities of other houses.
This is a common problem. I have two expensive German bench top machines, from Wabeco. One turret mill, about 8k, and one lathe, about 3.5k, bought 10 years ago. The motors in both are low cost chinese jobs, with various problems. I plan to fit 3 phase brushless motors to these, but all the time they are running, I have other stuff to do.
However, a 3 phase motor needs to be physically larger for the same power than a DC/AC brush motor, so it may not be a trivial replacement.
The inverters I like are from Control Techniques, but there are lots of options e.g. Parker…
I have a Unimat 3 sitting on my desk as I write this. These motors have given much grief over the years – just Googloe ‘Unimat 3 motor’! Despite which mine (40+ yrs old) still works perfectly.
This is a brush motor and one possibility for the leakage current might be carbon dust from the brushes. Could you remove the brushes and blow it out with compressed air? If it is a winding short, the transformer is only going to delay the inevitable. There are lots of home brew motor solutions – see the Google search.
