DIYbanter - View Single Post - Help needed. Zero crossing with RC snubber problem

Jamie

wrote:

On 27 Öå, 03:24, Jamie
t wrote:

michael nikolaou wrote:

Hi

I have a 12 v relay driving an large 220 volt AC relay . Across the contact
of the driver relay i placed one RC snubber circut (27NF with 100 R
resisitor in series) to help with some spikes that were influencing the low
voltage driver circuits.
The driver circuit is able to detect mains zero crossing and fire the
driver relay at an angle i choose .
From what i read the best point to switch off the power relay is at zero
crossing . I did that and i show a large spike up to 1 KV at the relay
contact followed by a decaying 500hz waveform to 0 volts . After some
experimentation the best point came exactly when switching off at the peak
of the mains voltage .At this point there is smooth decaying waveform to 0
volt after 5 periods of 500 HZ but no overshoot. The relay presents no
arcing. If i remove the snubber and make the experiment the best place to
switch is zero crossing but i also see large SHARP spikes up to 500 Volts
Peak.
My question is
The switching with snubber must be made at zero crossing or at the peak of
an ac voltage waveform ?
What is the behaviour of the circuit ?.
As i understand any large spikes can harm the X2 capacitor i'm using so
what is the best operating practise ?.

Any help will be appreciated

Michael

WHen you say relay, I assume you mean a mechanical contact?

if so, It takes time for the contacts the release. If you turn it
off at what you detect as the zero crossing point, the contacts most
likely will not actually release until some where in some mid point .
Many contactors are fast but not fast enough to open before current
can get a charge going.
That's just my evaluation of what you're doing.

By you signaling to turn off the relay at a peak, the contactor will
most likely not open until it gets near the zero crossing point.

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I observe all signals with a scope . I see actually the "driver relay"
signal .So all my observations
about timing are correct . My notion was different though .I expected
to turn off at zero crossing
voltage and have no arc .Actualy when the transition happents at the
peak of voltage then i see only a
decaying waveform ,no overshoot, for which i suspect since i know the
capacitor and measure the
frequency one could calculate the total inductance value (cables+relay
coil). Since its clear that V(emf)=-L*di/Dt
as pointed out by your emails also then the correct point to switch
off the driver relay is at peak voltage
since current lags voltage by 90 deg in any inductor.
Now i only have to find out how much can a relay type deviate from the
measurements i have made.
Thank's for all you help guys
if you're trying to save the contact life you can assist it with a SSR
across the terminals.
The SSR will conduct just prior before the contacts close, this will
create a shunt on the contacts. when the contacts finally make, they
will remove the load from the SSR.
When opening the contacts, the SSR will switch to save the day and
unlatch at the base line. If you decide to employ this, you need to have
a snubber tide across the SSR because of slight delays of the SSR, you
could damage it when the contacts open on a high peak.
Just select a SSR with the same turn on voltage as the contactor and
tie them to the same control voltage.

--
"I'd rather have a bottle in front of me than a frontal lobotomy"

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