|202 forum posts|
Trying to find the fault that is causing the "differentiator" to trip in our consumer box when I plug my Unimat into the socket has resulted in my terminally breaking the diode that allows low speed.
|82 forum posts|
Looks like a DS 1.8 16A general purpose rectifier - 1.7A If / 1600V PIV
(Possibly used to half-wave rectify the incoming mains to give reduced speed, based on your description, but I have no knowledge of the Unimat circuit).
|John Olsen||24/11/2020 03:25:02|
|1125 forum posts|
Yes, that diode gives the reduced speed on the half way position on the switch. It is unlikely to be the original problem since even if it went short all that would happen is that you would get full speed all the time. It is more likely that you have a problem with an interference suppression capacitor, which you are likely to find inside the motor itself. They are intended to reduce the interference from sparking at the commutator, and would usually be connected to ground, so if they are short, or leaky, or sometimes just too large they can trip an earth leakage breaker.
If that is the problem, make sure that any replacement is of the correct voltage and service rating as they are across the mains and so require to be rated for 240V AC. So a DC rated capacitor is not suitable, even if it has a higher voltage rating.
It is also possible that the motor windings have gone leaky to ground. Unimat motors get worked quite hard and it is easy to get them too hot. That is why I have two dud ones lying around, and the machine now has a small three phase motor with a VFD.
6681 forum posts
+1 to that.
Not familiar with the Unimat either but in addition to, or instead of, a capacitor(s) at the motor, there's likely to be a capacitor suppressor at the mains input. Being highly stressed by being plonked straight across the mains, in part to remove voltage spikes produced by the motor they aren't ordinary capacitors. Best not to replace them from a junk box. Don't panic - X and Y rated capacitors are inexpensive and easily sourced, for example Farnell. (Search for 'Safety Capacitor, or suppressor and look for X Y in the description). The Unimat may be fitted with only an X, or only a Y, or both. The old one should be labelled.)
Suppressors often fail by going open circuit silently and only occasionally go bang. Whilst working up to fail they may start by leaking a slightly excessive earth current, eventually building up enough to cause trips. Best not to remove suppressors permanently but they can be by-passed for test purposes. If the suppressor is out of circuit and the lathe still trips the mains, the motor is chief suspect.
|202 forum posts|
Thanks for all of the replies - the function of the diode is as suggested to half-wave rectify the inbound ac and to provide the lower of the two speeds. I have never seen a diode marked with a comma before, most of those I have come across have been in the 1N... series or similar. I will hunt down a replacement through our local suppliers.
Thanks also for the comments on the potential (pun not intended! ) causes of tripping - the Unimat is fitted with an XY suppressor, or was until I removed it when a similar problem appeared a couple of years ago - It gets very little use and I didn't quite get around to replacing it.....
The only remaining capacitive devices are a couple of small ceramics at the brush plate which I will snip out to test, hoping that the fault lies here.
There is no measurable line-earth resistance to the ability of my digital meter - given that the on-off switch also appears sound and the tripping happens on inserting the plug into the socket, I would expect a neutral-earth fault. Also, there is no smell of hot or dead windings which I know only too well, nor any visible signs of destruction - even if this not exactly a scientific test.
Tedious and the garage has become cold at night and work is keeping me occupied in my(warm) office during the day, so I have limited time to fault-find. Suppressors and diode on order in the hopes that the problem is not windings related.
I will report back when I find something.
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.