Unimat 3 motor diode

[simondavies3Participant @simondavies3]

Hi All,

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.

Can anyone identify this – or propose an equivalent that I can hopefully find from my Big Box Of Electronic Scrap

[simondavies3Participant @simondavies3]

Identification

[Andy_GParticipant @andy_g]

Looks like a DS 1.8 16A general purpose rectifier – 1.7A If / 1600V PIV

https://www.web-bcs.com/diode/dc/da/DS1,8-16A.php?lan=en&cl=1

(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 OlsenParticipant @johnolsen79199]

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.

John

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Olsen on 24/11/2020 03:25:02:

… . It is more likely that you have a problem with an interference suppression capacitor, …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.

John

+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.

Dave

[simondavies3Participant @simondavies3]

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.

Simon

[John PenfoldParticipant @johnpenfold74932]

Old subject, but my little SL Unimat has, after all these years, stopped. The issue is that the 1A plug fuse is blowing. I have tested the safety capacitor resistance and that is nil between terminals.

Reading the messages about, the SillyOldDuffer thought that a replacement can still be found!

I am sitting here in front of this PC for some time and cannot find a 3 wire “safety capacitor” or “suppressor” in F-arnells or “RS” or all the other b/s on Google.

I am looking for an axial capacitor rated as 0.01uF 2x 2500pF – I cannot read the makers’ name as that is not legible.

Some help needed please.

J

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Penfold on 31/12/2021 14:59:29:

…

Reading the messages about, the SillyOldDuffer thought that a replacement can still be found!

I am sitting here in front of this PC for some time and cannot find a 3 wire “safety capacitor” or “suppressor” in F-arnells or “RS” or all the other b/s on Google.

I am looking for an axial capacitor rated as 0.01uF 2x 2500pF – I cannot read the makers’ name as that is not legible.

Some help needed please.

J

Finding a 3-wire axial suppressor capacitor is harder than I thought – there doesn't seem to be a way of filtering out all the 2 wire types, thousands of them!

If it's not absolutely vital it be axial, there's a suitable example on the first page on the Farnell link I posted, this one. It's 0.015uF 2x 2200pF which is close enough – the exact value isn't critical.

You can confirm the capacitor is responsible for blowing the fuse by removing it. For test purposes the motor will run without a suppressor.

Dave

[John PenfoldParticipant @johnpenfold74932]

Thanks for the prompt response.

Having read the supporting technical sheets, I have found a capacitor. Super, thanks for that.

I had to strip apart the motor to get to the capacitor and found a rough bearing. So I will change the bearings as well. I rather dread putting it back together as small hands and some craftiness required to get the wires in the proper places.

I will try putting it back together to confirm the capacitor failed.

J

[Gary WelchParticipant @garywelch79699]

Hi John. Could you share a link to the capacitor you found? I have the same problem. Thanks

[John PenfoldParticipant @johnpenfold74932]

Hi Gary

From R/S Components –

KEMET PZB300 Paper Capacitor, 275V ac, ±20%, 100nF, Through Hole
Stock no.:121-5449

It is a little too big to fit inside the motor so I added extra lengths of wire to take outside the motor cover. I will try to add a picture of what I have done.

The motor is running fine and back on the lathe.

J

[John PenfoldParticipant @johnpenfold74932]

Here are the pictures – rather big I think. J

[John PenfoldParticipant @johnpenfold74932]

The side view. J

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