[Amps] Solid state relays..again

Thu Feb 13 10:08:40 EST 2014

That's why I put step start in my large Astron power supplies. No more 
thump. No more blown up power switches.

73
Jim W7RY

On 2/13/2014 4:01 AM, Ken Durand wrote:
> " And this, my dear friends, is why equipment with steel boxes and big
> transformers inside can make that "WHummpppp" at the moment it's switched
> on.
>
> Of course, in equipment controlled by a simple mechanical power switch, it's
> impossible to decide when exactly in the cycle the switch will close. It's
> this a lottery how close we will get to the peak voltage point, or the zero
> crossing. That's why sometimes the "WHummpppp" is strong, sometimes weaker,
> and sometimes even non-existent."
>
> Thanks for that bit of wisdom Manfred. I never could figure out why
> sometimes I get the "WHummpppp" and other times the sound would be quite
> different or non-existent like you described.
>
> Ken
> N4zed
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Manfred
> Mornhinweg
> Sent: Wednesday, February 12, 2014 12:56 PM
> To: amps at contesting.com
> Subject: Re: [Amps] Solid state relays..again
>
> Hello,
>
>> Once in a while that thing turns on with a WHummpppp and a low
>> frequency ringing that just trails off.
> Let's see if I can explain this in a way everyone can understand.
>
> When a power transformer is operating normally, the magnetic flux density in
> its core fluctuates in a sine wave function. The peaks of this sine wave are
> usually roughly where saturation of the core begins, but is not yet too bad.
> Typical values for power transformers would be between 1 and 1.3 tesla of
> flux density. Those who learned electromagnetics at least 40 or 50 years
> ago, probably are more used to the antiquated gauss unit - anyway, 10,000
> gauss is 1 tesla.
>
> The sine wave of the magnetic flux is in the same phase as the magnetizing
> current in the primary, which is 90 degrees lagging the line voltage. This
> means that when the line voltage crosses zero, flux density is maximum, and
> when the line voltage is maximum, flux density is zero.
>
> Whenever the line voltage is positive, flux goes up, and whenever line
> voltage is negative, flux goes down (accepting some simplistic conventions
> on what is positive line voltage and increasing flux). So, at the time the
> voltage waveform crosses zero going from negative to positive, the flux
> density is at its negative peak. With the voltage being positive from there
> on, the flux density goes up - that is, it goes from -1 tesla to -0.9 tesla
> to -0.5 tesla, etc, reaches zero, then gets positive, keeps climbing, and by
> the end of the voltage semicycle the flux density has become +1 tesla. Then
> the voltage goes negative, making the flux density start going downwards
> again.
>
> This should be clear enough, I hope. Now let's see what happens when you
> switch on a transformer.
>
> If you do that precisely at the peak of the line voltage, which is the time
> when the flux density should be zero, and given that it _is_ zero in the
> transformer that was off, the transformer simply starts operating, without
> any sort of strange behaviour, overcurrent, spikes, or anything. This would
> be the optimal situation, from the transformer's point of view. But if you
> power up the transformer at any other time, things are not as good. The
> worst situation is switching on the transformer at the voltage zero
> crossing. In this case, the core is at zero flux density and now gets a full
> semicycle of voltage in one and the same polarity. So, instead of starting
> from -1 tesla to go to +1 tesla, it will start from 0 tesla and thus will
> try to go up to 2 tesla in that first semicycle! And 2 tesla causes total
> saturation of the core. When a core saturates, several things happen. One is
> that the inductance of the windings drops drastically, producing a strong
> current pulse in the primary. And another is that the core no longer
> concentrates most of the flux, so that a significant amount of the total
> flux leaves the core, and flows through the air around it, and specially
> through anything magnetic in the vicinity. If the transformer is housed in a
> steel cabinet, the panels of that cabinet will attract a huge part of this
> leaked flux.
>
> During the next several cycles, the resistance of the windings, losses of
> the core, etc, play together to gradually center the magnetic flux around
> zero. That is, while in the first cycle the flux moved between zero and 2
> tesla, during the second cycle it might fall between -0.2 and
> +1.8 tesla, in the third cycle it might go from -0.35 and +1.65 tesla,
> and so on. It can take ten or more cycles to center the flux well enough to
> make the transformer work normally, without spreading flux around any
> longer.
>
> The cabinet panels will thus pick up lots of flux initially, and then
> gradually less, over several cycles. And they will act like electromagnets,
> while they are picking up flux. This makes them move, proportionally to the
> flux they get. And since the flux is alternating, the panels will vibrate.
>
> And this, my dear friends, is why equipment with steel boxes and big
> transformers inside can make that "WHummpppp" at the moment it's switched
> on.
>
> Of course, in equipment controlled by a simple mechanical power switch, it's
> impossible to decide when exactly in the cycle the switch will close. It's
> this a lottery how close we will get to the peak voltage point, or the zero
> crossing. That's why sometimes the "WHummpppp" is strong, sometimes weaker,
> and sometimes even nonexistant.
>
> OK. Now somebody out there might say that it would be a good idea to make a
> circuit that always powers up an amplifier or power supply at the peak
> voltage, to avoid that "WHummpppp". But it ain't that simple... We aren't
> powering up just a transformer, but instead a combination of a transformer
> and filter capacitors. The filter caps would prefer to be powered up at the
> zero crossing... they don't agree with the transformer at all!
>
> That's why large power supplies should have at least a step start circuit,
> or even better, a full soft start circuit. Full soft start can be combined
> with power factor correction in the same circuit.
>
> About solid state relays: They exist with TRIAC output, SCR output, MOSFET
> output or IGBT output. Each of them can be made in versions that switch on
> immediately when they get the control signal, or at the next zero crossing
> of the applied voltage. All SCR and TRIAC SSR's switch off as soon as the
> current through them falls to a certain level, after the control signal has
> ceased. Usually that's close to the first current zero crossing after the
> control signal ceases. But note that this is the _current_ zero crossing,
> not voltage! MOSFET and IGBT SSR's can be made to switch off immediately
> when the control signal ceases, or at the next current zero crossing.
>
> Handling inductive loads is more difficult than resistive ones, because of
> switch-off, not switch-on. When a SSR switches off, there is still a small
> current flowing in the load inductance. Even if small, it can be enough to
> induce a high voltage spike at the moment the SSR switches off, and this
> spike is easily strong enough to make the SSR switch on again, which
> collapses the voltage, leaves a tiny current flowing, so it switches off
> again, causing the next spike... leading to an oscillation, which places
> great stress on the small control areas of the semiconductor structure that
> forms a TRIAC or SCR.  This is what can kill SSR's or TRIACs when switching
> inductive loads.
> The trick to safely switch inductive loads is to install a snubber in
> parallel with the power device. It's typically as simple as a 0.1 µF
> capacitor in series with a small 100 ohm resistor. This will absorb the
> small residual current of the inductor, avoiding the creation of a voltage
> spike, and will give the power device time to switch off fully.
> But this network will also pass a tiny current even while the SSR is
> supposedly off. That's why not all SSRs are fitted with that snubber.
>
> Enough for now...
>
> Manfred
>
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