On 2/11/2014 2:27 AM, Jim Thomson wrote:
Message: 8
Date: Mon, 10 Feb 2014 19:46:26 -0600
From: "Paul Kraemer" <elespe@lisco.com>
To: <amps@contesting.com>
Subject: [Amps] Crouzet SSR - FAQs
There is a wealth of information here worthy of reading which may further
understanding of solid state relays, the application, what they can and
can't do and the myth of zero turn on related to inductive loads
http://www.crouzet-ssr.com/english/faq.shtml
Paul K0UYA
## Scroll down part way in the url...and u will see that the AC SSRs all
use ZERO CURRENT shut off.
They also state that SCR SSRs are better for high inrush and inductive loads......VS
the resistive load SSR’s.
I looked at P+B 70A SSRs years ago..with one in each leg of the 240 line.
The heat sink requirements were huge..
like 1 square foot PER 70A SSR device. And that was using .25” thick AL
plate.
P+B did have one good design example however....like a typ industrial
application. The example they used was a 150 HP
electric motor, running on 600V, 3 phase power. The SSRs could be series for
more V....and paralleled for more I. It all works,
BUT
I can see the worse case scenario would be the case where the amp is running 1.5 kw
out RTTY – FM..and the SSRs are then
shut off. The back EMF would be just wicked. Transorbs are built into some
SSRs. Transorbs and movs can also be
wired across the output of each SSR device.
The peak current every 8.3 msecs in normal operation is wicked.... like 4-6
times what you measure with a clamp on ammeter.
This is with a C input filter. Most SSRs I have seen still have a small
leakage current on their outputs.
In the end, I gave up on em....and used heavy duty contactors instead. 135A
CCS, 3 pole contactors, NIB, are still only
$50.00 each at the surplus place in the usa. They come with 220 vac
coils....so the coils can be run on anything from
208-240 vac easily..with no drop resistor required. They also come with 3 x
sets of AUX contacts. You can do the
same with 20-50A 3 pole contactors.
I went with a 60A 3 pole contactor for my rotary 3 phase converter. I
understand they have improved the SS 3-phase converters, so I may go
that route for the lathe and cut off saw. I doubt I'm going to build an
amp big enough to require 3 phase <:-)) Just something big enough to
run digital for extended periods with out over heating.
There are ... well, call them industrial recyclers on e-bay that have
the contactors and coil modules. Nice thing about those contactors is
the coil. They come in a molded module to fit the relay and you can get
them in most standard voltages. It's either one or two screws and drop
in the new coil.
The first time you look at new ones, the price will scare you. IIRC the
60A 3 pole was around $500-$600.
A friend had a 60A one he'd picked up at a garage sale for a buck or
two. They had it in a small NEMA enclosure and that thing was full of
...stuff.
Not sure what it had been used for but, It needed a bit of cleaning.
(understatement) The contacts were in good shape.The only problem was
the coil was for 440 and I have 240. I don't remember what I paid for
the coil off e-bay, but it was a tiny fraction of a new one.
If you buy off e-bay, make sure the contacts are good and not pitted. Go
with a place that guarantees them.
73
Roger (K8RI)
We paralleled all 3 x poles with 1 inch x 1 inch Angle AL to make 1 big 400A
spst contactor. Ditto with a 2nd contactor.
1 assy used in each leg of the 240 V line. A 3rd assy used for the step start.
For a laugh, I bought several 3 pole, 300A
CCS contactors. These used 120 vac for the coil.... but 1st went to a FWB..with
no cap. The raw output is 108 vdc, which
is what is actually applied to the solenoid coil.
SSRs have their place...... but with the big heat sink requirements in some
cases, I really see no real practical advantage...except
a SSR can be shut off faster....which will eliminate follow on current. A HV
fuse in series with a 50 ohm glitch resistor in series with
the B+ is ample for most cases. A 2nd HV fuse can be wired in series between
the plate xfmr sec and FWB. If you are real hardcore,
you can use an adjustable plate overcurrent setup.....to open off the coil to
the contactor in the 240 vac primary.
Jim VE7RF
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