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Universal Motor 2 speed Rectifying Diode Kaput (1990 Unimat 3 Lathe)

ElMike

May 30, 2023
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Hi - This is a motor from a virtually Mint Unimat Modelling Lathe, having been in storage since circa 1980/90. It is Gefeg 220v/50Hz AC German made motor 80% duty, 220v/50Hz AC. It has a 2 speed switch (95/65W) with a rectifying Diode Marked 'DS 1.8' '16A' - Also 'R30A'.

It now runs at a single speed at both switch settings, which suggests the Diode has Shorted.

I am a rank amateur with Semi Conductor Specs and I am battling to locate a suitable replacement. I saw an earlier thread on this Motor that had some useful comments but looking for confirmation on a few suggestions made there.

"Any diode rated >= 400 V, >= 1 A should work. AN 1N4007 with 1000 V / 1 A is at the lower limit, current wise.

An 1N5407 is good for 3 A.

A P600M-E3/54 is good for 6 A, 12 times the rated motor current. With a surge current of 400 A this diode should easily take the inrush current of the motor."

I gather the last one would be the safest, if it is small enough to fit in the limited space but how do these three compare to the original specs?

Thanks in advance.

Circuit diagram below

U3 Motor Wiring.jpg Motor Diode.jpg
 

Minder

Apr 24, 2015
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I would go with the ;larger one, just to be on the safe side, My guess is a 1n4007 would not last long, there is high inrush to a series wound motor.
 

ElMike

May 30, 2023
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I would go with the ;larger one, just to be on the safe side, My guess is a 1n4007 would not last long, there is high inrush to a series wound motor.
My thoughts as well but I am inclined to aim at the P600.

A little further research had uncovered another forum on exactly the same subject (2012) stating that the original Diode DS 1,8 16A is actually rated at 7amp 1800V and also suggested the P600 P600M-E3/54 rated at 6A 1000V (Anything larger apparently are bigger tabbed types). It also suggested the likely in-rush could be around 400V.

That 7A 1800V rating seems seriously high. Any thoughts? Is this, as has been suggested already, a bit of 'Old world' Teutonic Overkill or would this make the motor more robust/resilient?

If the quoted rating is correct then the P600 would seem an obvious choice.

Original - 7A 1800V
1M4007 - 1A 1000V
1M5407 - 3A 800V (or 1M5408 - 3A 1000V)
P600 - 6A 1000V
 

Bluejets

Oct 5, 2014
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It also suggested the likely in-rush could be around 400V.
It's applied voltage, not inrush.
Inrush will refer to the "inrush current" ......
The 400v is rating the diode to withstand the applied peak inverse voltage of around 350v on a 240 AC supply system.(when the sine wave reverses)
No problem going higher (1000v) but any more would be just more cost, no extra advantage.

Otherwise your choice seems appropriate as Minder suggested.
 

ElMike

May 30, 2023
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May 30, 2023
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Thanks Folks > I have ordered the P600 - 6A 1000 as advised. Happy with the choice.

However, a German friend has suggested a couple of others (which I think might be unnecessary overkill). Interested in your take on the suggestions (for future Ref)

VS-20ETS12-M3 - 20A 1200V
DS145-16A - 48A 1600V (Pricey)
 
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