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Posted

Hi Andy. I'm a bit confused. The diode is marked DS 1,8-16A, so I am presuming that the 1,8 (European) is 1.8 here in Australia. You then say 7A is the key but the numbering says 16A. What should I be looking for?

1 hour ago, Neverenoughwatches said:

Still looks enough on the brushes, maybe stretchout the springs so the brushes have better contact with the commutator. Commutator looks like it has seen better days though, did it clean up ok ?

Commutator cleaned up fine. Just really black from carbon. There are wires in there that are coloured and everything was just black. It was no wonder that the motor started tripping the RCD and that I heard it arc internally when I turned it on just recently.

As the motor is still in bits, I will take some photos as to what it looks like now.

Posted (edited)
41 minutes ago, Michael1962 said:

Hi Andy. I'm a bit confused. The diode is marked DS 1,8-16A, so I am presuming that the 1,8 (European) is 1.8 here in Australia. You then say 7A is the key but the numbering says 16A. What should I be looking for?

The DS 1,8-16A is just a part number although the 16A does appear to match with the 1600V.

As you're operating at 240V (340V Peak to Peak) I suggested 600V VRRM (Voltage Repetitive Reverse Maximum) to give a safety margin.

The 7A is from the IFRMS column (I Current Forward Root Mean Squared) this would normally be 3 to 4 times the motor rated current for inrush.

image.png.93cd1e4b105512a170dcae7495cfa1a1.png

30 minutes ago, HectorLooi said:

I have never seen a diode in that package before. Are you sure it's a diode?

Where in the motor was that "diode" found?

Here's the full data sheet for the diode in question.

https://elektronik-lavpris.dk/files/sup0/126620DSdioderBBC.pdf

It's just in one of the supply legs. Check out this post.

https://cr4.globalspec.com/thread/3527/Diode-speed-control-and-uneven-brush-wear

 

 

Edited by AndyGSi
Posted

You're a wealth of information Andy. Thanks very much.

So based on the numbering on the diode that I have, I need a diode that is capable of handling 1.8A @ 1,000V?

Does it matter if I get a diode with a higher rating than this?

Posted

Your diode is rated at 7A RMS forward current. The "DS 1,8" is part of the part number, not a current rating.  The "16A" is also part of the part number and in this case means it's the variant of the "DS 1,8" that is rated at 1600V. Andy mentioned that this may be overkill but Electric motors do produce repetitive voltage spikes that can be significantly larger than the supply voltage and I'd be reluctant to go below this. Australian supply voltage is nominally 230V but the motor might generate spikes as high as 1100V to 1300V.

The "DS 1,8" is rated at 7A RMS forward current, so your replacement should have at least this forward current rating. More than this is fine.

You may be able to get an exact match for the diode from here: https://www.1sourcecomponents.com/partinfo/DS1%2C8-16A/BBC.htm

However, I would probably be looking to get something from Australia. Try RS, https://au.rs-online.com/web/p/schottky-diodes-rectifiers/7690518?gb=s or https://au.rs-online.com/web/p/schottky-diodes-rectifiers/7614013?gb=s or https://au.rs-online.com/web/p/schottky-diodes-rectifiers/1952483?gb=s would suit, I think. It's been nearly 3 years since I last had them ship me parts and they then charged $12.95 for delivery.

Alternatively you might go cheaper with eBay IF you trust that the part supplied does actually have the specs they claim. I don't think I'd do that for a power diode in an application like this, but it depends on how risk averse you are and how tight money is.

Posted

I also found this,

https://maker.pro/forums/threads/universal-motor-2-speed-rectifying-diode-kaput-1990-unimat-3-lathe.299232/

and the diode they mention here is one that Jaycar (about 15 minutes from where I live) actually have in stock.

https://www.jaycar.com.au/6a10-6a-1000v-diode/p/ZR1024#

Due to my ignorance of things electrical, would this diode be acceptable to replace the one from the motor? My concern is the difference in voltage. The original has a rating of 1,600 volts where this one is only 1,000 volts. However, I did notice that the question was asked whether the designers were just being overly cautious and over-engineered it.

The two diodes are completely different in apperance though, but I doubt that matters?

Posted

Ok. Everything from the motor label.

Type: Ug7750-25

95/65W

S3 80%

5000/3000/min

Intermittent duty 8 min on, 2 min off

That's everything that would be considered relevant to your question Andy. I hope anyway.

