Re: snubber resistor power rating?

From: John Smith (kd5yikes_at_mindspring.com)
Date: 11/28/04 

Date: Sat, 27 Nov 2004 20:43:44 -0600

"Terry Given" <my_name@ieee.org> wrote in message
news:5a8qd.14297$9A.276874@news.xtra.co.nz...
> John Smith wrote:
>
>> "Terry Given" <my_name@ieee.org> wrote in message
>> news:e20qd.14186$9A.271659@news.xtra.co.nz...
>>
>>>Arie de Muynck wrote:
>>>
>>>>"Neil Preston" ...
>>>>
>>>>
>>>>>So, how would typical resistors react to such a pulse? (In this
>>>>
>>>>application,
>>>>
>>>>
>>>>>it might occur as often as once per second.) Would a 1 watt resistor be
>>>>>sufficient?
>>>>>Would it be subject to internal arcing or other degradation?
>>>>>Would there be any performance/reliability difference in various types
>>>>
>>>>such
>>>>
>>>>
>>>>>as carbon comp, carbon film, metal film, MOX flame proof, wirewound,
>>>>>etc?
>>>>>Is there a rule of thumb for the ratio of the power rating to the
>>>>
>>>>intensity
>>>>
>>>>
>>>>>of the energy pulse?
>>>>
>>>>
>>>>I've once made the mistake of just calculating the power dissipation and
>>>>using a film resistor (triac, inductive load, snubber use). They showed
>>>>beautiful little sparks after a year of 1 Hz switching. Replacing them
>>>>with
>>>>(cheaper) carbon composite resistors solved the problem.
>>>>Lesson: never ignore the PEAK dissipation and current that may occur,
>>>>and
>>>>check the data*** of even a simple item like a $0.03 resistor.
>>>>
>>>>Regards,
>>>>Arie de Muynck
>>>
>>>Hell yes. I learned this very early on, when placing damping resistors in
>>>series with Y caps in big EMI filters. One day we noticed a flash of
>>>light when we switched the prototype on. We switched it off, pronto. A
>>>thorough set of diagnostics found no problem, but we weren't
>>>hallucinating so kept looking. And the 1R 2W PR02 resistor I had in
>>>series with the 100nF Y cap was open circuit. As were ALL of them, in all
>>>of the prototypes. We then asked the peak-power question, which was
>>>something like (400V*1.41)^2/1R = oh ***, 314kW. And a
>>>carbon-composition resistor solved the problem. HVR make some real good
>>>ones :)
>>>
>>>Cheers
>>>Terry
>>
>>
>> Our three-phase, 460VAC, thyristor-controlled, DC and AC motor speed
>> controllers had snubbers and MOV transient suppressors. All our
>> controllers used the same snubber values. In two different sites (Denver
>> and Puerto Rico), one of the MOVs would explode and the customer would
>> send the unit back for repair. After noticing that the same unit had been
>> returned more than once for the same problem, we got to looking into the
>> cause more closely.
>>
>> It turns out that the firing of the thyristors in conjunction with the
>> snubber values and line inductance can _create_ a transient, and
>> repetitive transients will destroy MOVs eventually. The controllers would
>> operate fine for a day or three, then fail. Simulation showed that we
>> needed to change the snubber values based on the controller model current
>> rating.
>>
>> We had no hard data showing that the line impedance in Denver and Puerto
>> Rico was higher than in other places, but, as I recall, simulation showed
>> that it most certainly could be the cause.
>>
>> Pardon me for posting a little off-topic, but I thought it might be
>> useful information.
>>
>> John
>
> Hi John,
>
> Thats a sneaky one. Heres a far less subtle problem:
>
> I had a 2am call from Germany a few years back. One of our 400kW drives
> kept blowing up - the Germans built it into a machine, which they sent to
> Korea (IIRC), whereupon it promptly blew the hell out of the VDR board.
> They replaced it and boom. They replaced it and boom. They called me.
> Turns out Korea had some whacked form of 3-phase power distribution (open
> delta? I forget - I used to have a power systems of the world book) in
> which the phase-earth voltage was equal to, not 1/sqrt(3) of, the
> phase-phase voltage. Poor old mr VDR lasted a few seconds then pop. The
> solution - sidecutters and no VDR.
>
> Cheers
> Terry

I had similar problems in west Texas with some irrigation machines. You run
into all sorts of things when you work with 3-phase power. I hated it.

John

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