" 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@contesting.com] On Behalf Of Manfred
Mornhinweg
Sent: Wednesday, February 12, 2014 12:56 PM
To: amps@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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http://ludens.cl
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