 

Posted (edited)

It's likely that the diode you specify would work fine and may well go forever.

The challenge is that just calculating based on the motor continuous power rating does not take into account the much higher inrush current when it is first turned on, but the difference between a 7A rating and a 6A rating is likely to be inconsequential: both are higher than the current is likely to be barring exceptional circumstances. The 1kV rating is also likely to be ok at least most of the time, but the switching of the inductive motor coils that happens at the commutator can give high voltage spikes that might exceed 1kV especially if the brushes are a bit iffy. (These spikes are the source of most of the sparks around the commutator.) Even a tiny model train 12V motor can produce voltages over 100V that can kill control circuitry if it isn't protected from them. The worst that's likely to happen with that, however, is that you cook your diode and it goes open circuit and needs to be replaced again.

Weighing up a two dollar part that you can get on Monday vs a $20 part you have to wait for, I'd probably be calling on Jaycar, but if it does fail again in short order you'll know why.

The different shape of the diodes is of no particular significance. Make sure you install it the right way around, though. The stripe end is the cathode. That corresponds to the left (broken/missing) lead on the existing diode.

Edited by GPrideaux
add shape comment +
Posted

In case You didn't understand what is the diode for, it is for the 'Half power mode'. The motor is 95W, but when switched to 65W, the diode is in series with the motor thus only half of the 'waves' of mains power supply the motor. There is no matter in what direction (anode/cathode) the diode will be placed. Also, no danger for the motor if the diode goes out of order: if it is 'shorted', then just full power will be in both modes, and if it 'opens', then motor will not work in half power mode.

Posted
4 minutes ago, HectorLooi said:

How long would a MOV last with repeated spikes in this application?

It would depend how big the spikes were but I wouldn't expect spikes above the 710V that often.

Posted
25 minutes ago, Michael1962 said:

So the MOV is superior in its performance than a diode for protecting the motor?

Have I got that right?

The diode has nothing to do with protection the motor and is just the means of giving a slower speed option as described by @nevenbekriev.

The idea in your case is that the MOV would prevent any possible spikes over 710V which would help protect the motor and in turn protect the 1000V Diode.

The downside to the MOV is that if it has to supress a lot of spikes then it will decay over time.

The post has got me thinking what spikes would be created by such a motor and I think I've got an old sewing
machine motor at work which is similar so will be looking at doing some testing when I return in the New Year.

Posted

Ah OK. Is your suggestion to put both the dice and the MOV in the circuit? I misunderstood you, I believe. I am not sure if there is enough room inside the motor housing to fit everything in.

Posted
49 minutes ago, Michael1962 said:

Ah OK. Is your suggestion to put both the dice and the MOV in the circuit? I misunderstood you, I believe. I am not sure if there is enough room inside the motor housing to fit everything in.

Yes both Diode & MOV but as my earlier post I wasn't sure about space.

How and where do the wires connect to the brushes?

 

Posted (edited)

Hi Andy,

I thought it would be best if I posted this and not tried to explain it myself.

Read 240VAC where it says 120VAC

I'm off to Jaycar in a minute to get the diode. I am getting 2 in case I make a mess of putting the first one in. Pennies in cost.


image.png.04f13613c76cbc041ae7e1ae6f1e86de.png 

Edited by Michael1962
Posted

That item is inside the motor enclosure packed in with everything else. It looks like it is fabricated in parts and then the wiring is inserted into the capacitor housing and then the filling compound added to everything. It does not look like a normal capacitor that I am used to. It is slightly bulged around the centre of the plastic outer. It all fits in inside the housing with the switch and diode in the compartment you see in the second photo.

 IMG_2252.thumb.JPG.07a34269a5f5b6966d5f633d839ef03d.JPG

IMG_2253.thumb.JPG.05c798a14e9881c28a4730e62aa05eb1.JPG

Posted

All fixed now for the grand sum of $5. Could have been $2.50, but I bought two diodes in case I botched the first one. Tight, but it all fits and it works more to the point.

In the first photo you can see the diode sitting just under the switch with the capacitor out of the way

IMG_2254.thumb.JPG.4ca21a1fd19da9859ce1234c682d86a5.JPG

Then with everything tucked away. Bench tested and have two different motor speeds. Just the plastic cover to put back on and then bolt it back to the lathe.

IMG_2255.thumb.JPG.4a97b84c54f002d5a6626bdf67b4553b.JPG

Really appreciate all the help guys. Thanks to everyone for their input.

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