On Wed, 6 Apr 2011, Rafael Skodlar wrote:
> Date: Wed, 06 Apr 2011 09:45:45 -0700
> From: Rafael Skodlar <ra...@linwin.com>
> Reply-To: "Enhanced Machine Controller (EMC)"
> <emc-users@lists.sourceforge.net>
> To: emc-users@lists.sourceforge.net
> Subject: Re: [Emc-users] PSU
>
> On 04/06/2011 07:32 AM, Dave wrote:
>> On 4/6/2011 9:18 AM, andy pugh wrote:
>>> I have 400V servo drives, 350V motors and 240V single-phase mains power.
>>>
>>> Is there any reason that my servo power supplies need to be anything
>>> more than a rectifier and caps? I will probably add a PTC and an input
>>> fuse too.
>>>
>>> Would a relay/resistor to discharge the caps on power-off be usual?
>>>
>
> Perhaps but not enough when you are dealing with huge surges. A
> commercial mill had problems with burnt relay contacts which had to be
> replaced all the time. The owner was not amused with the downtime and
> cost of replacement parts. I installed RC snubber circuit which solved
> the problem. That was over 15 years ago.
>
>> Do you have any concerns about the drives regenerating into the power
>> supply during decels and causing a bus overvoltage - I'm assuming that
>> the drives can regenerate... perhaps they cannot?
>
> Disconnecting large currents will cause arching, ripple on power lines
> in other circuits, radio noise, etc.
>
> Try "snubber circuit" on search engine to find volumes on this subject.
> I'm sure others have solved these problems and circuits are readily
> available.
>
>> I don't know why a relay/resistor wouldn't be a better solution for
>> discharging the caps. Power goes off, the relay closes and the caps
>> discharge..
>>
>> Dave
>>
>
> Relays are very slow in comparison to semiconductors. Better use
> combinations of other components like varistor inverse-series Zener
> diodes (transorbs), FETs, etc. which are not too expensive these days to
> create an effective circuit that will protect your driving systems.
> Circuits allow you to "ramp up" the discharge rate rather than simply
> short circuit it through the load.
>
> Perhaps start with a calculator for your situation:
> http://www.daycounter.com/Calculators/Snubbers/Snubber-Design-Calculator.phtml
>
> I wish snubber website http://www.snubberdesign.com/why-snubbers.html
> was available when I was looking for this kind of information.
>
> Just keep the smoke in your circuits.
>
> --
> Rafael
I think there's a mis-understanding of the purpose of the relay here. It is
not for snubbing or braking in normal operation (these are handled by
semicoonductor devices) but for fast but controlled discharging of the filter
capacitors on the servo DC bus when primary power is removed (ether by power
down or E-STOP). The relay/resistor combination can do this much faster that a
reasonably bleeder, say .5 seconds or so. This makes the power supply safer
and provides braking of all servo axis in a fault condition.
Peter Wallace
Mesa Electronics
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