Ulf,
I can't specifically address your issue, but I ran into a problem a few years
ago when designing a remote SSR to take the high-current inrush load off a SPST
On/Off power switch. I had always thought zero-crossing SSRs to be the premium
type of SSR that could be used as a 100% replacement in random-switching SSR
applications. Roughly around the time I designed the SSR remote switch, I
found the Crydom white paper shown in the link below.
The content in the paper may be known among others in the group, but I did not
realize that care must be exercised when using zero-crossing SSRs with highly
capacitive and inductive loads. The reason makes sense after reading the
material.
Here's one "takeaway" from the paper:
"Random turn-on relays are usually recommended for these types of loads as the
phase shift between voltage and current may cause problems with some
zero-crossing relays. However, most Crydom zero-crossing SSRs today work well
with motor loads, but historically we have always recommended random turn-on
relays. Phase-angle and burst-fire relays are not suitable for inductive loads."
http://www.crydom.com/en/tech/newsletters/solid%20statements%20-%20ssrs%20switching%20types.pdf
Paul, W9AC
-----Original Message-----
From: Amps <amps-bounces@contesting.com> On Behalf Of Ulf Tjerneld
Sent: Wednesday, January 9, 2019 1:42 PM
To: GEORGE WALLNER <aa7jv@atlanticbb.net>; amps@contesting.com
Subject: Re: [Amps] SSR in amp PS not working as expected
Hi George,
I think the main recommendation is to use random fire rather than
zero-crossing. But I’m new to the subject, so I’m open for arguments.
/Ulf
Sent from Mail for Windows 10
From: GEORGE WALLNER
Sent: den 9 januari 2019 14:25
To: Ulf Tjerneld; amps@contesting.com
Subject: Re: [Amps] SSR in amp PS not working as expected
Ulf,
I have used 100-s of SSR on various ship-board systems. They work great
provided they are used correctly and are of the right specifications, and a few
precautions are taken. Generally speaking, there are two kinds: zero crossing
and random switching. For your application you should use zero crossing. Also,
you should add 270 VAC varistors (MOV) between "neutral" (non-switched leg) and
both the input and output. In an RF environment it also makes sense to add a
10nF capacitor parallel with the control input and both outputs (from ground).
Also, for 240 V applications, I always use 480 V SSR-s. Just to have enough
headroom: switching inductive loads can create high voltage transients. An SSR
witll have about 1.8 V of forward voltage drop when on. At 10 Amps that is 18 W
of dissipated heat. You want to mount it either on a large metal surface or a
heat-sink.
It is better to find the right SSR for the application, rather than finding an
SSR and then trying to use it in an application. You need to know the
specifications, as there are many variations.
73,
George,
AA7JV/C6AGU
On Mon, 7 Jan 2019 19:31:21 +0100
Ulf Tjerneld <ulf@sm0nor.com> wrote:
Thanks a lot for all this information. I got some help from the local ham
community here too. Most of the feedback I get is to avoid them and stay with
traditional relays. I have learned a lot recently 😊.
However, I don’t give up that easily so I will investigate this a bit further.
If someone on the reflector has experience of SSR:s that actually do work,
please advise! I know of at least one power supply project published in QEX
where zero crossing SSR:s are used (W8ZR, QEX sometime back in 2013). At least
the Chinese ones he proposes are very cheap, about 10 dollars, so I’m inclined
to order a couple of them and try them out. But the delivery time from China is
something like 60 days, so I might give because of that ☹.
PS… the snubbing network didn’t work. Well, maybe a bit, I actually measured
103 volts on the 105 volt secondary. But still a very loud hum that I don’t
like.
73’s Ulf
Sent from Mail for Windows 10
From: Manfred Mornhinweg
Sent: den 6 januari 2019 23:26
To: amps@contesting.com; ulf@sm0nor.com
Subject: Re: SSR in amp PS not working as expected
Ulf,
there are two fundamentally different kinds of SSRs. One type uses thyristors
(TRIACs or SCRs), the other uses back-to-back MOSFETs.
These two (or three!) types have very different behavior. Not only that MOSFET
SSRs can handle AC and DC while the others are usable with AC only, but also in
that their handling of inductive loads is very different. Generally TRIACs are
poor with inductive loads, antiparallel SCRs are much better, and MOSFET SSRs
might be best if rated for inductive loads, but that needs a special circuit
inside them to avoid damage from inductive kick.
Thyristor SCRs will switch on either the instant you apply a control signal, or
the next voltage zero crossing after that, depending on their type, and will
always switch off at the current zero-crossing. With a purely inductive load
(and the primary of a very lightly loaded transformer is very close to that),
the current zero-crossing is 90 degrees out of phase with the voltage
zero-crossing. This leads to some head-scratching.
TRIAC SSRs will often trigger far more easily in one polarity than in the
other. This is likely the cause for your transformer hum: It's getting only a
semicycle instead of the full grid waveform, so there is a huge DC component
present, and the transformer core gets hopelessly saturated, with the magnetic
flux leaking out of the core.
Another possible reason for hum is that the SSR is switching on too much after
the zero crossing, so that there is a hard voltage step inside each half cycle.
The harmonics contained in such a step tend to be very loud.
You will need to look for SSRs rated for highly inductive loads, and preferably
of the back-to-back MOSFET type. Or else use old-fashioned mechanical relays.
One last thing: In some cases you can cure such thyristor misbehavior by adding
snubber networks. You might want to try this first. Simply add a
100 ohm resistor in series with a 100nF capacitor, of adequate AC voltage
rating, directly across the output side of each SSR, right at the SSR. With
some luck that might tame them.
Manfred
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http://ludens.cl
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